1Prof. John P. WaltersDep't. of Chemistry

St. Olaf CollegeNorthfield, MN 55057

507-646-3429walters@stolaf.edu

Fall, 2000Chemistry 382/378Role-Playing Lab in

Instrumental AnalysisExperiment #05

Polarography

Perhaps the most fascinating thing about solution electrochemistry is the fact that you literally stickone end of a wire into the solution you want to analyze, and connect the other end to yourelectronics and computer. There hardly could be a more graphic illustration of the interfacing ofcomputers, electronics, and chemistry than this. Of course, the wire is not simply a wire. It musthave certain properties in the solution, such as resistance to attack and the ability to conduct

electricity.

Even more fascinating than the ensembleof instruments collected to doelectrochemistry is one of the morehistorical wires that is stuck into thesolution. This is the dropping mercuryelectrode, an illustrative example ofwhich is shown here in Figure 1.

Mercury is placed in the leveling bulb atthe top of the tri-pod stand. The bulb isheld in an aluminum plate. A length oftygon tubing is tightly fit to the bottom ofthe bulb, and is dropped down into theelectrochemical cell. A short section ofcapillary tubing is tightly fit into thetygon. Because of the very small bore ofthis tubing, mercury flows slowlythrough it, forming a drop at the end thatdetaches every few seconds when itsweight is greater than what the surfacetension will support.

Mercury is itself a fascinating metal. Itnot only is an excellent conductor, it alsois a liquid at room temperature. And,other metals dissolve in it to formamalgams. The down side to usingmercury is its toxicity, which is severe ifinhaled, ingested, or absorbed throughthe skin. Portions of the Fisher ScientificMSDS on mercury follow, and should bethoroughly read before any lab work iscontemplated. Take care of this now,please.

Leveling BulbMercury Fil l

Tri-Pod Stand

AluminumPlate

TygonTubing

Epoxy Painted Shelf

CellSolution

Mercury Drop

Capillary

Tight Fit

Tight Fit

Figure 1 An illustrative dropping mercury electrode.

2MERCURY HAZARDS IDENTIFICATION

EMERGENCY OVERVIEW

Appearance: Silvery, reflective liquid.

WARNING! CAUSES SKIN IRRITATION. MAY CAUSE ALLERGIC SKINREACTION. THIS SUBSTANCE HAS CAUSED ADVERSE REPRODUCTIVE ANDFETAL EFFECTS IN ANIMALS. MAY BE ABSORBED THROUGH THE SKIN.CAUSES DIGESTIVE TRACT

IRRITATION. MAY CAUSE CENTRAL NERVOUS SYSTEM EFFECTS. MAY CAUSESEVERE EYE IRRITATION AND POSSIBLE INJURY. CAUSES SEVERERESPIRATORY TRACT IRRITATION. INHALATION OF FUMES MAY CAUSEMETAL-FUME FEVER.

Target Organs: Blood, central nervous system.

Potential Health Effects

Eye:Contact with eyes may cause severe irritation, and possible eye burns. Vapors may causeeye irritation.

Skin:May cause skin irritation. May be absorbed through the skin in harmful amounts. May causeskin sensitization, an allergic reaction, which becomes evident upon re-exposure to thismaterial.

Ingestion:May cause gastrointestinal irritation with nausea, vomiting and diarrhea. May cause effectssimilar to those for inhalation exposure.

Inhalation:Causes respiratory tract irritation. Inhalation of fumes may cause metal fume fever, which ischaracterized by flu-like symptoms with metallic taste, fever, chills, cough, weakness, chestpain, muscle pain and increased white blood cell count. May cause central nervous systemeffects including vertigo, anxiety, depression, muscle incoordination, and emotionalinstability. May cause severe respiratory tract irritation.

Chronic:Chronic exposure to mercury vapor may cause permanent central nervous system damage.Early symptoms include weakness, fatigue, anorexia, loss of weight, and disturbances ofgastrointestinal function. Unintentional tremors of the fingers, eyelids, lips, and entire bodymay occur at later stages. Behavioral and personality changes, increased excitability, loss ofmemory, insomnia and depression may occur.

3FIRST AID MEASURES

Eyes:Immediately flush eyes with plenty of water for at least 15 minutes, occasionally lifting theupper and lower lids. Get medical aid immediately.

Skin:Get medical aid. Immediately flush skin with plenty of soap and water for at least 15minutes while removing contaminated clothing and shoes.

Ingestion:Never give anything by mouth to an unconscious person. Get medical aid immediately.Wash mouth out with water.

Inhalation:Remove from exposure to fresh air immediately. If not breathing, give artificial respiration.If breathing is difficult, give oxygen. Get medical aid.

Notes to Physician:Treat symptomatically and supportively. The use of DMSA or BAL as antidotal treatmentshould be determined only by qualified medical personnel (Medical Toxicology, 1988).

ACCIDENTAL RELEASE MEASURES

Spills/Leaks:Vacuum or sweep up material and place into a suitable disposal container. Wear a selfcontained breathing apparatus and appropriate Personal protection.

HANDLING and STORAGE

Handling:Wash thoroughly after handling. Remove contaminated clothing and wash before reuse. Usewith adequate ventilation. Minimize dust generation and accumulation. Avoid breathingdust, vapor, mist, or gas. Avoid contact with eyes, skin, and clothing. Keep containertightly closed. Avoid ingestion and inhalation.

Storage:Store in a cool, dry, well-ventilated area away from incompatible substances. Keep awayfrom metals (such as wedding or engagement rings).

PRODUCT NAME: MERCURY (METAL)FORMULA: HG

FORMULA WT: 200.59CAS NO.: 07439-97-6

NIOSH/RTECS NO.: OV4550000

HEALTH 4 EXTREME (POISON)FLAMMABILITY 0 NONEREACTIVITY 1 SLIGHTCONTACT 3 SEVERE (LIFE)

HAZARD RATINGS ARE 0 TO 4 (0 = NO HAZARD; 4 = EXTREME HAZARD).

4Clearly, mercury is toxic. But, as long as there are no spills, as long as the whole electrode todeliver the mercury to the solution has been constructed, assembled, and tested prior to the time youhave to use it, and provided there are safe methods available to you to fill and empty the cellcontaining the solution into which the mercury flows, then the polarography experiment can be donewith only normal safe lab practices. That is the case here. When you come into this lab, you willfind two polarographic systems set up for you. You will have the opportunity to use them, and tochange the chemistry of the solutions being analyzed. But their assembly will have all been done for

you.

The Apparatus Pictured

The complete apparatus is pictured in Figure 2.

At the top of the apparatus is a leveling bulbpartially filled with mercury (Figure 3).

Contact is made to the mercury by inserting acopper wire into it. Over time, the mercury will show somecopper contamination, but for our uses here it will benegligible.

Mercury flows out of the leveling bulb downward througha length of tygon tubing into the cell that contains the DMEcapillary. The tygon tubing filled with mercury and enteringthe cell from above is visible in the center of Figure 4. Thethree fingered clamp holds the top of the capillary tube,which is inserted firmly up inside the larger tygon tubing.Clearly, the tygon glass seal has to be firm and durable. Itis the stiction of tygon on glass that makes this so. Tygonis, unfortunately, somewhat oxygen permeable.

Figure 2 The DME apparatus

Figure 3 The mercury leveling bulb

Figure 4 Mercury into the DME

5The bottom of the cell into which the DME is insertedholds the solution being analyzed. This is shown inFigure 5. Because the glass capillary and the glass cellwalls are equally clear, the DME as such is hard to see inthis picture.

There is a danger that in handling the solutions, the cellparts (top and bottom) might become accidentallyseparated. Since the bottom of the cell has a rather largepuddle of mercury in it after even a few hours of use, thiscould be a disastrous happening. Thus, the cell parts arefirmly held together with a cam type of clamp, and thecam is held in an engaged position by a plastic cable tie.

This means that the cell can only be filled and emptied bya combination of syringe (to fill it) and vacuum aspirator(to empty it). In this way, it is never necessary to take itapart and come into any kind of contact with the mercuryit may contain.

The vacuum pump that is used to aspirate out the contentsof the cell is shown in Figure 6. This pump pulls a vacuumsufficient not only to draw the solution out of the cell intothe waste bottle (Figure 7), but also excess mercury thataccumulates over time in the bottom of the cell.

This means that the waste bottle forms a keypart of the mercury safe handling system. Allmercury that is in it is typically covered by alayer of acidified water waste. The same istrue when the mercury is in the polarographiccell. The leveling bulb is partially stopperedat the top. Thus, mercury vapor is alwaystrapped in glass or under water, throughoutthe whole apparatus.

Figure 5 The bottom of the DME cell

Figure 7 The waste bottle for sample + Hg

Figure 6 The cell aspiration pump

6Nitrogen is used to deaerate the solution. This is done byfirst bubbling nitrogen through the solution (calledsparging) to displace the dissolved oxygen. After a shorttime, most of the oxygen has been removed and replacedwith nitrogen, which produces no electrochemical signal.After that, a flowing layer of nitrogen is directed over the topof the solution to keep it from absorbing oxygen from the air.

A two pronged gas delivery tube is used to control thenitrogen. It is shown in Figure 8. Two hoses come to it.When nitrogen flows through the front hose, it is directeddown into the solution. When it flows through the backhose, it is directed over the top of the solution.

The nitrogen pathway is selected with a small three waytoggle valve as shown in Figure 9.

When the valve is one position, gas flowsthrough one tube. When in the other position,gas flows through the other tube. Practice withthese valves is needed to master their use.

The toggle valves just switch the path of thegas. Its flow rate is set by a needle valvemounted on one of the legs of the tri-pod standas shown in Figure 10.

Typically, the toggle valve is first set to direct

the nitrogen down into the solution in the cell. Duringthis time, the needle valve is turned off. The needle valvethen is very carefully opened until a gentle stream ofbubbles is observed passing through the solution. Thisstarts the deaeration. It usually takes between 5 and 10minutes to deaerate a solution. After this, the togglevalve is switched to the position to direct the same gasflow across the top of the solution. Once set, the needlevalve is not changed.

When the system is shut down after a days run, thedeaeration gas is turned off, the

solution is aspirated out of the cell, and a small amount of deionized wateris played over the DME capillary tip to rinse it. Then the DME is allowed todrop for a few minutes. After that, the mercury flow is shut off bytightening a pinch clamp on the tygon tubing so it completely restrictsmercury flow (see Figure 11). In this way, the DME is cleaned and will beready to run the next day. It is never stored under water or in any electrolytesolution. Should it become plugged, it has to be discarded and a new oneplaced in the tygon. This is an elaborate, and undesired, job.

Figure 8 The nitrogen inlet tubes

Figure 10 The needle valve

Figure 9 A nitrogen toggle valve

Figure 11 Clamp

7The DME apparatus is on acart, and the potentiostat isplaced in a plasticware boxon the back of the cart. Adigital voltmeter is used tomonitor the potentiostat inputand output voltages. A largeribbon cable connects the 50pin terminal block on the sideof the cart to a companionMac, 640AV computercarrying LabVIEW and asingle NB-MIO-16L A/Dboard. Solutions to beanalyzed are placed on asmall shelf at the front of thecart. All of this is shown inFigure 12.

This completes theapparatus.

The Potentiostat

As shown in Figure 13,the potentiostat is madefrom 3 operationalamplifiers. The topamplifier receives theoutput from the D/Aconverter as a voltage andapplies it to the counterelectrode in theelectrochemical cell. Thereference electrode probesthe voltage droppedbetween the counterelectrode and the workingelectrode, picking off afraction of it close to theworking electrode. Thisvoltage is applied to thesecond amplifier. The 2ndamplifier plus theresistance between thecounter electrode and thereference electrode form

the feedback loop for the first amplifier. This sets the voltage applied to the working electrode to beessentially the same as the output of the D/A converter, no matter what the solution or referenceelectrode resistances are. Current in the working electrode is measured with the current follower, asdescribed on the next page.

Figure 12 The potentiostat and computer interface cable

-

+

Counter Electrode

Reference Electrode

Working ElectrodeVoltage Follower

Current Follower

-

+

-

+

Voltage Follower with Gain

Input from D/A Converter

Output to A/D Converter

+ 9

- 9

+ 9

+ 9

- 9

- 9

Figure 13 Basic 3 Electrode Potentiostat Circuit

8The key operating principle of the threeamplifier potentiostat shown in Figure 13 is thevirtual ground.

In Figure 14, we see that a current is the resultof a potential acting through a resistance. If oneend of the resistance is returned to the sameground point to which the low potential point ofthe battery is connected, then the current isdetermined only by the size of E/R.

In the bottom of Figure 14, we see that a currentfollower has as its input only a current,generated in any way that happens to beappropriate. When the amplifier as such isbalanced, then the potentials at the inverting andnon-inverting inputs are very close to the samevalue, whatever that value is. In a currentfollower, the non-inverting in[put is physicallyconnected to ground. Thus, the inverting inputis very close to zero potential, or it is virtually atground potential. We call this then a virtual

ground. A current into that point will be determined only by the effective potential that operatesthrough the effective resistance.

In the potentiostat, the potential is that applied by the voltage follower with gain to the workingelectrode and reference electrode. The resistance is complex and depends on the diffusion thatoccurs in solution. The current however, still is determined by just the applied potential and theeffective resistance. The current follower holds the potential of the working electrode at virtualground to ensure that this is so. This is illustrated in Figure 15 below.

i

R

E

-

+

( )i

R

Figure 14 Virtual ground in a current follower

LabVIEW PotentiostatV/t, i/t, and i/VGraphic Displays

LabVIEW PotentiostatDriver and Scan

Selection/Controls

NI NB-MIO-16L A/D,D/A and DIO Board toOutput to Potentiostat

VoltageFollowerw/Gain

CE RE WE

NI NB-MIO-16L A/D,D/A and DIO Board to

Input from Potentiostat

CurrentFollower

Electrochemical Cell

E

D

C

B

A

F

Figure 15 The 3 amplifier potentiostat connected to the three electrode electrochemical cell

difference between twoelectrodes, and the polarity of oneelectrode relative to the other. Themagnitude of the potential difference(DE) applied between the referenceelectrode and the working electrode isshown to increase progressively infour steps in both insets A and B ofFigure 16.

But, in inset A, we view the referenceelectrode as becoming more positive.In inset B, we view the workingelectrode as becoming more negative.In either case, the effect is the same.As the potential difference between thetwo electrodes increases, the referenceelectrode becomes more positivebecause the working electrodebecomes more negative.

In the potentiostat, where the currentresults from a potential difference

between the reference and working electrodes, the current follower holds the potential of theworking electrode at virtual ground, relative to the reference electrode. Thus, its measured potentialremains at approximately zero magnitude relative to the ground reference point from which thereference electrode potential is also measured. But, its polarity will always be negative relative to thereference electrode, simply because the voltage follower always drives the counter electrode positiverelative to that same ground. The voltage follower does this because it is wired that way.

In a word then, when the potentiostat voltage follower drives the counter electrode positive, and thecurrent follower holds the working electrode at virtual ground, the effect is the same as driving theworking electrode negative, and holding the counter electrode at ground. The potential differencebetween the two electrodes will increase as the output of the voltage follower increases. This ismagnitude. But the polarity of the working electrode will remain negative.

When we use the LabVIEW potentiostat, we talk about scanning the potential of the workingelectrode anodically (positive relative to the reference) or cathodically (negative relative to thereference). When the potentiostat output becomes more positive, we scan cathodically. When itbecomes less positive, we scan anodically. In both cases, the potential of the working electroderelative to ground remains constant, and very close the zero, as a virtual ground point. Setting aninitial anodic potential means setting the potentiostat output negative. This has the same effect assetting the working electrode to a positive (anodic vs. the reference) voltage. Setting an initialcathodic potential means setting the potentiostat output positive. This has the same effect as settingthe working electrode at a negative (cathodic vs. the reference) potential.

+

- - - -

+

++

+

- - - -

+

++

A

B

(0)

(0)

E E E E

E E E E

Reference Electrode

Working Electrode

Reference Electrode

Reference Electrode

Reference Electrode

Working Electrode Working Electrode Working Electrode

Reference Electrode

Working Electrode

Reference Electrode

Reference Electrode

Reference Electrode

Working Electrode Working Electrode Working Electrode

1 2 3 4

1 2 3 4

Figure 16 The relation between magnitude and polarity

10

The actual circuitry for the potentiostat is to be assembled within a plasticware box following thecircuit diagram shown in Figure 17.

Three type 741 operational amplifier chips are to be used. They should be placed in theexperimenters socket more or less as shown, paying particular attention to their orientation withregard to the pin one dot.

The plasticware box has connections that have to be made to the polarograph and tri-pod electrodestand. It will be up to you to identify the leads on the polarograph that connect to the SCE referenceelectrode, the DME working electrode, and the platinum counter electrode. All of these connectionsare shown as an electrical schematic in Figure 18, and also are shown as a pictorial diagram shownas Figure 19 on the next two pages. When you have identified these cables and connectors, you willneed to connect them to the proper outputs of the Figure 17 potentiostat in the plasticware box.Tutorial assistance likely will be needed, and will be available.

The D/A input and the A/D output of the potentiostat have to be connected to the 640AV Mac thatholds the NB-MIO-16 board and LabVIEW software. This is not difficult, but is best done withsome tutorial assistance. The connections needed are shown in Figure 20.

-9 vDC Battery

+9 vDC Battery

+9 vDC Power Bus (red colored all connected)

-9 vDC Power Bus (black colored all connected)

Ground Bus

(green colored)

2 3 4

5678

OUT toCounter

Electrode(Pt)

IN fromReferenceElectrode

(SCE)

IN fromWorking

Electrode(DME)

OUT to A/DConverter

(Channel 0)

GroundConnector

GroundConnector

External Power Supply Input Jacks(Disconnect internal batteries first!!!)

2 3 4

5678

2 3 4

5678

Modular Plug-In Feedback

Resistor

Battery On-Off Switch

In from D/AConverterChannel 0

GroundConnector

Figure 17 Wiring and connecting the plasticware potentiostat

11

11 22 33 44

55667788 22

33

66

77

44

( - )

(+)

(-9 vDC)

(+9 vDC)

(out)

( in)

( in)

2 3 4

5678

Pinouts for the type 741 Operational Amplifier

-

+

-

Counter Electrode

Reference Electrode

Working ElectrodeVoltage Follower

Current Follower

-

+

-

+

Voltage Follower with Gain

Input from D/A Converter

Output to A/D Converter

+ 9

- 9

+ 9

+ 9

- 9

- 9

Figure 18 Wiring the 741 operational amplifiers to make the potentiostat in the plasticware box.

12

+

-2

3

6

+9 v

-9 v

7

4

OUT toCounter

Electrode(Pt)

IN fromReferenceElectrode

(SCE)

IN from D/A Converter(Channel 0)

D/Aground

+

-

ground

2

3

6

+9 v

-9 v

7

4

IN fromWorkingElectrode

(DME)

A/DgroundOUT to A/D

Converter(Channel 0)

+

-2

3

6

+9 v

-9 v

7

4

OUT toCounterElectrode

(Pt)

D/Aground

PtSCE

DME

A/Dground

Figure 19 Physical layout of the connections between the potentiostat and the electrochemical cell.

13

SystemGround

Adc Input GrouND 1 2 Adc Input GrouND Adc In CHannel 0 3 4 Adc In CHannel 8 Adc In CHannel 1 5 6 Adc In CHannel 9 Adc In CHannel 2 7 8 Adc In CHannel 10 Adc In CHannel 3 9 10 Adc In CHannel 11 Adc In CHannel 4 11 12 Adc In CHannel 12 Adc In CHannel 5 13 14 Adc In CHannel 13 Adc In CHannel 6 15 16 Adc In CHannel 14 Adc In CHannel 7 17 18 Adc In CHannel 15 Adc In SENSE Line 19 20 DAC channel 0 OUT DAC channel 1 OUT 21 22 EXTernal REFerence dAc Output GrouND 23 24 DIGital i/o GrouNDbyte A DigIO bit 0 25 26 byte B DigIO bit 0byte A DigIO bit 1 27 28 byte B DigIO bit 1byte A DigIO bit 2 29 30 byte B DigIO bit 2byte A DigIO bit 3 31 32 byte B DigIO bit 3DIGital i/o GrouND 33 34 +5 V dc @ 500 mAmp+5 V dc @ 500 mAmp 35 36 SCANned CLocK outEXternal STROBE out 37 38 EXTernal TRIG ger inEXTernal GATE in 39 40 EXTernal CONVert inpulses SOURCE in 1 41 42 count GATE on in 1gate counted OUT 2 43 44 pulses SOURCE in 2count GATE on in 2 45 46 gate counted OUT 2pulses SOURCE in 5 47 48 count GATE on in 5gate counted OUT 5 49 50 Freq. division OUT

A/D Channel 0Input Twisted Pair

D/A Channel 0Twisted Pair

DigitalGround

DropKnocker

Line N2Line

VacuumLine

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

SystemGround

A/D Channel 0Input Twisted Pair

D/A Channel 0Twisted Pair

D/A Channel 1Twisted Pair

DropKnocker

Line

Vac.Line

N2Line

DigitalGround

ReferenceJumper - do not

remove

Figure 20 NB-MIO-16L board and terminal block connections

14

The connections shown in Figure 20 are of three types. The A/D twisted pair receives the output ofthe current follower in the potentiostat and carries it to the NB board as an input signal. The D/Atwisted pair carries the DAC-0 channel as an output to the potentiostat. These leads are color coded,and can be easily traced between the connector and the potentiostat. This should be done prior tousing the apparatus.

The drop knocker, nitrogen gas on/off vale, and aspiration vacuum pump are all switched on or offby solid state relays controlling the primary 110 vAC power to them. One set of solid state relaysused for this are shown in Figure 20. The digital lines from the NB board are then used as outputsto turn on or off the solid state relays. This is an excellent way to control devices that drawsignificant amounts of AC power.

When you are in the lab, trace the lines that run from the 50 pin terminal block to the devices shownin Figures 20 and 21. You will have to stretch to do this, since the solid state relays are underneaththe cart that holds the polarograph. Also trace out the ribbon cable that connects the terminal block tothe NB board in the Mac 640AV computer. The rest of the work involves the LabVIEW software onthat computer, so it is important that you have a good physical picture of what is being controlledbefore you start building the LabVIEW potentiostat controller.

+ -+ -

+ red- black + red- black

AC in

AC out

AC in

AC out

CircuitBreaker

AC LineSwitch

Switch 1 Switch 2

110 vAC Power Cord

SSR 1 SSR 2

Nitrogen Line Vacuum Pump

Figure 21 Solid state relays used to control nitrogen valve and vacuum pump devices

15

The LabVIEW potentiostat is a VI that is best viewed in terms of how the DME is operated.Consider the sequence portrayed in Figure 22 below.

Add Solution to Cell

Start Mercury Drop

Wait for Drop toGrow to Desired Area

Read Currentat DME vs. Ground

Detach Drop withDrop Knocker

Apply Current toDrop Knocker

Solenoid

1

2

3

4

5

6

7

89Repeat CycleUntil Done Change Potential

Set Potentialto Initial Value

Figure 22 Pictorial of DME sequence

16

The experiment starts when the pinch clamp on the mercury flow line to the capillary is opened andthe mercury drops start free falling (1). Then, solution is added to the cell (2), and the potential setto the desired starting point (3). Time is then allowed to pass while the mercury drop grows to somesize less than what it would take to freely detach (4), when the current is read (5). After this, thesolenoid controlling the drop knocker is powered (6), and the drop knocker plunger taps the side ofthe cell, causing the drop to fall off (7). The potential then is set to a new value (8), and the wholecycle is repeated (9) for as many times as it takes to complete the experiment.

One of these cycles takes 2 to 3 seconds. The number of cycles used to record the data depends onthe size of the potential change between drops.

To better see the timing, refer to Figure 23

When the potential is first set (1), it is held constant for a time D t. This time is set by the time it takesthe drop to grow to some desired size (2). Before the drop is detached (4), the current must be read(3), at least once for each drop.

Set Potential at E

Read Current for Drop #1

Detach Drop #1

Read Current for Drop #2

Read Current for Drop #3

Detach Drop #2 Detach Drop #3

Allow Drop #1 toGrow for time D t

Allow Drop #2 toGrow for time D t

Allow Drop #3 toGrow for time D t

Are

a o

f D

rop

Applie

d P

ote

nti

al

Time

Time

D t

D t

D t

D E

D EStep Potential

Cathodic by D E

Step PotentialCathodic by D E

E + D E

E + 2D E

1

2

3

4

5

6

7

8

9

1011

12

13

14

Figure 23 Timing of DME sequence

17

To compensate for small variations in the surface geometry of the drop when it is large, severalthousand current readings are usually taken in a fraction of a second and averaged.

Then, after the drop has been detached, the potential is stepped by an amount D E. This usually is asmall potential, so that there is a need for many drops to be repeated to scan the total voltage from 0to -1.7 or so.

The whole potential scan can take several minutes, especially if the potential is scanned from 0 to -1.7 and back to 0, and the results overlaid to ensure reproducible electrode response. The solutionthen is changed, and another scan done on a new solution.

All of the timing and reading shown in Figures 22 and 23 is done by the LabVIEW potentiostat VI.The front panel of the VI (Figure 24 above) is designed to show the potential and the current in twoforms.

The applied potential is displayed as a function of time in chart 1. The current that results from thisapplied voltage is displayed as a function of time in chart 2. The more traditional polarographicdisplay is evident when current is displayed as a function of applied potential (rather than as afunction of time) in XY chart 3.

The XY chart is equipped with cursor controls (4) that later are used to measure the half wavepotentials and the diffusion limited current for qualitative and quantitative analysis.

1

2

3 45

6 789

1011

12

13 14 15 16 17 18 19 20 21 22

24

23

Figure 24 The front panel of the Potentiostat VI (numbered for explanation)

18

The front panel also contains the controls that are needed to cause the potential to scan as shown,plus other functions. For example, a switch is provided (6) to determine if the potential should bescanned (as shown) or simply held at some initial value (11). The scan polarity can be set to becathodic or anodic with switch #5. The range of potential scan is determined by the final voltagesetting (12). When the VI is used with solid electrodes (instead of mercury) the scan usually starts atsome anodic initial voltage. This is set by the slider (10) as an initial anodic offset voltage.

The scanning voltage waveform is made using a cyclic function generator. To get a wave such asshown here, only one half of a complete cycle is needed, and this is entered in control #14. Thereare other possible waveforms to use for the scan. All waveforms are selected from the menu incontrol #13.

The output potential does not scan smoothly. The D/A output is discrete, i.e., it occurs in steps. Thesize of the step, D E, and the time for which it is constant, D t, is determined by the settings incontrols #15 and #16. This determines how long the drop is allowed to grow before the current isread. Shortly after the current is read, the drop is detached.

The switches needed to turn on the drop knocker (17), the vacuum pump to aspirate out the cellcontents (18) and to turn on the nitrogen gas for deaeration (19) also are on the front panel of theVI. These switches are queried only when the VI is first started, so it usually is best to turn the scanswitch #6 off, turn on the other switches desired, and then start and stop the VI. A bit of practice inthe lab will make this more clear.

Saving data to a spreadsheet for subsequent plotting and other manipulations is important. A buttonis available (#20) to allow this. When it is depressed, the data shown in the XY plot (23-24) will bewritten out to a file as named in path control #21, but not until after the scan has been completed. Ifthe scan is interrupted before it is plotted on XY graph 3, then nothing is saved. If the file name isnot changed between runs, then the new data are appended to the old, i.e., the file is notoverwritten, but just becomes longer.

The controls for smoothing (8,9) will be mentioned later. Half wave potentials and related currentscan be read by adjusting the positions of the cursors with the fine position diamond control (22), orby simply dragging the cursor with the mouse. Readout (4) will show the cursor intersections.

The data shown on graph (3) in Figure 24 are for a solution containing 0.001 molar Cu+2 and 0.001molar Zn+2 in 0.1 molar KCl and 5 drops of 0.2% Triton X-100. Wave 23 is for copper, and 24 forZinc.

19

The VI Diagram

The diagram for the potentiostat VI as set up for polarography is shown in Figure 25 below. Thereare four parts to it. At the left are shown the sub-VIs used to generate the voltage scanningwaveform. In the middle is the FOR loop containing the frames used to do the timing sequence anddata acquisition shown previously in Figure 23. The right hand side contains the file writing andsmoothing routines. To build the VI, or understand it better, it is the central FOR loop that needs tobe understood first. The other functions follow from that.

The first frame in the central sequence is shown in Figure 26. It is used to turn on or off the vacuumpump or the nitrogen gas flow valve. This are wired into digital byte zero on device 5 using bits 1

and 2 of the byte.

Whether they are turned on or off dependson the state of the Boolean control shown.This is set by the switches 16 and 17 on thefront panel.

Thus, if you have decided to build this VI,you should lay out the front panel first, andthen find the appropriate controls to wire inthe diagram.

Figure 25 The LabVIEW VI potentiostat diagram as used for polarography.

Figure 26 Gas and vacuum digital lines

20

Two sequence frames are needed to operate the drop knocker, and these are shown in Figure 27above. The drop is detached in frame 1 when digital bit 0 is made true, and held that way for 0.1second. This causes power to be applied to the drop knocker solenoid, extending its plunger andcausing it to strike the side of the cell. In frame 2, the same bit is set false, opening the circuit to thedrop knocker solenoid. The plunger is spring loaded so that when power is removed from the coil itretracts inside it.

Some practical work was done to determine that power needs to be applied to the solenoid for 0.1second. Less than this leads to taps on the cell that are too light to be effective. Times longer thanthis cause the solenoid coil to heat up. In some cases, the coil can burn out if power is kept on it fortoo long.

AC

S

SolenoidP

I

S

P

AC

A

B

Figure 27 Sequence frames used to operate the drop knocker

21

Once the old drop has been knocked off,and before the new drop has had anappreciable chance to grow to a fragilegeometry, one step of the potential scaneither is, or is not, applied to the electrodesthrough the NB-MIO-16L D/A converter.

Figure 28 shows the case where we wishthe potential to increase cathodically. Theoutput of the function generator is simplypassed through the TRUE case andpresented to the AO sub-VI. Channel 0 hasbeen selected on device 5. The voltage outis read by the AI MULT PT Sub VI andaveraged before being displayed. Channel 1is used.

It is quite important that the voltage bepresented to the electrodes before themercury drop has had a chance to growappreciably. If it is larger, then the suddenapplication of a change in voltage to itssurface can cause the surface to stream andflow sufficiently that the drop will bedislodged.

Figure 29 shows the case where we do notwant to scan the voltage. Each element then in the function generatoroutput array is multiplied by zero. There then are no changes in thetiming.

Note that frame 3 is held up until the timerinterval set by the front panel controls haselapsed. It is during this time that the dropgrows. The drop area increases, but thepotential is held constant on it by thepotentiostat output.

When frame 3 has timed out, frame 4executes. Here, the current is read. The AImulti-point sub-vi takes 500 readings of thecurrent at 5000 readings per second, andpasses the resulting array to the averagingsub-vi. Such an approach is needed tocompensate for the rapid fluctuations in thesurface of the mercury drop, which is itself

Figure 28 Applying a cathodic scan

Figure 29 Not applyinga cathodic scan

22

flowing as the drop grows. A single reading would not be at all reproducible from drop to drop.The result is multiplied and offset for display,and passed out of the frame.

The manner in which the different waveforms aregenerated to make the scan is interesting. This isshown in Figure 31.

A sub-VI is used to generate the waveform. It

can output a triangle wave,, a sawtooth,

, a sine wave, , or a square wave,

. In our work, we almost always used thetriangle or sawtooth waveforms, although it

might be interesting to try a sine wave sometime.

The sub-VI outputs a complete waveform array at once. The number of points in the array, thevariation in amplitude between them, and the number of repetitive cycles of each are all set byexternal controls or constants, as is the symmetry of the waveform and its phase.

Note in Figure 31 that the Initial Voltage and Final Voltage controls simply set the amplitude ofthe selected waveform.

The number of output points from the function generator array is the same as the No. of Stepsinput control. The first operation on this array is to rectify it with an absolute value function. Thismakes all values positive, and has the effect of doubling the frequency. This is why the Cyclescontrol is set to half of what would intuitively seem to be the right number.

The potentiostat does not achieve any timing capability from the function generator sub-VI, since itoutputs a complete array of points, essentially instantaneously. Timing is achieved when the array ispassed to the FOR loop. The FOR loops in LabVIEW are self-indexing. If an array of 100 points iswired into one, then it will automatically execute 100 times. If the array has 200 points, then theloop will execute 200 times. Within the for loop, the Wait milliseconds clock delays the sequenceframe that outputs the potential (Figure 28 and 29), and this is where all of the timing originates.

For future development work, the array of waveform points is completely available. For example,the direction of the scan in this VI is always from anodic to cathodic. This can be changed bymultiplying the array points each by -1. The starting potential also can be set either anodic orcathodic by adding a constant to each. The scan can be made logarithmic, have other signals addedto it at higher frequencies (e.g., for AC polarography), or be pulsed for pulse polarography byoffsetting groups of points. This mode of primary signal generation is flexible and simple, a trueplus for the LabVIEW approach.

Figure 31 The waveform generator sub-VI

23

A particularly interesting feature of this potentiostat is the ability to smooth the sampled currentpoints before saving or plotting them. This kind of smoothing is referred to as digital signalprocessing, and provides noise cancellation over the complete scan. The multiple sampling andaveraging done when the current is first measured does not, since it applies to a single time in thelifetime of a single drop.

Smoothing is done here using a Savitzky-Golay algorithm, implemented in LabVIEW by DanGilmore of the Chemistry Department at the University of Arizona. This algorithm is based ondoing a running average over a variable number of points in an array. But, instead of just calculatinga simple average, each of the points in the set is first multiplied by a weighting factor that makes

the net average over the set approach a leastsquares error minimization optimum value. Thenumber of points in the weighted average is setwith an external control, but the coefficients areall pre-calculated and loaded permanently into aconcatenating routine, as shown in Figure 32 for19 points. This smoothing routine is both fastand effective.

The SG smooth is a sub-VI that is simple towire. The routine used here is shown in Figure33. The output also is an array, which may betaken directly to a plotting routine.

Figure 33 Wiring the SG smooth sub-VI

Figure 32 The Savitzky-Golay algorithm for smoothing data in an array.

24

How the Potentiostat Gathers and Presents Data to its Display

To see how the VI works to give a smooth display from a dropping electrode, where the drop areachanges continuously as a function of time, some expanded scale recorder tracings were prepared toaccompany the brief explanation given previously in Figure 23. These follow here.

In Figure 34 are shown the actual polarogram of an 0.001 M solution of Cd+2 (center), anexpanded view of the individual drop behavior near the start of the wave (left), and the sampledcurrent/potential diagram obtained using the potentiostat VI (right). Observe the center polarogram.The current rises and falls as the potential is scanned, and as the area of the drop increases with timeuntil the drop knocker finally detaches the drop.

The behavior of individual drops (left inset) is interesting. The current rises as the drop areaincreases. Keep in mind that the applied potential is held constant during this time. Just before theend of the drop life the current is read. Then the solenoid of the drop knocker is actuated, and thedrop falls off. Note that there appears to be a small period of time when the current is rising,followed by a sudden increase. This is best understood by inspection of Figure 34B on thefollowing page.

Figure 34 Composite illustration of DME behavior

25

Here it is evident that after drop detachment the current begins its expected slow increase due toincrease in the drop area. But, the next potential then is applied, suddenly. This causes a surge inthe current (the so-called charging current) followed by a transient dip. The current then startsincreasing at the new potential as the drop area increases. The small noise transients near the end ofthe drop life are hard to diagnose, but may be associated with physical oscillations in the drop.

Figure 34B Expanded view of drop detachment transients

26

Analytical Lab Work

You will be working as a Software/Chemist team when doing this lab. There are two DMEpolarograph stands, and each team will have access to one of them.

There are several analytical projects that can be explored in this lab. They are:

1. Determine the concentration of an unknown sample, using a set of standards to make aworking curve.

2. Determine the effects of adding an unregulated amount of a maximum suppresser to thepresence of determinate error in determining a set of unknown samples.

3. Determine the effects of unregulated amounts of deaeration on the expected accuracy ofanalyzing a set of unknowns.

4. Determine the effects of variable amounts of acid in an unbuffered solution on the expectedaccuracy analyzing a set of unknowns.

5. Determine the effects of other cations present in a natural sample matrix on the expectedaccuracy analyzing a set of unknowns.

Project 1 - Determination of Unknown Concentration

This is the classic application for which the Nobel Prize was awarded. It is based on the fact thatunder diffusion control, in an unstirred solution, the limiting current is directly proportional toconcentration of the reducible species.

To help Chemist with meeting this objective, a set of cadmium solutions and an unknown havebeen prepared ahead of time for you. This means that all that Chemist has to do is load the cellwith the appropriate solution, add Triton X-100 maximum suppresser, deaerate, and let Softwaretake the scan and save the data. After that, Chemist would empty the cell using the aspiration pump,refill it with another solution, and repeat the process. Software would compile a working curvefrom the polarograms, and use it to determine the concentration of the unknown.

The recipe for the solutions is as follows:

Concentration Ml. to add X-100 to add N2 time Aspirate to0 in KCl blank ~ 50 6 drops 10 minutes waste bottle0.0004 M Cd+2 ~ 50 6 drops 10 minutes waste bottle0.0008M Cd+2 ~ 50 6 drops 10 minutes waste bottle0.0012 M Cd+2 ~ 50 6 drops 10 minutes waste bottle0.0016 M Cd+2 ~ 50 6 drops 10 minutes waste bottle

The unknown solution uses the same recipes as the standard solutions.

It will take some time to run these standards. During this time, all of the conditions in theexperiment must remain constant. One of the things that you should attempt to determine is overwhat length of time the experimental parameters do in fact remain constant. I did some work thissummer and found that the data shown in Figures 35 and 36 occurred. This convinced me that youshould be able to hold your experimental conditions constant enough to do this determination for atleast a period of 2 to 3 hours.

27

With data such as theabove at hand, the trendline function can be usedto draw straight linesthrough the limitingcurrent and residualcurrent regions, and fromthem the net diffusioncurrent calculated.Alternately, you maywant to use the cursorsthat are available on thefront panel of thepotentiostat VI, and setthem to read out the netdiffusion current.

An example is in Figure36. One cursor is put inthe residual currentregion. The other is put inthe limiting currentregion, but not whereoxygen would bereduced. Their differenceis the net limiting current.

Figure 35 Polarograms of Cd+2 at various concentrations for making a working curve.

Net Limiting Current

Oxygen Reduction

ResidualCurrent

Figure 36 How to use cursors to determine net limiting current.

28

When the data are taken properly, using either a spreadsheet or the cursor method, the results can beimpressive. For example, in Figure 37 is shown one working curve that was prepared frompolarograms taken both in the morning and the afternoon. A straight line with good regression dataresulted, even though the DME was stopped and the system shut down for lunch, and restartedfrom scratch after that. The DME characteristics, and all of the electronic settings, obviously heldover this long a period. Such performance cannot be guaranteed, but with careful techniques itshould be possible to determine several unknown concentrations over a full days work when thestandardization is done at the start of the day.

Figure 37 Cd working curve, taken from polarograms run over several hours

29

Project 2 - Effects of a Surfactant on Analytical Results

In preparing the solutions to use in meeting the first objective, you added 6 drops of a surfactantcalled Triton X-100 to each solution. In the past, other compounds like gelatin also were added.The function of these compounds is to coat the surface of the mercury drop as it forms with asubstance that will stabilize the flow that it undergoes while expanding. Mercury flows into the dropfrom the inside, and then circulates inside of it. It swirls outward as the drop grows, and causes thesurface itself to flow. This leads to mixing in the immediate vicinity of the drop surface, and eddiesand cavities form in the solution that produces irregular streaming along the drop. All of this canproduce strange, non-diffusion effects when the potential is sufficiently cathodic that the surfaceconcentration of the analyte is approaching zero.

When a surfactant is added to the solution, the streaming effects at the drop surface are minimized,and the polarographic wave approaches the limiting current smoothly. We expect then that there willbe better analytical results.

Consider the data shown in Figure 38 below.

The effect of adding X-100 is clear in Figure 38. To explore is how much of a difference this makeswhen an uncontrolled amount of X-100 is added to an unknown whose concentration is to bedetermined from a working curve made with standards that have reproducible amounts of X-100added. In other words, how critical is the concentration of X-100 in solution on the accuracy of themethod?

Figure 38 Effects of 6 drops of Triton X-100 on 0.0016M Cd+2 polarogram

30

Project 3 - Effects of Deaeration Time

Dissolved oxygen in test solutions has always been a headache in polarography, and in itsvariations. Oxygen is reducible in two steps according to the follow half-reactions:

O2 + 2H2O + 2e- H2O2 + 2OH

- E1/2 = -0.1

H2O2 + 2e- 2OH- E1/2 = -1.1

These two waves are irreversible, and drawn out over virtually the entire cathodic range. They alsoare pH dependent, and will change height with mechanical agitation of the solution as oxygen ismechanically displaced. Our book, unfortunately, is not clear on what the oxygen waves will look like at a DME in just asolution of supporting electrode. Figure 39, below, was scanned from Bard and Faulkner to giveyou an idea of what to expect in our lab.

The question posed in this objective is not if oxygen produces waves that clutter the cathodic regionof the polarogram that we use to analyze for Cd+2. Rather, it is how big a problem this will be? Willthe presence or absence of oxygen in solution make a difference in the accuracy with which we canmeasure the concentration of Cd+2 in a series of unknown samples?

Oxygen can be sparged from solution by bubbling a stream of nitrogen through it. There is alwaysa question of what flow rate of nitrogen to use, whether to saturate the nitrogen stream with water

Figure 39 Oxygen waves at a DME scanned from Bard and Faulkner

31

before passing it into the solution, how much to disperse it, how long to pass it through thesolution, and whether to blanket the solution with it after the deaeration has been done. Anotherimportant question is whether to deaerate the solutions to be analyzed in a separate part of theanalysis (say, in a set of bottles before adding them to the cell), or to wait until the actual analyticalaliquot is in the polarographic cell and deaerate it there.

An example of one way to explore this objective is shown in Figure 40. Here, a Cd+2 test solutionwas scanned both with and without deaeration. While there was little control over the design of thestudy, it did show where the greatest obvious effect of deaeration would be on the Cd wave.Clearly, you can come up with much better approaches.

When working on this objective, the question of timing the deaeration is at issue. Should the time beregulated, and, if so, how well? Is it enough to simply deaerate for more than some critical time? Ifcomplete deaeration is desired, will the time to achieve it make the total analytical time for a set ofsamples too long to be used in a quality control situation?

In exploring this effect, pose your questions well. I am especially interested in deciding if roboticsample preparation could be done, and, if so, could deaeration be done outside the polarographiccell as part of the sample preparation. I also wonder of there is a way to measure the Cd+2 diffusioncurrent so that deaeration is not needed at all.

When you use the apparatus that we have assembled here, try to avoid letting the way that we havebuilt deaeration into it influence your thinking too much. There certainly are other possibilities.

Figure 40 A simplistic look at the effects of deaeration on the Cd+2 wave

32

Project 4 - Effects of Solution Acidity

Consider the half reaction :

2H+ +2e- H2 E1/2 = - 1.5

This implies that small and variable amounts of hydrogen ion could have an effect on the Cd+2analysis. The fact that the half-wave potential is significantly larger than that for Cd+2/Cd doeshowever suggest that it might not be a problem. Most chemists know though that runningunbuffered solutions raises problems in general with CO2 absorption. So, we need some ideas ofwhat the effects of [H+] will be on the analysis.

To gain some experimental ideas, I suggest that you study the poster outside SC328. Some stripchart recordings that indicate at least one effect of [H+] are shown there. Also, since the supportingelectrolyte for the Cd+2 standards is KCl, the addition of small (0.001 M) amounts of HCl seemssensible.

Again, the idea here is not just to observe the reduction of H+. Instead, we need to know if controlof the amounts of [H+] in a series of natural samples is important. And, always remember, theremay be ways to measure the diffusion current that eliminate the need for such control.

Objective 5 - Effects of a Natural Sample MatrixIn Figure 41 above is shown the polarogram of a mixture of cations. This, in one step, shows what

the problems could be in using polarography to analyze natural samples. Suppose, for example, thatthe sample we were interested in monitoring was an effluent stream from some industrial processsuspected to be putting Cd+2 into the environment.

Figure 41 Polarogram of a mixture of reducible cations

33

If the sample were collected downstream from the source, the chances are excellent that it wouldhave dissolved some of the soil over which it was flowing. This would contribute transition metalsto the sample, among other things. We thus could expect variable, and largely unpredictable,contamination of the natural sample with Cu+2, Ni+2 and Zn+2 cations, since these are common tosoils. Since we would hope that there would not be significant Cd+2 in the sample, it might evenoccur that the contaminants would be present at a higher concentration than the analyte. Such is thenature of a matrix effect.

The central question is what effect variable amounts of these contaminants would have on theanalysis. Note in Figure 41 that the 0.1 M NH4Cl supporting electrolyte has buffered the solution tobe basic. This extends the range of the DME cathodic scan enough that all of the cations produce adetectable wave.

As you explore this objective, keep in mind that you can measure the diffusion current from theseions separately. You also can transfer the data to a spreadsheet, and in a series of columns literallysubtract out the current from one cation from the net current produced by more than one. These kindof manipulations presuppose a linearity between diffusion current and concentration for all of thecations, but, given the excellent behavior for Cd+2, this does not seem and unrealistic assumption.

Again, keep in mind the central objective. How does the presence of unpredictable amounts ofcontaminant cations, such as those shown in Figure 41, effect the accuracy with which Cd+2 can bedetermined?

Project 5 - A Class Project

The entire class can do an interesting project that relates to mixtures. Each lab section will have tomake up a set of solutions that constitutes one part of a larger mixture. The section will then take thepolarograms of each set of solutions, and pass the data on, via the Analytical Chemistry server, tothe next section. At the staff meeting in the following week, the data can be assembled to completethe project.

As an example of this kind of project, view the figures on the following pages. Solutions wereprepared that had a base concentration of copper, and to each were added increasing amounts ofzinc. The question that was to be answered was, Could varying amounts of copper be determinedin solutions that had varying amounts of zinc in them, and could varying amounts of zinc beanalyzed in solutions that had varying amounts of copper in them?. While the solutions were nothard to make, they took at least 20 minutes each to deaerate, and about 10 minutes to scan, so eachset of data took an afternoon to gather.

A class project that could be done involving the same kind of question would be to use cations otherthan copper and zinc. You would refer to a table of half wave potentials (in the Science Library) toselect these ions.

34

Quantitation of Cu+2 Limiting Current

-0.025

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.225

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ecathodic vs. SCE

Cur

rent

follo

wer

out

put

volt

age

(1 m

Am

p/d

ivis

ion)

Residual

10 ppM Cu

20 ppM Cu

30 ppM Cu

Figure 42 These data would be used to determine if Cu can be quantitated alone.

Variable Zinc, Fixed Copper, pH = 3.7 6/17/97 - John Walters - Bronze Analysis Development Study

-0.025

0.000

0.025

0.050

0.075

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0.125

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0.250

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Cur

rent

Follo

wer

Out

put

Volt

age

(0.5

mA

mp /

div

isio

n)

0 ppM Cu 0 ppM Zn

10 ppM Cu 0 ppM Zn

10 ppM Cu 10 ppM Zn

10 ppM Cu 20 ppM Zn

10 ppM Cu 30 ppM Zn

Figure 43 These data would be used to determine if Zn can be quantitated in the presenceof 10 ppM Cu.

35Variable Zinc, Fixed Copper, pH = 3.7

6/18/97 - John Walters - Bronze Analysis Development Study

-0.025

0.000

0.025

0.050

0.075

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ecathodic vs. SCE

Cur

rent

Follo

wer

Out

put

Volt

age

(0.5

mA

mp /

div

isio

n)

0 ppM Cu 0 ppM Zn

20 ppM Cu 0 ppM Zn

20 ppM Cu 10 ppM Zn

20 ppM Cu 20 ppM Zn

20 ppM Cu 30 ppM Zn

Figure 44 These data would be used to determine if Zn can be quantitated with the same workingcurve slope and intercept in the presence of 20 ppM Cu as was observed with 10 ppM Cu.

Variable Zinc, Fixed Copper, pH = 3.7 6/17/97 - John Walters - Bronze Analysis Development Study

-0.025

0.000

0.025

0.050

0.075

0.100

0.125

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ecathodic vs. S.C.E.

Cu

rren

t Follow

er

Ou

tpu

t V

olt

ag

e (

0.5

mA

mp

/d

ivis

ion

) 0 ppM Cu, 0 ppM Zn

30 ppM Cu, 0 ppM Zn

30 ppM Cu, 10 ppM Zn

30 ppM Cu, 20 ppM Zn

30 ppM Cu, 30 ppM Zn

Figure 45 These data would repeat the Zn quantitation question with 30 ppM Cu.

36

Reporting the Results

Different teams in the class will select these objectives. Many of them will be done on different labdays. To report out the results of all of them, a special Monday meeting will be scheduled to allowshort 10-minute presentations, with appropriate visual aids, of what the conclusions are.

Asking that an elected class representative present an answer to the question, Is polarography aneffective analytical method for the routine determination of sub-millimolar amounts of Cd+2 innatural samples, will emphasize whole class communication?

This is one of several lab section laterals that we will do this semester. Success in an experimentlike this requires that the people in one lab section communicate ahead of time to those in the nextwhat has happened to them and how well the results met their expectations. Good communicationcan be done by e-mail while the lab is being done. The analytical Chemistry server can be used todrop ship results, or they may be attached to an Eudora message.

The reason for such efforts is shown on the next page in the form of a survey done by Dr. RobertGentry of the University of Minnesota. No matter what you have done, how you share it withothers is the most valuable skill you can have.

37

Dr. Ron Gentry, Chair of the Chemistry Department, reported the following material to Chemistryfaculty and students of the University of Minnesota on October 4, 1994. In an accompanyingmemo, he stated:

A committee of the Council for Chemical Research this year polled major chemicalcompanies about the characteristics they look for when assessing M.S. or Ph.D. jobcandidates in chemistry and chemical engineering. Attached are the responses, which shouldbe interesting and helpful for those preparing for industrial interviews.

When seeking a graduate degree level (M.S. or Ph.D.) candidate in Chemistry or ChemicalEngineering, which of the following are most important characteristics?

Most Important -> Least Important5 4 3 2 1

Thesis Topic 10 28 20 10 1Thesis Advisor/Professor 13 26 20 8 2Academic Institution Granting Advanced Degree 16 39 12 1 1Grade Point Average - Graduate School 13 31 18 5 2Grade Point Average - Undergraduate School 12 31 20 6 0Undergraduate School 2 16 30 19 1Undergraduate Discipline/Curriculum 6 27 24 9 2Communications Skills 39 31 0 0 0Extracurricular Activities 0 7 33 23 6Foreign Language Skills 2 6 25 23 12Professional/Honor Society Membership 0 3 24 25 16Papers Presented (Number) 1 13 30 15 9Publications (Number) 1 18 26 14 10Computer Skills 2 30 29 8 0Participation in co-op Programs 3 23 22 16 5Full/Part-Time Employment in Major Field 11 24 23 10 1

II. Which of the following traits are Important?

Independence 21 31 13 1 0Self Motivation 57 11 0 0 0Good Problem Solving Skills 55 11 2 0 0Team Player 43 18 5 2 0Enthusiasm 36 24 6 1 0Decision Making 27 29 10 1 0Professionalism 26 20 17 3 2