Selected_Methods_Chapter37.pdf

CHAPTER 37

Selected Methods of Analysis

Chemistry is primarily an experimental science. This chapter presents a variety of

laboratory experiments, from classical titrations and gravimetry to instrumental

methods such as chromatography and spectroscopy. Detailed directions are given

for each experiment.

analyses. The methods have been chosen to introduce you to analytical tech-

niques that are widely used by chemists. For most of these analyses, the composi-

tion of the samples is known to the instructor. Thus, you will be able to judge how

well you are mastering these techniques.

Your chances of success in the laboratory will greatly improve if you take time

before you enter the laboratory to read carefully and understand each step in the

method and to develop a plan for how and when you will perform each step.

The discussion in this section is aimed at helping you develop efficient work

habits in the laboratory and also at providing you with some general information

about an analytical chemistry laboratory. Before you start an analysis, you should

understand the significance of each step in the procedure to avoid the pitfalls and

potential sources of error that are inherent in all analytical methods. Information

about these steps can usually be found in (1) preliminary discussion sections, (2)

earlier chapters that are referred to in the discussion section, and (3) the “Notes”

that follow many of the procedures. If, after reading these materials, you still do not

understand the reason for doing one or more of the steps in the method, consult

your instructor before you begin laboratory work.

The Accuracy of Measurements

In looking over an analytical procedure, you should decide which measurements

must be made with maximum precision, and thus with maximum care, as opposed

to those that can be carried out rapidly with little concern for precision. Generally,

measurements that appear in the equation used to compute the results must be per-

formed with maximum precision. The remaining measurements can and should be

made less carefully to conserve time. The words about and approximately are fre-

quently used to indicate that a measurement does not have to be done carefully. For

example, you should not waste time and effort to measure a volume to

0.02 mL

when an uncertainty of

0.5 mL or even

5 mL will have no discernible effect on

the results.

In some procedures, a statement such as “weigh three 0.5-g samples to the near-

est 0.1 mg” is encountered. Here, samples of perhaps 0.4 to 0.6 g are acceptable,

but their masses must be known to the nearest 0.1 mg. The number of significant

T his chapter contains detailed directions for performing a variety of chemical

37A An Introductory Experiment

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figures in the specification of a volume or a mass is also a guide to the care that

should be taken in making a measurement. For example, the statement “add 10.00

mL of a solution to the beaker” indicates that you should measure the volume care-

fully with a buret or a pipet, with the aim of limiting the uncertainty to perhaps

Time Utilization

You should study carefully the time requirements of the several unit operations

involved in an analysis before work is started. This study will reveal operations that

require considerable elapsed, or clock, time but little or no operator time. Examples

of such operations include drying a sample in an oven, cooling a sample in a desicca-

tor, or evaporating liquid on a hot plate. Efficient workers use such periods to perform

other operations or perhaps to begin a new analysis. Some people find it worthwhile

to prepare a written time schedule for each laboratory period to avoid dead time.

Time planning is also needed to identify places where an analysis can be inter-

rupted for overnight or longer, as well as those operations that must be completed

without a break.

Reagents

Directions for the preparation of reagents accompany many of the procedures.

Before preparing such reagents, be sure to check to see if they are already prepared

and available on a side shelf for general use.

If a reagent is known to be hazardous, you should plan in advance of the labo-

ratory period the steps that you should take to minimize injury or damage. Further-

more, you must acquaint yourself with the rules that apply in your laboratory for

the disposal of waste liquids and solids. These rules vary from one part of the coun-

try to another and even among laboratories in the same locale.

Water

Some laboratories use deionizers to purify water; others employ stills for this pur-

pose. The terms “distilled water” and “deionized water” are used interchangeably

in the directions that follow. Either type is satisfactory for the procedures in this

chapter.

You should use tap water only for preliminary cleaning of glassware. The

cleaned glassware is then rinsed with at least three small portions of distilled or

deionized water.

37A AN INTRODUCTORY EXPERIMENT

The purpose of this experiment is to introduce several of the tools, techniques, and

skills necessary for work in the analytical chemistry laboratory. The techniques are

considered one at a time, as unit operations. It is important to learn proper tech-

niques and to acquire individual skills before attempting additional laboratory

experiments.

37A-1 Using the Analytical Balance

Discussion

In this experiment, you will obtain the mass of five new pennies—first by deter-

mining the mass of each penny individually. Then you will determine the mass of

0.02 mL. In contrast, if the directions read “add 10 mL,” the measurement can be

made with a graduated cylinder.

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CHAPTER 37

Selected Methods of Analysis

all five pennies at once, remove one penny at a time, and calculate the individual

masses of the pennies by finding the difference. The pair of masses determined for

a particular penny by the two different methods should agree to within a few tenths

of a milligram. From the data, you will determine the mean and median values, the

standard deviation, and the relative standard deviation of the masses of the pennies.

You will then weigh an unknown aluminum cylinder and report the mass of this

unknown.

PROCEDURE

1. After you have been instructed in the use of the balance and have become famil-

iar with its use, obtain a set of pennies, an unknown aluminum cylinder, and a

pair of tweezers from the instructor.

2. Do not handle the pennies or the cylinder with your fingers; always use the

tweezers. If you are using a mechanical balance, be sure to have the balance in

the “off” or “complete arrest” position whenever removing anything from or

adding anything to the balance pan.

3. Before you begin to determine masses, zero your analytical balance carefully.

Select five pennies at random from the vial containing the pennies, and weigh

each penny on your balance. Enter the data in your laboratory notebook. Keep

track of the identity of each penny by placing each one on a labeled piece of

paper.

4. Check the zero setting on your balance. Place these same five pennies on the

balance pan, determine their total mass, and record it.

5. Remove one of the pennies from the balance, obtain the mass of the remaining

four, and record the mass.

6. Repeat this process, removing one penny at a time. Obtain the individual

masses by subtraction. This process is known as weighing by difference, which

is the way many mass determinations are done in the analytical laboratory.

7. Finally, check the zero on your balance, and find the mass of the unknown alu-

minum cylinder.

37A-2 Making Quantitative Transfers

Discussion

The following experiment is designed to provide experience in the correct use of

the volumetric flask.

PROCEDURE

1. Weigh a 50-mL beaker on a triple-beam balance or an appropriate electronic

top-loading balance.

2. Adjust the balance for an additional 0.4 g and add solid KMnO 4 to the beaker

until the beam is again balanced. If you have an electronic balance with a tare

function, depress the tare button to set the balance to zero. Then add KMnO 4

until the balance reads about 0.4 g. Note that chemicals should never be

returned to a stock bottle, as this may contaminate the bottle.

3. Dissolve the potassium permanganate in the beaker using about 20 mL of dis-

tilled water. Stir gently to avoid loss. This is nearly a saturated solution, and

some care is required to dissolve the crystals completely.

37A An Introductory Experiment

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4. Quantitatively transfer the solution to a 100-mL volumetric flask fitted with a

small funnel. To prevent solution from running down the outside of the beaker,

pour it down the stirring rod, and then touch the rod to the spout of the beaker to

remove the last drop. Add more water to the beaker, stir, and repeat the

procedure.

5. Repeat the procedure until no trace of the color of the permanganate remains in

the beaker. Note the number of washings that is required to quantitatively trans-

fer the permanganate from the beaker to the flask.

6. Rinse the last portion of solution from the stirring rod into the volumetric flask

with a stream of water from the wash bottle. Rinse the funnel and remove it.

Dilute the solution in the flask until the bottom of the meniscus is even with the

graduation mark. Stopper, invert, and shake the flask. Return it to the upright

position, and allow the air bubble to return all the way to the top of the neck.

7. Repeat until the solution is completely homogeneous; about 10 inversions and

shakings are required. Save the solution for Part 37A-3.

37A-3 Delivering an Aliquot

Discussion

Whenever a buret or pipet is used to deliver a measured volume of solution, the liq-

uid it contains before measurement should have the same composition as the solu-

tion to be dispensed. The following operations are designed to illustrate how to

rinse and fill a pipet and how to deliver an aliquot of solution.

PROCEDURE

1. Fill a pipet with the solution of potassium permanganate and let it drain.

2. Draw a few milliliters of distilled water from a 50-mL beaker into the pipet,

rinse all internal surfaces of the pipet, and discard the rinse solution. Do not fill

the pipet completely; this is wasteful, time-consuming, and inefficient. Just

draw in a small amount, tilt the pipet horizontally, and turn it to rinse the sides.

3. Determine the minimum number of such rinsings required to completely

remove the permanganate color from the pipet. If your technique is efficient,

three rinsings should be enough.

4. Again fill the pipet with permanganate solution, and proceed as before. This

time determine the minimum volume of rinse water required to remove the

color by collecting the rinsings in a graduated cylinder. Less than 5 mL are

enough with efficient technique. In the rinsing operations, was the water in the

50-mL beaker contaminated with permanganate? If a pink color shows that it

was, repeat the exercise with more care.

5. As a test of your technique, ask the laboratory instructor to observe and com-

ment on the following operation: Rinse a 10-mL pipet several times with the

solution of potassium permanganate you prepared.

6. Pipet 10 mL of the permanganate solution into a 250-mL volumetric flask.

7. Carefully dilute the solution to volume, trying to mix the contents as little as

possible.

8. Mix the solution by repeatedly inverting and shaking the flask. Note the effort

that is required to disperse the permanganate color uniformly throughout the

solution.

9. Rinse the pipet with the solution in the volumetric flask. Pipet a 10-mL aliquot

of the solution into a conical flask.

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CHAPTER 37

Selected Methods of Analysis

37A-4 Calibrating a Pipet

Discussion

The proper manual technique for calibrating an analytical transfer pipet is readily

learned with practice, care, and attention to detail. With the possible exception of

mass determinations, this experiment has the potential of being the most accurate

and precise set of measurements that you will ever make.

PROCEDURE

1. Clean a 10-mL pipet. When a pipet, buret, or other piece of volumetric glass-

ware is cleaned properly, no droplets of reagent remain on the internal surfaces

when they are drained. This is very important for accurate and reproducible

results. If reagent adheres to the inside of a pipet, you cannot deliver the nomi-

nal volume of the pipet. If you clean a pipet or any other glassware with alco-

holic KOH, use the bottle of cleaning solution only inside the sink and rinse it

off thoroughly before returning it to the shelf. Do not put the bottle of cleaning

solution directly on a bench top; it may ruin the surface. The solution is very

corrosive. If your fingers feel slippery after use, or if some part of your body

develops an itch, wash the area thoroughly with water.

2. Obtain a pipetting bulb, a 50-mL Erlenmeyer flask with a dry stopper, a 400-

mL beaker of distilled water equilibrated to room temperature, and a

thermometer.

3. Determine the mass of the flask and stopper and record it to the nearest 0.1 mg.

Do not touch the flask with your fingers after this weighing. Use tongs or a

folded strip of waxed paper to manipulate the flask.

4. Measure and record the temperature of the water.

5. Pipet 10.00 mL of the distilled water into the flask using the technique

described on page 45. Stopper the flask, determine the mass of the flask and the

water that it contains, and record the mass.

6. In the same way, add a second pipet of water to the flask; remove the stopper

just before the addition. Replace the stopper, and once again determine and

record the mass of the flask and the water. Following each trial, determine the

mass of water added to the flask by the pipet.

7. Repeat this process until you have determined four consecutive masses of water

that agree within a range of 0.02 g. If the determinations of the mass of water

delivered by the pipet do not agree within this range, your pipetting technique

may be suspect. Consult your instructor for assistance in finding the source of

the error, and then repeat the experiment until you are able to deliver four con-

secutive volumes of water with the precision cited.

8. Correct the mass for buoyancy as described on page 27, and calculate the vol-

ume of the pipet in milliliters.

9. Report the mean, the standard deviation, and the relative standard deviation of

the volume of your pipet. Calculate and report the 95% confidence interval for

the volume of your pipet.

37A-5 Reading Buret Sections

Discussion

The following exercise will give you practice in reading a buret and confirming the

accuracy of your readings.

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