acid base titration experiment 4

Exp. IV. CM1014 Fall 10 1

Acid-Base Titration

A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. In this experiment, you will first standardize (determine the concentration of) the NaOH solution which will be used throughout the experiment. The standard which will be used is a solid, monoprotic acid: potassium hydrogen phthalate (KHP). Once the NaOH has been standardized, you will use it to determine the unknown concentration of a HCl solution.

The formula for KHP is KHC8H4O4. It is a solid at normal lab conditions, can be highly purified, does not easily oxidize, and has a high MW, permitting good precision when weighing convenient sample sizes. It is therefore a useful primary standard which can make solutions of very well-characterized concentrations. KHP is a monoprotic acid, reacting 1:1 with NaOH:

KHC8H4O4(aq) + NaOH(aq) KNaC8H4O4(aq)+ H2O(l)

Sample calculation of a standardization run: A student is given a starting solution of NaOH to standardize, known to be approximately 0.1 M. It took 19.98 ml of the approximately 0.1 M NaOH solution to reach the equivalence point with a solution of KHP which was made up with 0.4168 g KHP. What is the concentration of the NaOH, to 4 s.f.?

0.4168 g/(204.2 g/mol) = 2.041 x 10-3 moles KHP

2.041 x 10-3 moles KHP x [1 mol NaOH/1 mol KHP] = 2.041 x 10-3 moles NaOH

2.041 x 10-3 moles NaOH/ 0.01998 L = 0.1022 M NaOH

Once the concentration of the NaOH titrant is well-characterized, it is used to titrate samples of acids of unknown concentration. In this experiment, the unknown acids are HCl solutions.

Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

Sample calculation of a run to determine the concentration of an HCl solution:A 10.00 ml sample of the HCl solution is titrated, using the standardized 0.1022 M NaOH solution. It took 20.04 ml of the NaOH to reach the equivalence point. What is the concentration of the HCl, to 4 s.f.?

0.1022 M NaOH x 0.02004 L = 2.008 x 10-3 moles NaOH

2.008 x 10-3 moles NaOH x [1 mol HCl/1 mol NaOH] = 2.008 x 10-3 moles HCl

2.008 x 10-3 moles HCl/ 0.01000 L = 0.2008 M HCl

The equivalence point will be determined by analysis of a titration curve, and compared to the value obtained by the color change of phenolphthalein, a commonly used indicator. Both work well if used correctly. A common error of beginning students using indicators, though, is “overshooting” the equivalence point, and hence calculating erroneously high values.

Volume NaOH (mL)

pH


When an acid solution is titrated with a basic solution, the pH of the acidic solution is initially low. As base is added, the change in pH is quite gradual until close to the equivalence point, when equimolar amounts of acid and base have been mixed. Near the equivalence point, the pH increases very rapidly, as shown in Fig. 1. The change in pH then becomes more gradual again, before leveling off with the addition of excess base. The inflection point on the pH vs. volume graph corresponds to the equivalence point. The equivalence point can be extracted, then, by finding the volume corresponding to the peak of the first derivative of the pH vs. volume graph, or the zero point of the second derivative graph.

In this experiment, you will use a computer to monitor pH as you titrate. The point of inflection in the pH curve will then be used to determine the equivalence point, and that determination will be compared to the value obtained from the color change of a common acid-base indicator, phenolphthalein. The volume of NaOH titrant used at the equivalence point will be used to both standardize the NaOH solution (Part 1), and to determine the molarity of the HCl sample (Part 2).

Fig. 1

Reading a buret:http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html

Buret

Look below for pictures of a buret. Note that the numbers get bigger as you go down the buret. This is different from the beaker or the graduated cylinder, because the liquid leaves the buret at the bottom.

Read at the bottom of the meniscus.

The smallest division in this buret is 0.1 mL. Therefore, our reading error is 0.01 mL. A good volume reading is 20.38 0.01 mL. An equally precise answer would be 20.39 mL or 20.37 mL.

How many significant figures does our answer have? 4! The "2", "0", and "3" we definitely know and the "8" we had to estimate.

Fig. 2 Reading a buret

http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html


The rings on the buret aid in reading the meniscus without parallax error. In the picture, note that the ring in the back of the buret can be seen, looking up a the 5 ml mark and down at the 7 ml mark.

PROCEDUREPART 1 Standardizing the NaOH

Work as partners. Let one partner dissolve the KHP and set up the buret while the other connects the computer, LabPro interface, ph probe, and checks the calibration. The LoggerPro software, example, and template should already have been downloaded from MyPoly CM1014 main site, under the LABS button. Make sure you have download ed and are familiar with the following :

EXAMPLE: Exp 4 pH vs. ml File: Sample_data_for_Acid_Base_lab.cmblExample of how to derive inflection point from pH curve via first and/or second derivative

TEMPLATE: Exp. 4 data input template, titration curve File: Exp_4.cmbl Template to use in the lab, for inputting experimental data.

Partner A

1a. Obtain a 50-mL buret. Use a buret clamp (or two utility clamps) to attach the buret to the ring stand with a waste beaker beneath, as shown in Fig. 6. Rinse the buret two times with a few mL of the ~0.1 M NaOH solution, draining the rinse into the waste beaker. Fill the buret a little above the 0.00-mL level of the buret with ~0.1 M NaOH solution. Drain a small amount of NaOH solution so it fills the buret tip (NO BUBBLES!) and leaves the NaOH at the 0.00-mL level of the buret.

2a. Weigh approximately 0.4 g of KHP in a tared beaker on the analytical balance, carefully recording the mass to 4 s.f. Add about 50 ml of distilled water to dissolve the KHP.

3a. Place the beaker on a magnetic stirrer and add a stirring bar. Carefully and slowly start the stir bar moving, and let all the KHP dissolve.

Parallax error

Using either a white card with a black bar on it or a plain white card (moving up from underneath) behind the buret will help highlight the bottom of the meniscus~chemlab/techniques/buret.html

Fig. 3 Fig. 4

http://www.dartmouth.edu/~chemlab/techniques/buret.html


Partner B

1b. Connect the computer interface as demonstrated by your instructor. Prepare the computer for data collection by opening the file from the MyPoly site, Exp_4.cmbl. The vertical axis has pH scaled from 0 to 14 pH units. The horizontal axis has volume scaled from 0 to 25 mL. Check to see that the Meter window shows pH value, indicating the probe and interface is communicating with the computer.

2b. Check the calibration of the pH probe by reading the pH of the available standard solutions:Red = pH 4 Yellow = pH 7 Blue =pH 10Between readings, rinse the pH probe with a wash bottle over a waste beaker. Rinse the pH probe any time it is transferred to a new solution, to prevent cross-contamination. GENTLY wipe the outside with a Kimwipe, and wick away rinse water around the VERY DELICATE bulb, as shown in the Fig. 5. Do not let the probe dry out. A quick and dirty technique is to keep the probe in an Erlenmeyer flask filled with water when not in immediate use.

Fig. 5 a & b Rinsing and drying the probe

a. Wash it with purified water . b. Dry it with tissue, GENTLY, both inside and outside.

3b. Calibration – use only if necessary

If your system reads more than 0.1 pH units off the standard solutions, calibrate by following the LoggerPro menu:

Experiment>Calibrate>box opens, choose Sensor Settings>Calibrate Now.

Use the pH 4 then the pH 10 standard, since that encompasses the widest range.

Submerge the tip of a clean probe in the pH 4 standard. You will see a voltage reading in the gray box. When it stabilizes, click KEEP, and enter pH = 4.00.

Rinse and daub dry the probe as shown in Fig. 6, and repeat the procedure with the pH 10 standard. Again, when the voltage stabilizes, click KEEP, and enter pH = 10.00.

Close the calibration.


Fig. 6 Experimental set-up

All together now - both partners

4. Use a utility clamp to suspend the pH Sensor on a ring stand as shown in Figure 6. Position the pH probe in the KHP solution and adjust its position so that it is not struck by the stirring bar. BE EXTREMELY CAREFUL OF THE GLASS TIP OF THE pH PROBE!

5. Move the buret so that it looks like Figure 6, with the buret set to dispense NaOH into the acid solution. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.

6. Add 2-3 drops of phenolphthalein indicator. Click Collect and monitor pH for 5-10 seconds. Once the displayed pH reading has stabilized, click Keep . In the edit box, type “0” (for 0 mL added). Press the ENTER key to store the first data pair for this experiment.

7. You are now ready to begin the titration. This process goes faster if one person manipulates and reads the buret while another person operates the computer and enters volumes.

NOTE: The suggested pH increments below are just that - rough values, suggestions. See Fig. 7, noting the small red balls marking each data point. Watch the graph as you introduce NaOH to the acid solutions. The red ball moves as the pH changes, freezing only when you hit KEEP to enter the volume. Use this moving ball to judge how much NaOH to add. Early in the titration, you will add a lot to get the red ball to move. As you get near the equivalence point, very little NaOH is needed move the red ball a lot. If that happens, STOP. Take the reading, and proceed more slowly.


Fig. 7 Sample titration curve

The display is controlled several ways. Right click on the graph and select Graph Options. Under the Graph Options tab, selecting Point Protectors will show the data points, and Connect Points will draw a line through the points. Left double clicking on a data point opens the Manual Column Options box. The Options tab in this box lets one set the size and color of the “Point Protectors”. The default settings are the small red balls, with no lines, as shown.

a. Add an increment of NaOH titrant (enough to raise the pH about 0.15 units, or to move the new red ball above the previous reading’s postion). When the pH stabilizes, again click Keep . In the edit box, type the current buret reading, to the nearest 0.01 mL. Press ENTER. You have now saved the second data pair for the experiment.

b. Continue adding NaOH solution in increments that raise the pH by about 0.15 units and enter the buret reading after each increment.

c. When a pH value of approximately 4.5 is reached, change to a one-drop increment. Enter a new buret reading after each increment. At one point, adding a drop of NaOH will cause the solution to change color from clear to pink. Write this buret volume on the data sheet in the box labeled “Volume of NaOH at color change (clear pink)”

d. After a pH value of approximately10 is reached, again add larger increments that raise the pH by about 0.15 pH units, and enter the buret level after each increment.

e. Continue adding NaOH solution until the pH value remains constant. 8. When you have finished collecting data, click Stop . Dispose of the beaker contents as

directed by your teacher. 9. Save a copy of the Table window to your computer, and place a copy in Assignments on

MyPoly, labeled with your name(s) and experiment name or number. (For example: Smith_Jones_Titration_Table1)


11. Save a copy of the Graph window to your computer, and place a copy in Assignments on MyPoly, labeled with your name(s) and experiment name or number. (For example: Smith_Jones_Titration_Graph1).

12. Repeat the procedure with a new mass of KHP. Save the tables and graphs as before, with the numbering changed. If necessary, add comments when loading the files in Assignments so the identity of the files is clear.

In the subsequent calculations, you will use the average of the molarities just determined in Part 1 as the accepted value for the concentration of NaOH in Part 2.

PART 2 Determining the concentration of a HCl sample

Using the same setup, the procedure in Part 1 will be repeated, but the sample for each run will be 10.00 ml of the HCl solution of unknown concentration instead of the KHP solution.

1. Obtain your unknown HCl sample. Record the sample number on the data sheet.

2. Using a volumetric pipet, pipet 10.00 ml of the sample into the beaker. Add about 50 ml of distilled water, and 2-3 drops of phenolphthalein indicator. Repeat the procedure of Part 1, titrating and recording pH and volume data using the Vernier interface and pH sensor. As before, write the volume observed when the solution changes from clear to pink, to the 0.01 ml. Do two runs, saving your table and graph and depositing copies in Assignments each time.

PROCESSING THE DATA

1. On the graph for each run, left click on the label of the pH axis. From the pop-up menu, choose First derivative. Enter Control-E. A pop-up box and sliding cursor will appear on the graph. Move this to highlight the peak value, and record the volume shown in the box for this maximum value. This is the equivalence point. Do this for each run, recording the volume of NaOH at the equivalence point in the appropriate box on the data sheet.

.2. Following the illustrative calculations on page one, fill in the data sheet. Show a sample calculation for Run 1 for Part 1 and for Run 1 for Part 2 in the areas provided.


DATA TABLES

PART 1 STANDARDIZATION OF NaOH

Run 1 Run 2

Mass of KHP, grams

Moles of KHP (MW=204.2 g/mol)

Volume of NaOH at equivalence point, from graph

Molarity of NaOH

Average Molarity of NaOH,from graph data

USE THIS VALUE IN PART 2 CALCULATIONS

Volume of NaOH at color change (clear -> pink)

Molarity of NaOH, based on color change

Average Molarity of NaOH, based on color change

Sample calculations, RUN 1

Moles KHP:

Molarity of NaOH, based on graph data:


PART 2 DETERMINATION OF HCl CONCENTRATION

Run 1 Run 2

Concentration of NaOH, from Part 1

Volume of HCl sample(better be 10.00 ml!)

Volume of NaOH at equivalence point, from graph

Molarity of HCl, calculatedfrom graph data

Average Molarity of HCl,from graph data

Volume of NaOH at color change (clear -> pink)

Molarity of HCl, based on color change

Average Molarity of HCl, based on color change

Sample calculations, RUN 1

Molarity of HCl, calculated from graph data


PRELABName____________________________________________Date_______________________

FOR FULL CREDIT, SHOW ALL WORK

1. A student is given a starting solution of NaOH to standardize. It took 20.36 ml of the NaOH solution to reach the equivalence point with a solution of KHP which was made up with 0.3998 g KHP. What is the concentration of the NaOH, to 4 s.f.?

2. A different student used different solutions than those used in Question 1. He titrated a 10.00 ml sample of his HCl solution, using a standardized 0.1011 M NaOH solution. It took 30.32 ml of the NaOH to reach the equivalence point. What is the concentration of the HCl unknown, to 4 s.f.?


POSTLAB

1. List your data results:Part 1Average Molarity of NaOH, from graph data _____________________Average Molarity of NaOH, based on color change _____________________

Part 2Average Molarity of HCl, from graph data _____________________Average Molarity of HCl, based on color change _____________________

a. How well do the results from the two techniques agree?

b. Do the results from the color change data (using the phenolphthalein indicator) tend to be higher, lower, or randomly fluctuate with respect to the results based on graph data? What can you deduce about your titration technique, based on this comparison?

acid base titration experiment 4

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