DETERMINATION OF CHLORIDE

8/7/2019 DETERMINATION OF CHLORIDE

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EXPERIMENT III

DETERMINATION OF CHLORIDE:

GRAVIMETRIC AND VOLUMETRIC METHODS

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INTRODUCTION

For any given analyte, numerous analytical methods are available in the literature, each

having unique advantages and disadvantages. The two most common methods for thedetermination of chloride ion are based on its quantitative reaction with silver ion to form

silver chloride. One method involves the isolation of the silver chloride precipitate by

filtration, determining its mass, and using stoichiometry to calculate the percent chloridein the sample. This is termed Gravimetric Analysis. Alternately, the chloride ion may be

titrated with a standard solution of silver ion and the volume of titrant used to calculate

the percent chloride in the sample. This Volumetric Analysis is often described as aprecipitate-forming titration.

In this experiment you will obtain an unknown sample, containing chloride ion, and youwill carry out both gravimetric and volumetric procedures using the same sample. In a

perfect world, bothe sets of data should result in identical values for the percent of

chloride in the sample as both methods are tried and true, and both have similar

precision and accuracy (2-3 ppt). In reality, your results for the two determinations willlikely differ. Your challenge is to determine if the difference is significant, and, most

importantly, you must decide what value to report as the best estimate of the true

percentage of chloride in your sample. You should use a blend of common sense, goodlab technique, statistical tests, and, perhaps, further replication to enhance confidence or

help to identify outliers.

The theory and practice of the Gravimetric Determination of Chloride is described in

Part I of this experiment and the Volumetric Determination of Chloride is presented in

Part II.

PART I: THE GRAVIMETRIC DETERMINATION OF CHLORIDE

Gravimetric methods are among the most accurate and precise methods of quantitativeanalysis. These advantages are counterbalanced by the fact that they are often time

consuming, require considerable attention to detail and are limited to sample size and

concentrations which yield a weighable quantity on a conventional analytical balance.

While many gravimetric methods have been at least partially replaced by newer, faster

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techniques, there remain several situations where the only suitable analytical techniques

are gravimetric. For this reason you should become experienced and competent with

gravimetric techniques, such as the gravimetric determination of chloride.

THEORY

Addition of a solution containing silver ion [usually an aqueous silver nitrate (AgNO 3)solution] to a dissolved sample containing chloride ion will quantitatively precipitate thechloride ion as solid silver chloride (AgCl), due to the small solubility product constant

of silver chloride (ksp = 1.82 x 10-10). The precipitate may be isolated from solution by

filtration, the material dried, weighed and the percent chloride (% Cl-) calculated from thestoichiometry of the reaction:

(aq)Cl+(aq)Ag-+ AgCl(s)

In addition to possible interferences common to all gravimetric methods, the presence of

any other halide will cause high results since they produce a silver halide which is evenmore insoluble than silver chloride. Also, silver chloride is light sensitive and excessive

photodecomposition will produce erroneous results, according to the reaction:

AgCl(s) hv ( gC l+A g ( s ) 221

The precipitate becomes violet-purple, due to the presence of finely divided silver metal,and results will be low. If silver ion (Ag+) is present, in addition to the above, the

following reaction will also occur as a result of the photodecomposition reaction:

(aq)6H+(aq)ClO+5AgCl(s)(aq)5Ag+O(l)3H+(g)3Cl-3

hv

22

++

If this reaction predominates, the results will be high. To minimize such errors, it is

recommended that unnecessary exposure to light be avoided.

PROCEDURE

Preparation of the Sample for Analysis

Obtain approximately 5 g of an unknown sample from your instructor in a clean, dry,and properly labeled weighing bottle. Dry the unknown material in the microwave oven

using medium power for two minutes. Weigh accurately, after cooling in a desiccator for

approximately 30 minutes, triplicate samples of approximately 0.5 g into numbered 400-mL beakers. Dissolve in deionized water and dilute each to approximately 150 mL

volume. Add approximately 0.5 mL of reagent-grade concentrated nitric acid (HNO3).

Preparation of Crucibles

Clean and suitability markthree sintered glass crucibles ofmedium(M) porosity. Yourinstructor will outline for you the best method of cleaning the crucibles, as the method of

cleaning is dependent upon their past history. Then bring the crucibles to constant weight

in the microwave oven using medium power. The first heating should be at least 30

seconds; let the crucible cool, heat for a further 30 seconds, cool, then finally heat for 4

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minutes. Re-heat for 2-minute periods until constant weight is obtained. The weight

should become constant to within 0.3 mg.

Preparation of Precipitating Reagent

An aqueous solution of 0.5 M silver nitrate (AgNO3) is available in the laboratory.

Assuming the unknown to be pure sodium chloride, calculate the volume of the silver

nitrate solution required to just precipitate all chloride ion in each sample. Include 10%excess in the final volume to compensate for the fact that the calculation is based on an

assumption that could be incorrect.

Precipitation

Add the calculated amount of silver nitrate solution slowly to each sample, preferablyfrom a buret, with adequate stirring, (remember each sample will require a different

volume of silver nitrate solution as each sample is of a different weight). Cover each

solution with a watch glass (remember to leave the stirring rod in the beaker), heat eachsolution to nearly boiling (hot plate) to coagulate the silver chloride precipitate. Let each

solution cool and settle, then test for complete precipitation as demonstrated by the

instructor. Add more precipitating reagent, if necessary, and re-check for complete precipitation. Allow each solution to digest (in the dark) at least 1-2 hours, or, if

necessary overnight.

Filtration

Filter each solution through the corresponding sintered glass crucible, adhering to the

guidelines for proper filtration, presented by the instructor. Be certain to "police" each

beaker to assure quantitative transfer. Wash each precipitate with a solution of 1:500nitric acid/water (nitric acid is added to prevent peptization) until the washings give a

negative test for Ag+. After you have a negative test for Ag+, place the sintered glasscrucible in the filtration apparatus and pull a vacuum on it for several minutes. It is

important that you get the filtrate as dry as possible before drying in an oven.

NOTE: Washings may be tested for their silver ion content by using the following

procedure. After you have filtered your precipitate and washed with an initial amount of

wash solution, carefully remove the filter crucible and touch the base to the surface of aclean, dry watch glass. Several drops of the residual wash will transfer to the watch

glass. If this does not occur, add a small amount (1 or 2 mL) of wash solution to the filter

crucible and wait for gravity to cause the transfer of some of the wash solution. Add 2-3drops of 12 M HCl to the wash solution on the watch glass. The formation of a cloudywhite solution indicates the presence of Ag+ in the wash solution. If this occurs,

reassemble and wash with ~10 mL of wash solution and retest. Repeat until a negative

test for Ag+ is obtained.

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Drying the Crucibles

Place the crucibles in a beaker covered with a ribbed watch glass (as with the emptycrucibles). Dry in the microwave oven atmedium power for 30 seconds; let the crucible

cool, heat for a further 30 seconds and cool, then finally heat for 4 minutes, cool in adesiccator and weigh. Reheat for 2-minute periods until constant weight is obtained.

Cleaning the Crucibles

After completion of the analysis, remove carefully, without damaging the sinter, the cakeof silver chloride from the crucible(s) and place the cake in the silver waste bottle. Place

the crucibles in a beaker and fill each crucible with concentrated aqueous ammonia

solution (Do this in a hood). Allow the crucibles to sit until all the silver chloride hasbeen dissolved by the ammonia. Rinse each crucible with several portions of deionized

water and dry in a microwave oven at medium power for 4 minutes or until dry.

CALCULATION OF RESULTS

Calculate your results as the % Cl- in your sample. Be certain to report each individualvalue, the mean value, the absolute deviation of each value from the mean, the relative

average deviation in parts per thousand, the standard deviation and the confidence

interval at an appropriate confidence level. A relative average deviation of > 5 ppt (5parts per thousand) should be considered unsatisfactory.

PART II: THE VOLUMETRIC DETERMINATION OF CHLORIDE

Precipitate-forming titrations are not common. However, they are widely used for

determination of the halides, chloride (Cl-), bromide (Br-) and iodide (I-), using silvernitrate (AgNO3) as the titrant. In this part of the experiment, chloride will be determined

by titration with silver nitrate using dichlorofluorescein as an indicator. This is

commonly known as Fajan's Method.

THEORY

Chloride present in a sample is quantitatively insoluble in a solution containing excess of

silver ion:

Ag+(aq) + Cl- (aq) AgCl(s) Ksp = 1.82 x 10

-10

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This reaction is the basis of a gravimetric procedure described in Part I of this

experiment. The same determination may be accomplished volumetrically if a standard

solution of Ag+ is available. The end point of the reaction may be determined through theuse of an adsorption indicator, dichlorofluorescein. Its function may be described as

follows:

If the reaction is run in neutral or basic solution some of the indicator will dissolve to

form the dichlorofluorescinate anion, which is represented as In-. Before the equivalence

point, with Ag+ as titrant, excess Cl- is present in solution. The excess Cl- is adsorbedonto the precipitate particles formed and the indicator anion is repelled by the negatively-

charged precipitate.

Ag+(aq) + 2Cl-(aq) AgCl:Cl-(s)

At the equivalence point, there is little or no excess Cl -, and just beyond the equivalence

point Ag

+

is in excess and becomes the primary adsorbed ion. The charge on theprecipitate changes from negative to positive and the indicator anion is adsorbed.

AgCl:Cl-(s) + Ag+(aq) AgCl:Ag+(s) + Cl-(aq)

AgCl:Ag+(s) + In-(aq) AgCl:Ag+In-(s)

yellow rose-pink

The color change is:

yellow rose-pink

It is believed that the indicator anion (yellow) forms a complex ion with Ag+, adsorbed on

the silver chloride precipitate, which alters its light-absorbing properties, and hence itscolor. The indicator function is critically dependent on the availability of a large

precipitate surface area to allow adsorption. The greatest surface area results from a

precipitate comprised of very small particles (colloidal). Stabilization of these colloidalparticles (recall that a colloid has a very high surface-to-volume ratio) is accomplished by

adding a protective colloid, such as dextrin.

It is important that the titration be conducted quickly in diffuse light as

photodecomposition of the silver chloride renders the solution purple, making it difficult

to discern the pale pink end point signal from the purple background.

PROCEDURE

Preparation of a Standard Silver Nitrate Solution

Obtain from the instructor approximately 8.5 g of AgNO3 in a clean, dry, weighingbottle. Grind the contents to a fine powder with an agate mortar and pestle. Return the

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AgNO3 to the weighing bottle, dry in a microwave oven at medium power for 2 minutes,

and accurately weigh, by difference, the contents into a 500-mL volumetric flask.

Dissolve with deionized water and dilute to the mark. Store in a dark area and calculatethe molarity of the solution.

Preparation of the Sample for Analysis

Using the unknown chloride sample from part 1, accurately weigh three samples of the

unknown into three 250-mL Erlenmeyer flasks (the sample size should be approximately0.3 g). Add approximately 50 mL of deionized water to each flask and swirl to dissolve.

Add 10 mL of a 2% dextrin suspension, 5 drops of dichlorofluorescein indicator, and

titrate each sample to the rose-pink end point.

CALCULATION OF RESULTS

Calculate your results as the % Cl- in your sample. Be certain to report each individual

value, the mean value, the absolute deviation from the mean of each value, the relative

average deviation in parts per thousand, the standard deviation and the confidence

interval at an appropriate confidence level.

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DETERMINATION OF CHLORIDE