Lab Equipment

Analytical Balance

Watch the movie on using an analytical balance. Analytical balances are used for very accurate, quantitative measurements of mass to the nearest 0.001 g. (Some read to 0.0001 g.) These are delicate instruments, subject to errors caused by vibration and drafts. These problems can be minimized with care and a certain amount of common sense.

Figure 1

For optimum accuracy, the balance should be level. If it is not, inform the laboratory instructor, who will make the necessary adjustments. Do not lean on the bench while operating the balance. This may cause vibrations that are transmitted to the balance. To begin any measurement on the analytical balance, close the draft shield doors and press the button or control bar that turns on the balance. The display should indicate zero (0.000) g. If it does not, inform your laboratory instructor.

To weigh a solid object that is not a reagent:

Open the draft shield door and gently place the object on the center of the pan. Close the door; the mass will be displayed. Record the mass. Never weigh solid or liquid reagents directly on the pan. Weighing paper or a container such as a beaker should be used for this purpose as described below.

To tare a container and weigh a reagent:

1

Open the draft shield door and gently place the container (or weighing paper) on the center of the pan. Close the door; the container mass will appear on the display. Record it in your data table.

2

Calculate a target mass (mass of desired chemical and tare mass).

3

Open the draft shield door and remove the tared container. With the container on the bench top, dispense the chemical.

4

Place the container back on the balance pan and check the mass. If you need to add more chemical, remove the container from the balance and add it, then check the mass again. Repeat this process until the target mass is reached.

5

Close the door and record the mass from the display. Carefully remove the container from the pan and close the door when finished.

One can also tare the container by zeroing the balance with the beaker or weighing paper on it. Then measure the mass of the compound directly, rather than obtaining its mass by difference. Directions for individual experiments will indicate which method is preferred. Generally, if the compound will undergo some chemical conversion in the container, then be reweighed, the method given in steps is preferred.

A few other tips on use of the analytical balance:

1

Do not dispense chemicals into a container while it is on the balance pan. This prevents spills in the balance chamber, which is difficult to clean. If you spill something near the balance, clean it up.

2

The target mass is just that, a target. It is very difficult to dispense an exact mass of chemical. Therefore, experiments are set up to require an approximate mass, but the experimenter records the exact mass of the chemical he/she dispensed. For example:

o The experiment states "obtain about 0.5 g of the unknown." The student finds the tare mass of the container to be 34.568 g. The target mass for container and chemical is 35.068. The student dispenses chemical according to Step 4 above. On his last addition, he overshoots the target mass by a bit. He closes the draft shield and records a mass of 35.142 g. He calculates the exact mass of unknown to be 0.578 g and uses this value in his calculations.

3

If you have overshot your target mass, do not put the excess chemical back into the reagent bottle. Retain it in the experiment, as in the example above, or put it in the waste container.

4

If you are doing a series of measurements of the mass of an object over a period of time, perform all measurements on the same balance.

5

Do not bump or place heavy objects on the bench after zeroing the balance.

6

Let hot objects cool before obtaining the mass.

7

Obtain the mass of hygroscopic (water absorbing) materials quickly.

Volumetric Glassware

In quantitative chemistry, it is often necessary to make volume measurements with an error on the order of 0.1%, one part per thousand. This involves using glassware that can contain or deliver a volume known to a few hundredths of a milliliter, or about 0.01 mL. One can then report quantities greater than 10 mL to four significant figures. Glassware designed for this level of accuracy and precision is expensive, and requires some care and skill to give best results. Four

main types of volumetric glassware are common: the graduated cylinder, the volumetric flask, the buret and the pipet. These have specific uses and will be discussed individually. There are some points that are common to all types, however. These involve cleanliness and how to read volumes accurately. Cleanliness is essential to good results. Chemically clean glass supports a uniform film of water, with no hanging droplets visible. Rinse your glassware thoroughly with deionized water when you are finished with it. If you are suspicious at all, wash it before you use it as well. With some types of glassware, one "conditions" the apparatus by rinsing with a few small portions of the solution one will be measuring prior to conducting the actual work. This prevents water droplets from diluting one's solution, and changing the concentration. More detail on how to do this will be given in the discussion of the individual pieces of glassware. All volumetric glassware is calibrated with markings used to determine a specific volume of liquid to varying degrees of accuracy. To read this volume exactly, the bottom of the curved surface of the liquid, the meniscus, should be located at the scribed line for the desired volume. It is often easier to see the meniscus if you put a white paper or card behind the apparatus. If your eye is above or below the level of the meniscus, your readings will be inaccurate due to the phenomenon of parallax. View the meniscus at a level perpendicular to your eye to avoid this as a source of error.

TC versus TD

Some volumetric glassware bears the label "TC 20°C" which stands for "to contain at 20°C." This means that at 20°C, that flask will have precisely the volume listed inside it. If you were to pour out the liquid, you would need to get every drop out of it to have that volume. Alternatively, some volumetric glassware bears the label "TD 20°C" which stands for "to deliver at 20°C." This means that at 20°C, precisely the volume listed will leave it when the contents are allowed to drain out of the vessel. It is not necessary to get every last drop and, in fact, it is inaccurate to blow the last bit out of a volumetric pipet.

Graduated Cylinders

Most students are familiar with graduated cylinders, which are used to measure and dispense known volumes of liquids. They are manufactured to contain the measured volume with an error of 0.5 to 1%. For a 100 mL graduated cylinder, this would be an error of 0.5 to 1.0 mL. Measurements made with a graduated cylinder can be reported to three significant figures.

Figure 2

Volumetric Flasks

Watch the movie on using a volumetric flask. The volumetric flask, available in sizes ranging from 1 mL to 2 L, is designed to contain a specific volume of liquid, usually to a tolerance of a few hundredths of a milliliter, about 0.1% of the flask's capacity. The flask has a calibration line engraved on the narrow part of its neck. It is filled with liquid so the bottom of the meniscus is on this engraved line. The calibration line is specific to a given flask; a set of flasks built to contain the same volume will have lines at different positions.

Figure 3

Volumetric flasks are used to make solutions with very accurately known concentrations. There are two ways to do this. One can start with a solid solute or with a concentrated stock solution. When working with a solid solute, one weighs the material to the desired accuracy and transfers it carefully and completely to the volumetric flask. If solute is lost in transfer, the actual concentration of the resulting solution will be lower than the calculated value. Therefore, one weighs the solid in a beaker or other glassware that can be rinsed with the solvent, typically water, and transfers it into the flask. Additional solvent is added, but not enough to fill the wide part of the flask. The solute is dissolved by swirling the flask, or by stoppering it and inverting it repeatedly. Once the solute is dissolved, more solvent is added to bring the volume to the mark on the flask. The last portion should be added very carefully, dropwise, so the bottom of the meniscus is at the mark. The flask is then stoppered and inverted a few times to completely mix the solution. When diluting a stock solution, the desired volume of solution is transferred into the flask via a pipet. The solvent is then added as described above. Obviously, the concentration of the stock solution must be accurately known to as many significant figures as one desires for the dilute solution. Also, the volume transferred must be known to the desired number of significant figures. Never fill a volumetric flask with solvent and then add solute. This results in overfilling the flask, and the volume will not be known accurately. It is sometimes useful to have some solvent in the flask before adding the solute. This is a good practice when dealing with volatile solutes. Volumetric flasks are not used for storage of solutions. Once the solution is prepared, it is transferred to a clean, labeled bottle or beaker. The flask is then washed and rinsed well. The last few rinsings should be with deionized water

Burets

A buret is a long, narrow tube with a stopcock at its base. It is used for accurately dispensing variable volumes of liquids or solutions. It is graduated in 0.1 mL increments, with the 0.00 mL mark at the top and the 50.00 mL mark near the bottom. Notice that the marks do not go all the way to the stopcock. Therefore the buret actually will hold more than 50.00 mL of solution. Burets with liquid capacities of 25.00 mL and 10.00 mL are also available.

Figure 4

Watch the movie on cleaning and conditioning a buret. For optimal accuracy and to prevent contamination, a buret must be clean. To test a buret for cleanliness, close its stopcock and pour a small volume (5-10 mL) of deionized water into it. Hold the buret at a slant, almost parallel to the desk surface. Slowly rotate the buret and allow the liquid to coat its inside surface. Then hold it upright; the liquid should settle to the bottom of the buret in sheets, leaving no droplets on the interior walls. If droplets form on the walls, wash the inside with a soap solution, and rinse with distilled or deionized water. Repeat the cleanliness test. Just before use, a buret should be "conditioned" to ensure that any water adhering to the inside walls is removed. Add ~5 mL of the liquid that is to be used into the buret. Rinse the walls of the buret, then drain the liquid through the stopcock. Repeat with a second volume of liquid. The buret can now be filled with solution. Do this carefully and avoid trapping air bubbles in the tube. You may need a small funnel. The liquid level can be above the 0.00 mL mark. Clamp the filled buret in place if this was not done prior to filling; it is sometimes easier to hold the buret while filling it. Open the stopcock and drain enough liquid to fill the buret's tip. Have a beaker for waste solution handy for this and similar operations. There should be no bubbles in the tube or tip of the buret. These will lead to volume errors. If there are bubbles in the tube, carefully tap the buret to free them. Use the stopcock to force bubbles out of the tip. It may be necessary to empty and refill the buret. Watch the movie on titration. When the buret is clean and bubble-free, drain the liquid until the meniscus (the bottom of the curved surface of the liquid) is at or slightly below the 0.00 mL mark. It is not necessary to align the meniscus exactly at the 0.00 mark since the difference between the initial and final volumes is the desired measurement. If there is a drop of liquid clinging to the buret tip, remove it by gently touching the tip to a glass surface, such as the edge

of the waste beaker or wiping with a Kimwipe. The volume of a drop is about 0.1 mL, the same volume as the buret's graduations. Find the bottom of the meniscus, and read the liquid level in the buret to the nearest 0.01 mL at that point. This will take a little practice. Remember, you are reading from the top down. Record this value as the initial volume. Although it is tricky to "read between the lines," remember that the last digit of a measurement is expected to have some uncertainty! One-fifth (1/5) of a division (0.02 mL) can be reproducibly estimated if the meniscus is between calibration marks, after a little practice. Now dispense the liquid you need. If you are using the buret to measure a set amount of liquid, determine what the final reading should be to obtain that amount. Dispense the liquid slowly into the receiving vessel. Remember, in a clean buret, water will coat the interior walls and drain slowly. After closing the stopcock catch any hanging droplet in the receiving vessel. It is part of the measurement at this point, so do not catch it in the waste container. Wait a few seconds for the meniscus to stabilize, then read and record the final volume to the nearest 0.01 mL. The difference between the initial and final readings is the volume you dispensed. When using a buret, it is easier to work with the exact volume dispensed than to try to dispense an exact volume. Plan your work with this in mind. Although burets are sometimes used as dispensers, they are far more frequently used in procedures called titrations. In a titration, one attempts to determine an equivalence point as exactly as possible. This usually involves the first persistent color change of an indicator. With a little practice, one can dispense fractions of drops (less than 0.1 mL) into the titration vessel, and reproduce results within 0.10 mL or less. Watch the movie on cleaning a buret. When finished using a buret, drain the remaining liquid and clean it carefully. Finish with several rinses of deionized water including the stopcock and tip. If solute dries in the buret, it can be very difficult to remove. Clamp the buret in the buret clamp upside down with the stopcock open so that it will dry for the next lab session.

Pipets

Watch the movie on pipeting techniques. A pipet is designed to deliver a known volume of a liquid. Their volumes range from less than 1 mL to about 100 mL. There are several types, which vary in accuracy and in the type of task for which they are optimum.

Figure 5

Volumetric pipets are meant to hold a single, specific volume. This type of pipet is a narrow tube with a "bubble" in its center, a tapered tip for delivery of liquid, and a single graduation mark near the top (opposite the tapered end) of the tube. Volumetric pipets, sometimes called transfer pipets, are the most accurate pipets. They generally deliver the specified volume ± 0.1%, an error of a few hundredths of a milliliter.

Most volumetric pipets are marked TD (to deliver) and are drained by gravity. If a drop remains on the tip of the pipet, it is touched gently to the receiving vessel to draw off the remaining liquid or wipe with a Kimwipe. This type of pipet is not designed have residual liquid forced out by blowing.

Mohr pipets, also called measuring pipets, are straight tubes with graduations (usually at 0.10 mL intervals) and a tapered end. Mohr pipets are not designed to be drained completely. The operator fills them to a certain level, then dispenses the desired amount of liquid. They are much like burets and can be used for small volume titrations. This takes a fair amount of practice, though.

Serological pipets are a hybrid of the two previous types. Like Mohr pipets, they are straight tubes with graduations. They can be nearly as accurate as volumetric pipets, and they are very convenient. They can be used to dispense various volumes. For example, an experiment may call for dilutions of a stock solution, requiring 2.5, 5.0, and 7.5 mL of solution. A serological pipet is an excellent tool for this sort of work. Most serological pipets are calibrated TD/Blow Out. They have a shaped tip, to hold a cotton plug, and horizontal bands near the top of the tube. They are drained by gravity, and the last drop is gently blown out with a pipet bulb into the receiving vessel.

Before use, a pipet should be rinsed a few times with deionized water. If water droplets remain on the inside, try cleaning the pipet with warm soap solution followed by several rinses of deionized water. A pipet should be "conditioned" after cleaning. First, obtain a small volume of

the solution to be dispensed in a beaker or flask. Never pipet directly from the stock solution bottle! Since you may contaminate this solution, plan on discarding it after conditioning is complete. Draw a small volume of the solution to be dispensed into the pipet, then turn the pipet sideways (parallel to the bench top) and slowly rotate it to coat the inside surface. Then allow the solution to completely drain. The pipet is now ready for transfers of the desired liquid. Filling a pipet takes a little practice; you may want to try it a few times with deionized water after cleaning it. Use a pipet bulb — never your mouth! — for this purpose. The bulb has a tapered rubber seal. It should never be fitted tightly onto the top of the pipet. Hold the bulb against the top of the tube, just tightly enough to get a seal. Squeeze and hold the bulb in the compressed form, lower the tip of the pipet into the solution of interest, and slowly release the pressure on the bulb. When the liquid has risen slightly above the calibration mark on the neck, quickly remove the bulb and place a finger (typically a thumb or an index finger) firmly on the top of the pipet. A gentle rocking or twisting motion of your finger should allow the solution to drain until the bottom of the meniscus rests at the calibration mark. Remove any droplet hanging on the tip by gently touching the tip to a glass surface, such as a beaker for waste solution. The contents of the pipet can now be drained into the desired container. Move the tip of the pipet into the container, remove your finger and allow the liquid to flow out of the pipet. A volumetric pipet will have one remaining drop that should be "touched off" by gently touching the tip of the pipet to an inside edge of the container. A small volume of liquid will remain in the pipet and should be left there. Serological pipets should have all liquid in the pipet expelled — typically with a slight pressure from the rubber bulb. Graduated pipets (serological or Mohr) are a little trickier to use than volumetric pipets, because there are more options in filling and reading them. Examine such a pipet before you use it and think through what you will do with it. Many graduated pipets have two scales. One scale has the highest values toward the dispensing tip, and is read like a buret. The other has the lowest values near the dispensing tip. This is easier to read when drawing liquid into the pipet for transfer to another vessel. After using a pipet, rinse it several times with deionized water. Draw up its full volume and allow it to drain. If you use the pipet repeatedly for several aliquots (samples) of the same solution, do not rinse the pipet between uses. You will just have to condition it each time. Clean it when you are finished, or before you start working with a different solution.

Significant Figures and Volumetric Glassware

As the preceding discussion points out, most volumetric glassware is accurate to a few hundredths of a milliliter, and is designed so a careful operator can reproduce measurements to this degree of precision. Therefore, measurements made with volumetric glassware are reported to 0.01 mL. Depending on the volumes used, three or four significant figures can be shown in data tables and carried in calculations.

Bunsen Burner

A Bunsen burner is one of the primary means of heating in the laboratory. A Bunsen burner is designed so that gas and airflow can be regulated separately and manually. Gas is delivered from the lab bench gas valve to the base of the Bunsen burner via a rubber tube. Gas flow is regulated with the small knob at the base of the burner and rotating the sleeve at the base of the burner to open or close air inlet holes regulates airflow.

Figure 6

When lighting a Bunsen burner, make sure the area around the burner is completely clear and that no flammable solvents are in use in the laboratory. Connect the tubing to the gas valve on the lab bench. Adjust the gas regulator knob on the bottom of the Bunsen burner for a moderate flow of gas and rotate the sleeve at the base of the burner so that airflow is almost completely closed off. Strike a match and hold it close to the side of the burner but not quite touching it while you turn on the bench gas valve. If you are using a striker, make sure that the striker is producing sparks, turn on the bench gas valve and then use the striker to produce sparks near the top of the Bunsen burner. If the burner fails to light, turn off the gas check the connection to the gas and the gas regulator and correct any problems before attempting to light the burner again. If the burner fails to light upon successive attempts, alert your lab instructor. The bluer the flame, the hotter it will be. This is achieved by allowing more air into the mixture. Adjust the gas and airflow to produce a flame of the desired size that has two distinct blue regions. The hottest part of the flame is the tip of the inner, dark blue cone. If the burner blows out, turn off the gas at the bench valve immediately. Readjust the gas and airflow to produce a better flame. To extinguish a Bunsen burner, simply turn off the gas at the bench valve.

Notes when using a Bunsen burner:

•

Never leave a lighted Bunsen burner unattended.

•

Never point the opening of the vessel being heated (test tube, beaker, etc.) toward you or others near the burner.

•

When flammable liquids are being used in the laboratory, a hot plate or a heating mantle must be used in place of a Bunsen burner.

Centrifuge

Centrifugation (high-speed rotation) will be used to rapidly settle precipitates in solution. When loading test tubes into the centrifuge, two tubes of approximately equal volume must be placed opposite each other to balance the rotor. If other groups are not ready to centrifuge, you can simply put deionized water into an extra test tube for balance. Note the letter or number near the position of your tube(s) so you can identify it (them) when centrifugation is complete. Never put stoppers or other items in the centrifuge with your tubes. Only test tubes are allowed in the centrifuge!

Figure 7

Always close the lid to the centrifuge while it is rotating. Many newer centrifuges have a safety switch that will not allow them to turn while the lid is open. Make sure that the lid is completely shut and the safety knob is turned to engage the safety switch. Then turn the timer knob to about 30 seconds. Turning the timer knob to zero or turning the safety knob on the lid will turn off the centrifuge. Do NOT open the lid to the centrifuge until the rotor has slowed down to prevent injury. Do NOT slow the rotor with your hand! This can result in injury AND it will stir up the solution you were attempting to separate by centrifugation. Wait for the rotor to come to a complete stop before attempting to remove your test tubes. After about 30 seconds of centrifugation most precipitates will have settled. And the liquid above it can be decanted (poured off).

Using MicroLabTM

MicroLab is a hardware device and software which, when connected to a detection probe (ex. a thermistor, pH electrode, voltmeter, or spectrometer) and a computer, allows the real time, in-situ data collection of several different kinds of experimental data. This data is stored for subsequent manipulation either by the spreadsheet functions included in the MicroLab software or other

spreadsheet software such as Excel. Detailed instructions for the calibration and use of each device will be provided as needed for the appropriate labs.

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Using Analytical Balances: Steps to be followed in Weighing by Addition:

Fig. 1 Analytical Balance

Fig. 2 Horizontal-level bubble gauge

1. Check to ensure that the horizontal position of the balance is level. Each balance is equipped with a level indicator. See Fig. 2. If not level see instructor.

2. Clean the balance pan with a brush.

3. Place a weighing boat, small beaker may also be used, on the centre of the balance pan and be sure to close the balance doors.

4. Tare the balance and wait until it reads zero grams.

5. Remove the weigh boat from the balance. Gently add solid sample to the weighing boat. Never add reagent while the weigh boat is in the balance!!!

6. Return the weigh boat to the balance and close the balance door. Record the weight to the nearest tenth of a milligram, i.e., four places after the decimal.

7. Remove the boat and sample. Transfer the sample to the appropriate piece of glassware.  

Steps to be followed in Weighing by Difference:

This technique is useful when several duplicate masses of a material must be weighed out.

1. Check to ensure that the horizontal position of the balance is level. Each balance is equipped with a level indicator. See Fig. 2. If not level see instructor.

2. Clean the balance pan with a brush.

3. Place a weighing boat on the centre of the balance pan and be sure to close the balance doors.

4. Tare the balance and wait until it reads zero grams.

5. Remove the weigh boat from the balance. Place approximately 1-4 grams of the material to be measured into the weighing boat. Never add reagent while the weigh boat is in the balance!!!

6. Return the weigh boat to the balance and close the balance door. Record the weight to the nearest tenth of a milligram, i.e., four places after the decimal.

7. Carefully remove some material from the weigh boat and place it in an appropriate container. Reweigh the weigh boat. The difference between the original weight and the final weight is equal to the weight of sample taken. Repeat as necessary.

Be sure to avoid the following common mistakes when using either the analytical or trip balance (where applicable). a)         Handling weighing bottles with bare fingers. b)         Weighing damp or dirty objects. c)         Leaving balance doors open. d)         Leaving weights on the balance when finished. e)         Forgetting to turn off the balance. f)          Removing the object from the pan while the balance is still on. g)         Weighing an object which is above room temperature.

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ANALYTICAL GLASSWARE:

In many laboratory experiments small amounts of reagents must be dispensed not only very accurately but also with a great deal of precision. In analytical experiments precise amounts of reagents are dispensed using glassware diagrammed in Fig. 3.

Graduated cylinders are the least precise of the analytical glassware but are suitable for most applications and there ease of use makes them a valuable piece of analytical glassware.

 

Buret Volumetric Pipet Measuring Pipet Volumetric Flask Graduated Cylinder

Fig. 3 Analytical glassware.

Pipets and burets must be kept absolutely clean in order to retain proper calibration. It is good practice in general to see that all glassware is absolutely clean. To test for cleanliness, fill the object with water and then empty it.  If the remaining water forms a uniform thin film on the walls with no beads, the object is clean.

To clean burets rinse repeatedly with distilled water(DW). Your buret has a Teflon bore stopcock: loosen the buret nut slightly to prevent freezing during storage.

Pipets and burets with hard-to-remove residues must be cleaned using soap and water. A buret brush may be used for cleaning a buret. Take care not to scratch the insides of the buret. Rinse well with distilled water after cleaning or use. Always rinse pipets well with distilled water after use and store them flat.

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Use of Pipets:

Pipets are calibrated to deliver (TD) a measured volume of a liquid.

Two types are commonly used in chemistry laboratories: volumetric (or transfer) pipets and measuring pipets. See Fig. 4.

Fig. 4. Volumetric and Measuring Pipets.

As the above diagram shows, a volumetric pipet has one calibration mark and is designed to deliver one fixed volume. Various capacities (i.e. 5 mL, 20 mL, 50 mL etc.) are available and are accurate to two digits after the decimal. The pipet shown above is calibrated to deliver exactly 10 mL at 20° C and should be recorded as 10.00 mL.

Measuring Pipets include Serological and Mohr pipets. They deliver various volumes to varying degrees of accuracy. The measuring pipet is calibrated along its length: either straight to the tip or along the straight part of the pipet only. These pipets are usually not as accurate as volumetric pipets. Both measuring pipets shown below are calibrated as "10 in 1/10" which means the pipet delivers a maximum of 10 mL and is calibrated in 0.1 mL divisions. Volume measurements should be recorded to two digits after the decimal.

The Mohr pipet is calibrated to deliver to the 10 mL baseline. The tip cannot be allowed to drain. The Serological pipet is usually calibrated to deliver the tip.

1. Squeeze the air out of the pipet bulb and press the opening of the bulb against the opening of the pipet. Do not push the pipet into the bulb. The tip of the pipet must be kept under the surface of the liquid the entire time suction is applied or air will be sucked into the pipet.

2. Rinse the pipet well with at least two portions of the desired solution.

3. Fill the pipet above the calibration mark using a pipet bulb.

4. Quickly remove the bulb and place your index finger (not your thumb) over the end of the pipet.

5. Wipe the pipet tip clean of any excess solution.

6. Drain the solution in the pipet down to the calibration mark by gently reducing the pressure of your index finger. You do not have to remove your index finger. Tip the pipet against the beaker to remove any excess solution. If your finger "leaks" and allows the level in the pipet to drop below the calibration line, moisten your finger slightly and try again.

7. Reset the pipet tip against the wall of the container into which the solution is to be transferred and allow the solution to drain.  Volumetric pipets are allowed to drain completely. Measuring pipets should not be allowed to go past the desired volume levels. Leave the pipet in this position for at least 10 seconds after all the solution appears to have drained out and touch the pipet tip to the side of the flask to remove any droplets. Remove the pipet. DO NOT  BLOW OUT THE SOLUTION REMAINING IN THE PIPET.

8. Rinse the pipet well with DW and store it when finished.

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Use of Volumetric Flasks:

Fig. 5 250 mL Volumetric flask. 

Volumetric flasks are available with capacities ranging from 5 to 5000 mL They are normally calibrated to contain (TC) a specified volume at 20° C. The volumetric shown above is calibrated to contain exactly 250 mL at 20° C and should be recorded as 250.00 mL.

The calibration mark is a ring around the narrow neck of the flask. A volumetric flask is used for making a solution whose concentration must be exactly known.

Some volumetric flasks have two calibration marks, TC and TD.  The TD mark is above the TC mark and includes the amount of solution that remains on the wall of the flask when emptied. All TD volumetric ware is calibrated to take into account the film of solution that remains on the vessel walls.  This is the reason that correct calibration is maintained only with clean glassware.

Volumetric flasks are properly filled when the bottom of the solution meniscus is exactly at the calibration mark. Always read or adjust the solution level in your glassware with your eye exactly level with the meniscus. This avoids a parallax error. On your buret use the calibration marks that completely encircle the buret to estimate when your eye is level with the meniscus.  Estimation of the meniscus bottom can be aided by placing a white card with a black line drawn horizontally across it behind the glassware and bringing the black line up to the meniscus.

1. Solid reagent (the solute) is transferred to the volumetric flask by using a funnel. The weighing boat is rinsed with the solvent used (usually distilled water) into the funnel. Then the funnel is rinsed with solvent while still inserted in the flask so that any remaining solute particles will be washed into the flask. In this way, the solute is transferred quantitatively - nothing is left behind.

2. After the solute has been added, the flask should be filled about halfway, and then swirled until all the solute has dissolved. Then solvent is added near to the calibration mark but not up to it.

3. Final addition is accomplished using a medicine dropper. If the liquid level goes past the calibration mark the solution must be discarded. Make sure your eyes are level with the liquid surface, and add solvent drop-wise until the bottom of the meniscus is touching the calibration line. It is important that the solution is well mixed. Stopper and invert the flask several times.

When using a volumetric flask to prepare a solution from a solid or pure liquid, it is usually best to weigh the desired substance by difference into an ordinary beaker. Dissolve the substance in the beaker with pure solvent (usually distilled water) being careful not to splash any solution out of the beaker. Transfer this solution to the volumetric flask quantitatively using a funnel. To avoid any solution running down the outside of the beaker, place a stirring rod across the top of the beaker. This rod should fit into the pouring depression on the beaker and extend about 1 cm beyond the lip. Use this rod to guide the solution into the flask. Rinse the beaker three times with solvent and transfer these rinses to the flask in the same manner. Fill the flask at least 95% full by adding solvent and mix thoroughly. Add enough solvent carefully and slowly to bring the solution level with the calibration mark using a medicine dropper for the last few drops. Again shake well to mix thoroughly. Never dilute to the mark if the solution temperature differs significantly from room temperature.

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Use of Burets:

Fig. 6 Diagram of a buret.

A buret is a glass tube with graduations that can be used to determine the volume of a solution added to a receiving vessel.

A buret is calibrated to be read from the top down.  For example, a 25 mL buret will have the zero mark at the top and 25 mL mark at the bottom, above the stopcock.  To determine the volume dispensed from the buret, use the graduation that is closest to the lowest point of the meniscus.  As an example, in the diagram, the meniscus is between the 11.4 and 11.5 mL marks. Thus the column of liquid dispensed would be recorded as 11.45 mL, assuming that the titration was begun at zero.

Using a buret is not difficult, however, good technique is needed to titrate accurately.

1. Be sure the buret is clean by rinsing several times with DW. Drain the buret well and rinse at least twice with the desired solution and discard the washings.

2. Fill the buret and drain to or below the 0.00 mL mark.

3. After a suitable time for drainage along the walls, wipe any excess solution from the buret tip and be sure that no air bubbles are trapped in either the tip or on the sides of the buret. If air bubbles are present in the tip, open the stopcock completely to force them out.  If there are air bubbles on the sides of the buret, gently tap the buret to get them to rise to the surface. Read the buret to 2 places after the decimal.

4. Deliver the required amount of solution and read the buret again. If you have emptied the buret rapidly, wait for the solution on the walls to drain before making a reading.

5. During titrations the titrant, solution in the buret, should be added slowly to the titrated solution.  A flow rate of 1-2 drops per second is recommended. It is essential that the titrant be mixed thoroughly with the solution being titrated throughout the titration.  Be sure to stir or swirl the solution into which the titrant is being added to insure complete mixing of the constituents.

6. When approaching the end point of a titration with a colour change that is not sharp, note down the volume and colour after each addition to avoid overrunning the endpoint. Approach the end point of all titrations with extreme care. After each addition contact the buret tip with the wall of the titration vessel to remove any drop of titrant remaining on the tip and be sure to wash down the sides of the receiving vessel.

7. Always estimate the  buret reading to 0.01 mL.

8. When finished, rinse the buret well with DW. Back

Measurements and Significant Digits

One of the most important

requirements of a good scientist is the ability to properly record measurements to the correct number of significant digits and with the correct units. Examples of the types of volumetric glassware and instruments encountered in the laboratory are given below with directions on how to correctly record measurements.

BeakersBurettesDigital BalanceErlenmeyer FlasksGraduated Cylinders

HydrometersMicropipettesMohr PipettesThermometersVolumetric Flasks

                                 

Beakers

Beakers are designed to hold a particular amount of liquid and are not designed to accurately measure volumes of liquids. If an approximate volume is needed with little accuracy, a beaker may be used to contain the liquid. Beakers come in various sizes with different calibrations. An examination of the 100-mL beaker shown reveals calibration lines every 10 mL between

20 and 80 mL. These measurements have a ±5% error and are therefore approximations.The 250-mL beaker has calibration lines every 25 mL between 25 and 200 mL. The error again is ±5%. The amount of liquid in the beaker is therefore 125 mL ±5%.

To determine the size of the beaker needed during an experiment, determine the maximum

amount of liquid the beaker needs to contain. Multiply that number by two and select a beaker size that comes closest to that number.

EXAMPLE: An experiment calls for 25 mL of one solution to be added to 30 mL of another solution. The beaker must safely hold 25 mL + 30 mL or 55 mL of liquid total. Multiplying 55 x 2 gives 110 mL. A 150-mL beaker

would be a good choice.Erlenmeyer Flasks

Erlenmeyer flasks are also designed to hold a particular amount of liquid and and are not designed to accurately measure volumes of liquids. Since the sides taper to a narrow opening, these flasks are useful for stirring solutions during titrations and synthetic work.

Erlenmeyer flasks come in various sizes. An examination of the

250-mL flask shown gives calibration lines every 25 mL between 50 and 200 mL. These measurements have a ±5% error and are therefore approximations.The 500-mL Erlenmeyer flask has calibration lines every 50 mL between 200 and 500 mL. The error again is ±5%. The amount of liquid in the flask is therefore 500 mL ±5%.

To determine

the size of the flask needed during an experiment, determine the maximum amount of liquid the flask needs to contain. Multiply that number by two and select a flask size that comes closest to that number.

EXAMPLE: An experiment calls for 25 mL of a solution to be titrated to its endpoint with 25 mL of another solution. The flask must safely

hold 25 mL + 25 mL or 50 mL of liquid total. Multiplying 50 x 2 gives 100 mL. A 125-mL Erlenmeyer flask would be a good choice.Graduated Cylinders

Graduated cylinders are calibrated to contain (TC) or to deliver (TD) a precise amount of liquid at a given temperature, usually 20 °C. Tolerances vary with the size of the graduated cylinder. Tolerance

s may or may not be given at the top of the cylinder neck. If a tolerance is not shown, a good rule of thumb is to use one half the interval division. The 50-mL graduated cylinder shown has 1-mL divisions and a tolerance of ±0.50 mL and is calibrated to contain the measured volume.

A graduated cylinder marked TC will hold the volume measured but will not deliver

                  

that volume to the container when transferred. Some of the liquid will remain behind in the graduated cylinder. If an exact amount is to be transferred, the graduated cylinder should be marked. TD.

MEASUREMENTS: Water and aqueous solutions will form a concave meniscus when placed in a graduated cylinder                     

as the water molecules are more strongly attracted to the glass than each other.  The bottom of the curved surface is read at eye level and the volume measurement is read to the proper number of significant digits.  Follow these steps to make the volume measurement with the proper number of significant digits:

1.  Determine the smallest division

                        Read scale at eye level.

marked on the graduated cylinder:  (1) find two adjacent markings that have a numeric label, (2) subtract and divide by the number of divisions between the numeric labels. 

2.  Numbers are scaled to increase from bottom to top of the graduated cylinder.  Using the scale markings, determine the value of the certain digits.

3.  Estimate the

distance the meniscus lies between markings as a decimal fraction.  Multiply the fraction times the division increment from Step 1.  Add this to the certain digits to provide the uncertainty in the measurement.  Record this number.EXAMPLE 1 

Step 1.  The labeled scale markings are 8 mL and 6 mL.  There are 10 divisions between the

numeric labels.  [(8-6)/10] mL = 0.2 ml is the increment value.Step 2.  The first certain digit is 6 mL since the meniscus is below 8 mL.  There are three smaller scale divisions below the meniscus: 3 x 0.2 mL/division = 0.6 mL  The known digits are (6 + 0.6 ) mL = 6.6 mLStep 3.  The meniscus lies 0.1 of the distance between the markings: 0.1 x 0.2 mL = 0.02 mL  The volume

Example 1.  10-mL graduated cylinder

should be recorded as (6.6  + 0.02) mL = 6.62 mLEXAMPLE 2

Step 1.  The labeled scale markings are 20 mL and 10 mL.  There are 10 divisions between the numeric labels.    [(20-10)/10] mL = 1 ml is the increment value.Step 2.  The first certain digit is 10 mL since the meniscus is below 20 mL.  There are seven smaller scale divisions below the meniscus:

Example 2.  50-mL graduated cylinder

7 x 1 mL/division = 7 mL  The known digits are (10 + 7) mL = 17 mLStep 3.  The meniscus lies on the scale division:  0.0 x 1 mL = 0.0 mL  The volume should be recorded as (17 + 0.0) mL = 17.0 mL

Burettes

Like graduated cylinders, burettes come in various volume sizes. Burettes are calibrated to deliver a very precise volume of liquid, often to the hundredths of a mL.  Care must be taken when filling the buret that the tip contains no air bubbles.

MEASUREMENTS: Water and aqueous solutions will form a concave

                 

meniscus when placed in a burette similar to a graduated cylinder. The bottom of the curved surface is read at eye level and the volume measurement is read to the proper number of significant digits.  Use the steps given for graduated cylinders to make the volume measurement with the proper number of significant digits. Note that, unlike the graduated cylinder, the

           Burette with holder and stand                              Tip with no air bubbles

numbers are scaled to increase from top to bottom.EXAMPLE 1

Step 1.  The labeled scale markings are 14 mL and 15 mL.  There are 10 divisions between the numeric labels.    [(15-14)/10] mL = 0.1 ml is the increment value.Step 2.  The first certain digit is 14 mL since the meniscus is below 14 mL.  There are zero smaller scale division

Example 1

above the meniscus: 0 x 0.1 mL/division = 0.0 mL  The known digits are (14 + 0.0 ) mL = 14.0 mLStep 3.  The meniscus lies 0.5 of the distance between the markings: 0.5 x 0.1 mL = 0.05 mL  The volume should be recorded as (14.0  + 0.05) mL = 14.05 mLEXAMPLE 2

Step 1.  The labeled scale markings are 20 mL and 21 mL.  There are 10 divisions Example 2

between the numeric labels.    [(21-20)/10] mL = 0.1 ml is the increment value.Step 2.  The first certain digit is 20 mL since the meniscus is below 20 mL.  There are eight scale divisions above the meniscus: 8 x 0.1 mL/division = 0.8 mL  The known digits are (20 + 0.8 ) mL = 20.8 mLStep 3.  The meniscus lies 0.9 of the distance between the markings: 0.9 x 0.1

mL = 0.09 mL  The volume should be recorded as (20.8  + 0.09) mL = 20.89 mL

Mohr Pipettes

A Mohr pipette differs from a volumetric pipette in that the barrel has a series of marked lines, much like a buret, to measure the amount of volume dispensed.  A pipette pump is used to draw liquid into the barrel and

10-mL Mohr pipette

1-mL Mohr pipette

dispense into a container.  To insure the correct volume of liquid is dispensed, the tip of the pipette is touched to side of the container to release the final drop.  Be careful to check the scale markings on Mohr pipettes.  The last calibration mark on the 10-mL pipette shown is 9.5 mL.  The entire volume is drained from the barrel if 10.00 mL is required.  The last calibration mark on the 1-mL

pipette is 1.00 mL.  The liquid is drained until the meniscus reaches 1.00 mL if 1.000 mL is required.

MEASUREMENTS: Water and aqueous solutions will form a concave meniscus when placed in a pipette similar to the burette. The bottom of the curved surface is read at eye level and the volume measurement is read to the proper

                          

number of significant digits.  Use the steps given for graduated cylinders to make the volume measurement with the proper number of significant digits. Note that, like the burette, the numbers are scaled to increase from top to bottom.

EXAMPLES: 

Step 1.  The labeled scale markings are 0.4 mL and 0.5 mL.  There are 10 divisions

between the numeric labels.    [(0.5-0.4)/10] mL = 0.01 ml is the increment value.Step 2.  The first certain digit is 0.4 mL since the meniscus is below 0.4 mL.  There are four smaller scale division above the meniscus: 4 x 0.01 mL/division = 0.04 mL  The known digits are (0.4 + 0.04 ) mL = 0.44 mLStep 3.  The meniscus lies 0.3 of the distance between the

markings: 0.3 x 0.01 mL = 0.003 mL  The volume should be recorded as (0.44  + 0.003) mL = 0.443 mL

Micropipettes

Micropipettes are used to accurately measure and dispense small volumes of liquid, usually in the 1 µL - 1000 µL range.  Micropipettes work on an air displacement principle and use detachable tips to

hold the liquid sample.  Adjustable volume micropipettes, like the one, shown have a window to display the volume required for pipetting.

MEASUREMENTS: Note the volume units given on the micropipette.  The micropipette shown measures volumes in the 100-1000 µL range.  The window indicates that the liquid volume in

the tip is 400.0 µL (0.4000 mL).

Volumetric Flasks

Volumetric flasks are calibrated to contain a  precise volume of solution and are, therefore, often used to prepare solutions needed for quantitative analysis.  The neck of the flask has a calibration mark to indicate the fill level for the volume of solution needed.  The bottom of the

meniscus is lined up with this mark to insure accuracy.

MEASUREMENTS: Note the volume given on the volumetric flask.  Tolerances are usually within a few hundredths of a mL.  When filled to the calibration mark, the flask shown would contain 100.00 mL (0.10000 L) of solution.

Digital Balance

Accurate mass measurements are made using a digital balance.  The maximum capacity of the balance is typically found on the front label.  Typical laboratory balances measure in grams (some may be set to measure in mg or kg) and feature a tare function. Tare allows the user to zero the instrument to cancel the mass of a

container (such as a weighing dish) from the reading of the instrument.  This allows the actual mass of the product being weighed to be read directly from the digital display.  A draft shield is often placed over the balance pan to minimize air currents and stabilize readings.

MEASUREMENTS: The mass is read directly from the digital display.  The

balance shown is set to display mass in grams to three decimal places.  All digits must be recorded for each weighing performed during the laboratory .  Note that the last digit in the display is an estimated digit.Hydrometers

Hydrometers are used to measure the specific gravity of a liquid. Hydrometers work on Archimedes principle that a submerge

d or floating body experiences a buoyant force equal to the mass of the liquid volume it displaces. Since this force depends on the mass and volume of liquid displaced, it is directly proportional to the specific gravity of the liquid.  The more dense the liquid, the higher the hydrometer floats.  A specialized hydrometer known as a urinometer can be used to measure

               

the specific gravity of a urine sample.

          urinometer                                    urinometer in a urine sample

MEASUREMENTS: If a hydrometer is floated in a liquid of known temperature, the specific gravity of the liquid can be read from

the scale on the stem of the hydrometer. The density of the liquid can then be determined from the equation for specific gravity. Figure 1 shows the use of a hydrometer to measure specific gravity. If attractions exist between the hydrometer surface and the liquid being measured, a meniscus occurs.  This is a common occurrence when the solution contains water and

the hydrometer is made of glass.  The reading is taken at the lowest level of the meniscus. Note that the scale markings increase from top to bottom.

EXAMPLE:

Step 1.   The labeled scale markings are .95 and 1.000.  There are five divisions between the numeric markings.  (1.000-0.95)/5 = 0.01 is the increment value.Step 2.  The first

significant digit is 0.95 since the meniscus lies below 0.95.  There are three smaller scale divisions above the meniscus: (3 x 0.01) = 0.03  The known significant digits are (0.95 + 0.03) = 0.98 Step 3.  The meniscus lies 0.2 of the distance between the markings: (0.2 x 0.01) = 0.002  The specific gravity should be reported as (0.98 + 0.002) = 0.982Thermomet

ers

Thermometers are used to measure the temperature of a system.  A thermometer has two important elements: the temperature sensor in the bulb of the thermometer in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value on the scale.   To protect against                                            

the hazards associated with mercury, many lab thermometers are filled with alcohol.  In the laboratory the Celsius temperature scale is used.  Temperatures ranges vary from one thermometer to another.  It is important to select a thermometer in which the expected temperature is within the range of the scale markings.

-20 -110 °C thermometer                  -10 - 260 °C thermometer

MEASUREMEN

TS: Use the steps given for graduated cylinders to make the temperature measurement with the proper number of significant digits.

EXAMPLE

Step 1.  The labeled scale markings are 80 °C and 90 °C.  There are 10 divisions between the numeric labels.[(90-80)/10] °C = 1 °C is the increment value.Step 2.The

first certain digit is 80 °C since the meniscus is below 90 °C.  There are seven smaller scale divisions below the liquid: 7 x 1 °C/division=7 °C.  The known digits are (80 + 7) °C = 87°CStep 3.The meniscus lies 0.5 of the distance between the markings: 0.5 x 1 °C = 0.5 °C. The temperature should be recorded as (87  + 0.5) °C = 87.5 °C