Chapter 7

 

 

 

 

 

PRECIPITATION  TITRATIONS

 

                 In previous sections, different titrimetric procedures that take place in solution were discussed.  A special type of titrimetric procedures involves the formation of precipitates during the course of a titration.  The titrant react with the analyte forming an insoluble material and the titration continues till the very last amount of analyte is consumed.  The first drop of titrant in excess will react with an indicator resulting in a color change and announcing the termination of the titration.

 

Argentometric Titrations

 

                 The most widely applicable precipitation titrations involve the use of silver nitrate with chlorides, bromides, iodides, and thiocyanate.  Since silver is always there, precipitation titrations are referred to as Argentometric titrations.  This implies that this type of titration is relatively limited.

 

      According to end point detection method, three main procedures are widely used depending on the type of application.  These are:

 

 

 

a.  Mohr Method

 

                 This method utilizes chromate as an indicator.  Chromate forms a precipilate with Ag+ but this precipitate has a greater solubility than that of AgCl, for example.  Therefore, AgCl is formed first and after all Cl- is consumed, the first drop of Ag+  in excess will react with the chromate indicator giving a reddish precipitate.

 

2 Ag+  +  CrO42-  =    Ag2CrO4

 

                 In this method, neutral medium should be used since, in alkaline solutions, silver will react with the hydroxide  ions forming AgOH.  In acidic solutions, chromate will be converted to dichromate.  Therefore, the pH of solution should be kept at about 7.  There is always some error in this method because a dilule chromate solution is used due to the intense color of the indicator.  This will require additional amount of Ag+ for the Ag2 CrO4 to form.

 

b.  Volhard Method

 

                 This is an indirect method for chloride determination where an excess amount of standard Ag+ is added to the chloride solution containing Fe3+ as an indicator.  The excess Ag+ is then titrated with standard SCN- solution untill a red color is obtained which results from the reaction:

 

Fe3+ + SCN-  =   Fe(SCN)2+

 

      The indicator system is very sensitive and usually good results are obtained.  The medium should be acidic to avoid the formation of Fe(OH)3 .  However, the use of acidic medium together with added SCN- titrant increase the solubility of the precipitate leading to significant errors.  This problem had been overcome by two main procedures:

 

      The first includes  addition of some nitrobenzene, which surrounds the precipitate and shields it from the aqueous medium.  The second procedure involves filtration of the precipitate directly after precipitation, which protects the precipitate from coming in contact with the added SCN- solution.

 

C.  Fajans Method

 

                 Fluorescein and its derivatives are adsorbed to the surface of colloidal AgCl.  After all chloride is used, the first drop of Ag+ will react with fluorescein (FI-) forming a reddish color.

 

Ag+  +  FI-=   AgF

 

                 Since fluorescein and its derivatives are weak acids, the pH of the solution should be slightly alkaline to keep the indicator in the anion form but, at the same time, is not alkaline enough to convert Ag+ into AgOH .  Fluorescein derivatives that are stronger acids than fluorescien (like eosin) can be used at acidic pH without problems.  This method is simple and results obtained are repoducible.

 

 

 

Applications

 

 

Experiment  17.  Determination of Chloride by Precipitaiton Titration

 

Background

 

                 As mentioned in the text, chloride reacts with silver ion to form a precipitate.  The end point can be detected by one of three methods, namely Mohr, Volhard or Fajans.  This experiment is devised for the application of these detection methods.

 

Reactions

 

Ag+  +  Cl-   =    AgCl      (s)

Ag+  +  SCN-  =   Ag SCN  (s)

 

Reagents and Chemicals

 

a.  Provided

 

      1.  Chloride solution of unknown concentration.

      2.  Predried and desiccated AgNO3 .  be careful !!

      Do not allow this substance either in the solid or solution to come in contact with your skin.  Black stains that will persist for almost two weeks will be formed.

      3.  0.1% Dichlorofluorescein in 75% (v/v) ethanol.

      4.  5% Dextrin solution.

      5.  5 M HNO3 solution.

      6.  Potassium chromate indicator.

      7.  Predried and desiccated KSCN.

      8.  Nitrobenzene.

      9.  Ferric alum indicator.

 

b.  Need Prepartion

 

      1.  Standard AgNO3 that is approximately 0.05M.  (250 mL).  Prepare by dissolving a precalculated amount in 250 mL of distilled water.

      2.  Standard KSCN solution that is approximately 0.05 M (250 mL).  Prepare by dissolving a suitable amount in 250 mL of distilled water and standardize against standard AgNO3 solution using ferric alum indicator.

 

Procedure

 

a.  Using Mohr Method

 

1.  Pipet exactly 25 mL of the chloride unknown into a 250 mL conical flask.

2.  Add 50 mL of distilled water and 1 mL of K2CrO4 indicator solution.

3.  Titrate against standard AgNO3 solution and record the volume to two significant figures after the decimal point.

4.  Repeat steps 1-3 two more times and calculate the concentration of chloride in the sample as ppm NaCl.

 

b.  Using Volhard Method

 

1.  Transfer exactly 25 mL of the chloride sample into a 250 mL conical flask.

2.  Add 50 mL of distilled water, 10 mL of the HNO3 solution provided, and exactly 40 mL of standard AgNO3 solution.

3.  Add 5 mL of nitrobenzene and shake vigorously.  (you may need to seal the flask with parafilm).

4.  Add 4 mL of the ferric alm indicator and titrate against standard KSCN solution and record the volume to two significant figures after the decimal point.

5.  Repeat steps 1-4 two more times and calculate the concentration of chloride as ppm NaCl.

 

c.  Using Fajans Method

 

1.  Pipet exactly 25 mL of the chloride unknown sample into a 250 mL conical flask.

2.  Add 5 mL of dextrin solution followed by 8 drops of 0.1% dichlorofluorescein indicator.

3.  Titrate against standard AgNO3 solution till a permanent change in the color of indicator is observed.  Record the volume of AgNO3 solution consumed.

4.  Repeat steps 1-3 two more times and report your results as ppm NaCl.

 

Report of Results

 

a.  Mohr Method

 

Vol of Chloride

Vol of AgNO3

ppm NaCl

 

 

 

 

 

 

 

 

 

X =

 

s =

 

 

 

 

 

 

 

 

b.  Volhard Method

 

Vol of AgNO3

Vol of KSCN

Vol AgNO3 Eq to Chloride

ppm NaCl

 

 

 

 

 

 

 

 

 

 

 

 

X =

 

s =

 

 

c.  Fajnans Method

 

Vol of Chloride

Vol of AgNO3

ppm NaCl

 

 

 

 

 

 

 

 

 

X =

 

s =

 

 

 

Remarks and Observations

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Experiment  18.   Determination of Sulfate by Precipitation Titration

 

Background

 

                 Ba2+ reacts with sulfate to form insoluble BaSO4 precipitate.  In the presence of alizarin red S indicator, the first excess of Ba2+ react with the indicator yeilding a pink complex.  The titration is fast, reliable, and offer results comparable to those obtained by gravimetry.

 

Reactions

 

Ba2+  +  SO42- =  BaSO4  (s)

 

Reagents and Chemicals

 

a.  Provided

 

      1.  Predried and desiccated sulfate sample.

      2.  Predried and desiccated BaCl2.2H2O.

      3.  5% HCl solution (V/V).

      4.  Methanol.

      5.  0.2% Aqueous solution of alizarin red S.

 

b.  Need Preparation

 

      1.  250 mL of the sulfate unknown.  Prepare by accurately weighing about 1.000 g of the sample.  Transfer to 250 mL measuring flask, dissolve in distilled water, and complete to the mark.

      2.  250 mL of the BaCl2. 2H2O standard solution that is about 0.05 M.  Appropriate amount of BaCl2 . 2H2O should be dissolved in some distilled water, the pH is adjusted to 3-3.5 by dropwise addition of HCl, and the volume is completed to the mark.

 

Procedure

 

1.  Transfer exactly 25 mL of the sulfate unknown into a 250 mL conical flask.

2.  Add approximately 25 mL of H2O and 40 mL of methanol followed by 2 drops of the indicator.

3.  Add HCl solution provided dropwise till the indicator attains the yellow color.

4.  Perform a trial titration with rapid addition of standard barium chloride and find the approximate volume of BaCl2  .

5.  Repeat steps  1-3 and titrate against standard BaCl2 till about 90% of BaCl2 necessary is added then add 3 more drops of the indicator.

6.  Continue titration with dropwise addition of BaCl2 and vigorous swirling.  Allow about 5 seconds between addition of each increment until a pale pink color is achieved.

7.  Record the volume and repeat steps  1-6 (excluding step step 4) two more times.

8.  Report your results as %S in the samplle.

 

 

Report of Results

 

Vol of Sulfate

Vol of BaCl2

Wt of S in 25 mL

 

 

 

 

 

 

 

 

 

X =

 

s =

 

 

% S in the sample  =

 

Remarks and Observations