Determination of chloride by Volhard

method

Done by: Samyah Alanazi

Introduction• Precipitation titration s a perfect method for

determine halogens and some metal ions. It involves formation of slightly soluble precipitate at end point. Argentometry, type of Precipitation titration, is derived form a latin word Argentum , which means Sliver. So, Argentometry is a type of titration involving the silver ion

• Methods of Argentometric titrations :• It involves precipitation titrimetry:• A- Mohr’s Method.• B- Fajan’s method.• C-Volhard’s method.

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+  +  CrO4

2-  =    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 dilulet 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.

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.

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:

•  

Cont. • 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

• The Volhard’s method was first described by the Jacob Volhard , a German Chemist , in 1874.

• It is a back titration procedure, used for determining the anions that precipitate with silver.

Advantages• 1- It is useful where the titration have to be

performed at low pH.

• 2- The strong acidic environment give advantage for halide analysis because ions like carbonate ,oxalate and arsenate do not interfere.

• 3- Give accurate result due to back titration.

limitations

• Can not be used where solution has to be neutral.• Time consuming.

• Sometimes adsorption of Ag+ may give false end point.

Principle

• It depends upon the precipitation of Cl- as AgCl by the addition of a known excess of standard AgNo3 THEN the remaining unreacted AgNo3 ia back titrated against standard KSCN using Ferric Ammonium Sulphate as indicator.

Equations

Ag+ + Cl‾ AgCl + Ag+ (excess) (white ppt)

Ag+ (excess) + SCN ‾ AgSCN (back titration)

Fe+ ++ + SCN ‾ FeSCN ++ (brown color) end point

ProcedureStandardization of Potassium Thiocyanate (KSCN) :20 ml AgNO380 ml distilled water, mix well2 ml con.HNO3, mix well 2 ml Ferric ammonium Sulphate, mix wellTitrate with Potassium thiocyanate solution until get light brown color (end point).Calculate the normality of KSCN.

Titration of unknown solution of chloride: 10 ml unknown solution of chloride 2 ml HNO3 ??, mix well 20 ml AgNO3, mix well (note the white ppt) Shake well for few minutes to coagulate the precipitated AgCl. 40 ml D.W, mix well ?? Filter through a dry filter paper, rejecting the first few millimeters of

the filtrate. ?? Transfer 60 of the filtrate into a clean glass stoppered flask. add 2 ml Ferric ammonium sulphate indicator, mix well Titrate with Potassium thiocyanate until get the light brown color. Calculate the concentration of unknown chloride.

Facts

1- Concentrated Nitric Acid is added because the titration must be done in acidic medium to prevent the precipitate of Iron(III) as hydrated oxide (iron hydroxide) A- In alkaline medium, Ag+ will be ppted as AgOH and Fe OH3.

B- HNO3 prevent ppted of other Ag3 salts like carbonate and sulfide because these salts are soluble in HNO3.

C- HNO3 helps the coagulation of AgCl ppt and that facilitate its filtration or coating.

2- To dilute sample.

3- Silver thiocyanate solubility is slightly lower than solubility of silver chloride, and during titration thiocyanate can replace chlorides in the existing precipitate:• AgCl(s) + SCN- → AgSCN(s) + Cl-.

Solutions• To avoid problems we can filtrate precipitated

AgCl before titration. However, there exist much simpler and easier procedure that gives the same result. Before titration we add some small volume of a heavy organic liquid that is not miscible with water (like nitrobenzene, chloroform or carbon tetrachloride). These liquids are better at wetting precipitate than water. Once the precipitate is covered with non polar liquid, it is separated from the water and unable to dissolve or to prevent this error the precipitate must be filtered off before titration with SCN- .

• Precipitate solubility is not a problem during determination of I- and Br-, as both AgBr and AgI have much lower solubilities than AgSCN.

Calculations

1- Calculate the concentration of KSCN:

Ex: If volume of KSCN = 23

 

M1 x V1 = M2 x V2

MAg+ x VAg+ = MSCN‾ x VSCN ‾

0.1 x 20 = M SCN‾ x 23

M SCN‾ = 0.087 M

2- Calculate the concentration of chloride :

Ex: If volume of KSCN = 0,2 ml

Total m moles of Ag+ = M x V

T = 0.1 x 20 = 2 m moles Un reacted m moles of Ag+ (m moles of KSCN used) = MKSCN

x VKSCN

U = 0.087 x 0.2 = 0.017 m moles Reacted m moles of Ag (m moles of Chloride) = T – U

= 2 - 0.017= 1.983

= 1.983 m moles / 10 ml

= 0.1983 m moles / ml