E2 · Web viewPrepare 250 mL of a .01 M solution of KHP (primary) and 250 mL of a .01 M solution of NaOH (non-primary). The weight of KHP should be recorded with care as it will determine

E2

Wet Techniques

C2507 Intensive General chemistry – Spring 02 – E2: Wet Techniques

E2 – Wet Techniques

OVERVIEW

Wet techniques (or classical methods of analysis) is the name given to a series of

laboratory measurements that include qualitative reaction analysis and quantitative

analysis, represented by gravimetry (analysis by weight) and volumetry or titrimetry

(analysis by measurement of the volume of a known concentration solution)1.

In this laboratory exercise you will be initiated to the concept of measurement and

its inherent uncertainty. While working collaboratively you will personally become

acquainted with:

a) Calibration of precision glassware

b) Preparation of solutions of standards (primary and non-primary) and titration

c) Qualitative analysis of a mixture of five cations

The tasks of part a) will be distributed among the group members. Part b) will be

performed individually, while part c) will be performed in pairs. Each member is

responsible for submitting a write-up for a), b) and c).

OBJECTIVES

After finishing this session and completing the write-up, you should be able to:

Appreciate the difference between general and precision glassware

Use error analysis to express your results

Prepare solutions of known concentration

Perform titration

Conduct the gravimetric analysis of a salt

Identify and separate cations in a mixture

1 Schwedt G., The Essential Guide to Analytical Chemistry, 2nd Ed, pp38-43, John Wiley and Sons, New York (1999).

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E2a – Calibrating Precision Glassware

CLEANING

Volumetric glassware must be scrupulously clean before use. The presence of streaks or

droplets is an indication of the presence of a grease film.

a) Scrub with detergent solution. Rinse with tap water. And finally rinse with a small

portion of distilled water.

b) If the grease film persists, rinse it with Chromerge solution. Return the rinsing to the

bottle (this solution must be reused until it becomes green colored due to the presence

of CrIII ions). Rinse with enough tap water followed by distilled water as before.

c) Volumetric glassware should never be dried with hot air or by heating it. Dry by

letting the glassware stand in inverted position. You can also rinse with a small

portion of acetone (very volatile).

SAFETY NOTE: Chromerge solution is potassium dichromate dissolved in

concentrated sulfuric acid. It is extremely corrosive; you should use

Neoprene gloves at all times and avoid its contact with clothes.

CALIBRATION

Volumetric ware is manufactured either “to contain” (TC) or “to deliver” (TD)2.

Volumetric flasks (TC)

A volumetric flask has a large round bottom with only one graduation mark positioned on

the long narrow neck. The position of the mark facilitates the accurate and precise

reading of the meniscus. When filling a volumetric flask, be sure to stop the transfer of

liquid when the liquid level is about an inch below the graduation mark. If you have a

mixture in your flask, cap it with a stopper and turn it upside down three times to insure

good mixing. Then use a Pasteur pipette to add liquid slowly to the mark. Overfilling

the flask above the graduation mark ruins the volume measurement. In this case, the

2 Skoog D.A., West D.M., Analytical Chemistry, 2nd Ed, pp89-91, Holt, Rinehart and Winston (1965).

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content inside the volumetric flask should be discarded. Once liquid is filled to the mark,

cap the flask with a stopper and mix as before.

Pipettes (TD)

In this course you will use graduated and volumetric pipettes. Graduated pipettes deliver

varying amounts by making use of incremental markings while volumetric pipettes

deliver specific, fixed amounts. The graduated pipette is a narrow tube while the

volumetric pipette possesses an enlarged portion below the mark in order to reduce the

speed of suction. A pipette bulb should be used to withdraw and deliver liquids.

Burettes (TD)

A burette is a long glass tube with a stopcock near the tip, which controls the rate of flow

of liquid down the constricted tip of the burette.

Before use, burettes must be rinsed with a small portion of the liquid to be measured.

Hold it horizontally and rotate it to allow the liquid to go over the entire inside, and

then drain the liquid down the tip.

When filling a burette, if you use a funnel, make sure it is very clean. If you see air

bubbles, eliminate them by draining a few mL of the liquid down the tip while

holding the burette diagonally.

Graduated cylinder (TD)

A graduated cylinder has incremental markings. When filling it, stop the transfer of

liquid before the desired graduation mark and adjust the volume of liquid with a Pasteur

pipette.

Depending on the glassware, the gravimetric calibration procedure at room temperature

consists of:

a) Determining the mass of distilled water that fills the TC-ware until the bottom of the

meniscus coincides with the graduation mark; or collecting the amount of water

delivered by the TD-ware in a tared beaker. The procedure should be repeated at

least three times to assure reproducibility.

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b) The exact temperature of water must be measured with a 0.1°C precision

thermometer before each measurement.

c) The calibrated volume will be calculated using the density of water at the calibration

temperature (CRC Handbook of Chemistry and Physics).

Warning: The Mettler balance has a capacity of 510g.

STATISTICAL ANALYSIS OF DATA

Distribute the four pieces of glassware: Volumetric flask, Pipette, Burette and Graduated

cylinder among the group members. Each group member should calibrate one object.

Your Ta will give you instructions about the volumes to measure.

Input the data from your notebook into the spreadsheet corresponding to each piece of

glassware. 1. Record the name of the manufacturer. Fill up the information as in the

example below.

Measured

Volume (mL)

Measured mass

(g)

Temperature of

water (°C)

Density (g/mL) Calculated

volume (mL)

10.00 9.99

10.00 9.98

2. The spreadsheet with the data from the entire class will be posted on the website.

Compute the average, variance, and standard deviation for each piece of glassware.

Compute the same statistic for the mass determined with the Mettler balance

3. Find the uncertainty given by the glassware manufacturer and the Mettler Company.

4. Compare the values of the computed statistic with the uncertainty provided by the

manufacturers. Comment on the statistic given by the manufacturer.

5. Discuss the precision of the calibrated volume for each piece of glassware and for the

Mettler balance.

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E2b – Volumetry

Introduction

In this experiment you will review the preparation of solutions of known concentration

and titration techniques. A good reference book for the review of Acid/Base concepts is

Oxtoby, Gillis, Nachtrieb; Principles of Modern Chemistry, 4th Ed.; Saunders College

Publishing, 1999 (available in the chemistry library).

Each group will obtain an unknown monoprotic weak acid. You will work individually

and then compare your results for the unknown with your group members.

Primary versus non-primary standard

Tare two watch glasses and on each mass a small amount of NaOH and KHP

Leave the watch glasses exposed to the air for 1 hour.

After 1 hour, weigh the watch glasses and record your observations.

Preparation of standards

Prepare 250 mL of a .01 M solution of KHP (primary) and 250 mL of a .01 M solution of

NaOH (non-primary). The weight of KHP should be recorded with care as it will

determine the concentration of the solution. The concentration of the NaOH solution is

obtained by titration with the KHP solution using phenolphtalein as an indicator. (see

appendix A for tips on titration techniques). Titration of the NaOH solution should be

repeated at least two times to insure accurate and precise result.

Titration of an unknown monoprotic weak acid

Use the standardized NaOH solution prepared above to titrate the unknown provided to

the group, using phenolphtalein as an indicator. Titrate at least three times.

Determination of the pKa of the unknown

Add to a fresh solution of unknown monoprotic weak acid half the volume necessary to

neutralize it. Measure the pH using the Corning pH meter.

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SAMPLE WRITE-UP (Include sample calculations with error analysis)

Primary versus non-primary standard

At t=0 WNaOH = WKHP =

At t=1hour WNaOH = WKHP =

Observations:

Preparation of standards

mKHP = CKHP =

mNaOH = CNaOH (approximate) =

Standardization of NaOH:

Veq 1 =

Veq 2 = CNaOH =

Veq 3 =

Titration of an unknown monoprotic weak acid

Unknown #:

Veq 1 =

Veq 2 = Cacid =

Veq 3 =

Results from group: Input the results of all group members (yours included). Calculate

the concentration of the acid and perform error analysis. Comment on the precision of

the result and the possible sources of error.

Determination of the pKa of the unknown

Explain how you can obtain the pKa from your experimental result. Compare your result

with your group members. Could you identify the acid you were given?

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E2 - QUALITATIVE ANALYSIS FOR CATIONS

INTRODUCTION

The serpent Ouroboros, this composition which in its ensemble is devoured and melted, dissolved, and transformed by the fermentation or putrefaction. It becomes a deep green and the color of gold is derived from it. It is from it that is derived the red called the color of cinnabar. This is the cinnabar of philosophers. Its stomach and back are the color of saffron, its head is a deep green. Its four feet constitute the tetrasomie. Its three ears are the three sublimed vapors….A serpent is stretched guarding this temple and he who has subdued it commences by sacrificing it, then roasts it, and after removing its flesh up to the bones makes of it a step to the entrance of the temple. Mount upon it and thou shalt find the object sought, for the priest at first a man of copper has changed color and nature and has become a man of silver; a few days later, if thou wish, thou wilt find him changed to a man of gold.3

Qualitative analysis can be traced back to the beginning of chemistry with the work of

alchemists. This experiment will not be as tedious as the preparation of the cinnabar of

philosophers, instead we will use the formation of colored precipitates to identify cations

in solution in the same way alchemists did in their time.

Take white lead, one part, and any glass you choose, two parts, fuse together in a crucible and then pour the mixture. To this crystal, add the urine of an ass and after forty days you will find emeralds. Ibid. p. 160.

You will be given 0.1 M solutions of Zn2+, Pb2+, Ni2+, Al3+, and Co2+, all necessary

reagents, and a General Procedure for identifying the ions. For each test, you should

observe and record the formation or dissolution of any precipitate, and note any color or

consistency changes in the solution or supernatant liquid in a Reaction Chart. Since

your observations from these tests will be essential for the creation of a successful Cation

Analysis Scheme, be sure to watch each reaction carefully. Once you have designed a

separation scheme, you will use it to analyze an unknown cation solution, which contains

one to five of the aforementioned cations.

3 Stillman, J. M. The Story of Alchemy and Early Modern Chemistry; Dover: New York, 1960, p. 171

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TIPS ON TECHNIQUES

Semi-micro Qualitative Ion Analysis: Qualitative ion analysis is concerned with

identifying the ions in a solution or solid. In contrast, quantitative analysis deals

with the exact quantity of ion present. The procedure in this experiment

introduces you to semi-micro techniques that only use a few mL (or less) of

sample solutions and test reagents. Although it is not essential to use exact

volumes of reagent in this case, it is wisest to use the number of drops specified in

the procedure. In order to avoid confusion, label your test tubes and arrange them

in the same order as listed on the reaction chart. It is also advisable to perform a

given test on all the known cations before going on to the next test. Be sure to

thoroughly mix test reagent and cation solutions before recording observations.

You may observe the formation of a precipitate (ppt.), which is a solid that falls

out of solution. The supernatant (snt.) is the liquid phase of this solid-liquid

mixture.

Separating reaction products: The separation of the precipitate and the

supernatant is achieved by spinning the mixture in the centrifuge such that the

solid becomes packed against the bottom of the test tube. After this is done, the

color of the precipitate is easier to see, and the supernatant can be simply poured

off.

Decant versus discard: After centrifuge separation of the precipitate and

supernatant, you may be instructed to either decant or discard the supernatant.

Both terms describe the removal of the supernatant from the precipitate (which is

most easily done by pouring off the supernatant while slowly inverting the test

tube.) Decant, however, specifically implies that the supernatant is to be poured

into another test tube and saved for further testing. Discard, on the other hand,

implies that the supernatant should be disposed of.

Washing precipitates: Following the removal of snt., you may need to wash the

precipitate free of residual supernatant. To do this, add no more than 2/3 of a test

tube full of distilled water, mix thoroughly, and centrifuge. Then pour off the

water supernatant One wash should sufficiently clean the precipitate provided

that most of the original supernatant had already been removed.

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Monitoring the pH: The pH plays a key role in most tests. Add acid or base to

the solution dropwise as directed and check pH as needed. To monitor the pH,

first cut a 1.5 inch piece of pH paper. Then cut the strip into small segments and

spread them on a three-inch watch glass. Take care not to wet the pH paper with

water, which would make it useless. Touch the solution you want to test with the

stirring rod and wet a piece of pH paper with it. The paper will change color

according to the pH of the solution. Overlay the watch glass over the dispensing

box to compare the color change you observe with the chart on the box. Record

the color and the corresponding pH. To assure reproducibility, repeat the test.

Contaminants: It is obviously important to have test tubes and stirring rods free

of any residual cations or reagents from previous tests.

SAMPLE CATION ANALYSIS

A systematic cation analysis consists of a three-steps operation:

A. Conduct the reactions according to the given General Procedure

B. Record the results of the reactions on the Reaction Chart

C. Draw a Cation Analysis Scheme based on the Reaction Chart

The following procedure is an example of the three step procedure you will follow to

complete this experiment. Note that the general procedure of tests for this group of

cations is not the same as that of the cations you will test for! It is only an example.

A. General procedure for the qualitative analysis of a mixture containing the

cations Ag+, Pb2=, Cu2=, Ba2+ and Fe3+:

1. To 10 drops of test solution, add 5 drops 3M H2SO4.

2. To 10 drops of test solution, add 5 drops 6 M HCl. For those cations which

form a ppt. with HCl, wash the ppt., in preparation for test 3.

3. To the ppt. from test 2, add about 1.5 ml of distilled water and heat in a boiling

water bath for 10 minutes while mixing. (This is done to determine if the ppt. will

dissolve.) Centrifuge and decant the hot snt. Add 1 drop 2M K2CrO4 to snt.

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4. To 10 drops of test solution, add 2 drops of 6M NaOH. If a ppt. is formed, mix,

centrifuge, and discard snt. This ppt. is also used for test 5 and 6. (No ppt.

formation means that test 5 and 6 need not be done.)

5. To the hydroxide ppt. in test 4, add 20 drops of 15M NH3. Mix and centrifuge.

6. To the washed hydroxide ppt. from test 4, add 4 drops of 6M HCl. Mix. Then

add 2 drops of 1M NH4SCN.

B. Reaction chart tabulated while executing the general procedure:

______solution

turnsbloody-brownsolutionturns

dull yellowpartially

dissolves torose-pinksolution

______dissolvestotan-yellowsolution

dissolveslime solution

5. Add 15M NH3hydroxidedissolves

deepblue soln.

hydroxidedissolves

clear solutionN.R.______

rustygel ppt.N.R

.marineblue

gel ppt.whiteppt.(then clear)temporary

brownish-greythickppt

.

___ ______

_________

yellow ppt.N.R.

ppt.dissolves

ppt.remains

N.R.N.R.

Thickwhiteppt. Flaky white

ppt. N.R.

N.R.partiallywhiteppt.

N.R.chalkywhite ppt.

6a) Add 6M HCl to test 4 ppt.

6b) Add 1M NH4SCN

Ag+Test / Reagent Pb+2 Cu+2 Ba+2 Fe+3

1. 3M H2SO4

2. 6M HCl

3a) Heat ppt. from test 2 3b) Add CrO4

-2

4. 6M NaOH

chunkywhiteppt.

N.R.

to

to

to

C. Cation analysis scheme based on the above reaction chart:

The scheme must be designed to account for the possibility of all five cations being

present if it is to be applied to an unknown solution of cations (containing from one to

five of the cations Ag+, Pb2=, Cu2=, Ba2+ and Fe3+).

Notice that a starting solution of about 5 mL is all that is used throughout the

scheme; i.e., the original cation solution is treated with reagents for selective precipitation

and repeated separation of cations until each cation is isolated and characterized. If a

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final confirmation test comes out negative, that cation is most probably not present in the

unknown solution. Furthermore, note that not every test performed in the general

procedure was needed in this final analysis scheme. Overall, the acid and base

preparations were used for initial precipitations, while more specific reagents like K2CrO4

and NH4SCN were saved for final confirmation tests for Pb2+ and Fe3+, respectively.

The analysis scheme proposed below is not the only way to analyze such a mixture.

As an exercise (pre-lab question), you should sketch an alternative analysis scheme

starting with precipitation with NaOH instead of HCl.

Figure 1. Analysis scheme for a mixture of cations Ag+, Pb2=, Cu2=, Ba2+ and Fe3+

CATION ANALYSIS FOR Zn2+, Pb2+, Ni2+, Al3+, and Co2+

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EQUIPMENT AND REAGENTS:

0.1 M solutions of each Zn(NO3)2, Pb(NO3)2, Al(NO3)2, Ni(NO3)2 and Co(NO3)2

3M NH3 Concentrated NH3

3 M HNO3 6 M NaOH

0.2 M K3Fe(CN)6 0.1% Aluminon

0.3 M KI pH 7 buffer solution

(DMG) reagent (1% dimethylglyoxime in 95% EtOH)

WASTE

All waste from this lab should be disposed in your waste beaker. Do not pour anything

down the sink!

PROCEDURE

A. Cation Analysis for Zn2+, Pb2+, Ni2+, Al3+, and Co2+

You will be provided with 0.1 M solutions of each Zn(NO3)2, Pb(NO3)2, Al(NO3)2,

Ni(NO3)2 and Co(NO3)2. Perform the following tests (1-8) on each cation solution before

going on to the analysis of the unknown. Record all observations on a reaction chart such

as the one shown below:

1. To 10 drops of test solution, add 3M NH3 (NH4OH) dropwise until the pH is

between 6 and 8. Then add 6 more drops of 3M NH3.

2. To 10 drops of test solution, add 3 M HNO3 until the solution is acidic (pH<3).

Then add 6 drops of 0.2 M K3Fe(CN)6 (potassium ferricynanide) and record

your observations.

3. To 10 drops of test solution, add 3 M HNO3until pH < 3. Then add 6 drops of

0.3 M KI.

4. To 10 drops of test solution, add 6 M NaOH until pH > 8. Then add 10 more

drops of 6 M NaOH. If a hydroxide ppt. forms, mix and centrifuge. Discard

snt. and wash the ppt. with distilled water once for test 5 or 6.

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5. To the hydroxide ppt. from the previous test with 3M NH3 (NH4OH), add 20

drops of concentrated NH3. The point is to see if the ppt. dissolves. Mix and

centrifuge, and add more concentrated NH3 if necessary to confirm dissolution.

6. Repeat the test with 3M NH3 (NH4OH) to obtain a washed hydroxide ppt. Add

15 drops of 3 M HNO3 to this point and mix.

7. To a few drops of the original test solution, add 3 drops of the Aluminon

reagent (0.1% Aluminon). Then, add pH 7 buffer solution dropwise and

observe.

8. To a few drops of test solution, add dimethylglyoxime (DMG) reagent (1%

dimethylglyoxime in 95% EtOH) and observe.

9. Repeat any tests you are uncertain of.

TestInitial pH

3M NH3

I -(acidic)

3-[Fe(CN) ]6(acidic)

6M NaOH

3conc. NHin

AluminonTest

DMGTest

Zn2+ Pb2+ Ni2+ Al3+ Co2+

3M HNO3

OH ppt.-

OH ppt.-in

B. Designing a cation analysis scheme for Zn2+, Pb2+, Ni2+, Al3+, and Co2+

Based upon the results of general procedure tests, devise an analysis scheme for a

solution containing all five ions. (This scheme will be used to analyze your unknown

solution which may contain one or more - up to five - of the cations you have already

tested). As demonstrated in the sample cation analysis, the main idea is to separate the

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cations - in the precipitate or the supernatant - and confirm their presence with a specific

test reagent reaction or by process of elimination.

The following questions should be helpful if you are having problems formulating a

flowchart:

Given a solution of Pb(NO3)2, how would you decide whether Pb2+ is present?

Given a solution containing only Zn2+ and Al3+, how you would confirm the presence

of each?

Given a solution containing only Ni2+ and Co2+, how would you confirm the presence

of each?

When applying your scheme, keep in mind that:

You must decide how much test solution and reagent to use. Try about 5 mL of

original unknown solution, about 2.5 ml of the initial acid or base precipitation,

and a few drops of reagent.

You will want the precipitation reactions to be complete (think about why.) Thus

after centrifuging, add another drop or two of reagent to see if any more ppt.

forms. Mix, re-centrifuge, and repeat until no new ppt. forms.

The pH must be checked when specified. If the solution is acidic and the

subsequent step requires a basic environment, you will need to first neutralize the

solution and then add excess base. Since the solution volume may double in this

case, do this step in a small beaker or flask and under the fume hood. Note that

the solution may become hot. Do not dilute the cation solution more than

necessary (think about why.)

WRITE-UP:

Your write-up will simply consist of the Reaction Chart and the Cation Analysis Scheme.

Include also a short discussion about the efficiency of the method. Were all the tests

conclusive?

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Documents

E2 · Web viewPrepare 250 mL of a .01 M solution of KHP (primary) and 250 mL of a .01 M solution of NaOH (non-primary). The weight of KHP should be recorded with care as it will determine