Define Negative Correlation

 

1- Mohr's Method :-

This method determines the chloride ion concentration of a solution by titration with silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver chloride forms.

    Ag+(aq) + Cl–(aq) → AgCl(s)

The end point of the titration occurs when all the chloride ions are precipitated.

Principle of Mohr's method:-

The principle of precipitation titration is- Amount of added precipitating agent = the amount of a compound that is precipitated.

AgNO3 solution is used to titrate the chloride. As an indicator, a soluble chromate salt (K2CrO4) is used. This results in a yellow solution, when the chloride precipitation is complete.

NaCl + AgNo3 ⇆ AgCl + NaNo3

After all, chlorides have precipitated as silver chloride, the first excess of Ag+ reacts with the indicator to form red silver chromate as a second precipitate.

2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s)

Mohr’s method is a direct method of titration in which a red precipitate of silver chromate is formed at the endpoint. This method is needed to be performed in the neutral to alkaline condition (pH of 08.00).

Procedure of Mohr’s method:-

Requirements: 

Burettes, conical flask, pipette, analytical/digital balance, 0.1 M silver nitrate solution, potassium chromate solution, and distilled water, etc.

• Fill the burette with a prepared solution of 0.05 N AgNO3.
• Pipette out 10.00 ml of sodium chloride solution (NaCl) and pour in a stoppered conical flask.
• Add 01.00 ml of 2% neutral potassium chromate indicator.
• By shaking the liquid continually, titrate it with 0.05 N silver nitrate solutions, until the red color formed by each drop of AgNO3 solution begins to fade more slowly; this indicates that the majority of the chloride has precipitated and the endpoint is approaching.
• Drop by drop; titrate until a faint but distinct, brick red color appears that does not disappear with vigorous shaking. The endpoint of this titration is a brick red color.

2- Volhard's Method :-

The volhard method is an indirect or back titration method in which an excess of a standard solution of silver nitrate is added to a chloride containing sample solution. 

The excess silver is then back titrated using a standardized solution of potassium or ammonium thiocyanate with ferric ion as an indicator.

The reaction involved in Volhard method:-

If analyte contains chloride anions, the reaction will be as follows.

To titrate Ag+; determination of Cl- involves a back titration

First, Cl- is precipitated by excess AgNO3

Ag⁺ (aq) + Cl^- (aq)     AgCl(s)

In the presence of Fe3+, excess Ag+ is titrated with KSCN

Ag⁺ (aq) + SCN^- (aq)  AgSCN(s)

When Ag+ has been consumed, a red complex forms

Fe3⁺ (aq) + SCN^- (aq)   [FeSCN]²⁺(aq)

Procedure of Volhard titration method:-

• Procedure for determination of chloride ion concentration by titration (Volhard’s Method)
• Fill the burette with 0.1 M potassium thiocyanate solution and adjust the zero.
• Measure 100 ml of the sample and pour it into a conical flask.
• Add 01.00 ml of ferric ammonium sulphate solution as an indicator.
• Using the potassium thiocyanate (KSCN) solution, titrate the unreacted silver ions.
• Because of the ferric thiocyanate complex, the endpoint is the appearance of a dark red color.
• Repeat the titration three times to get an accurate result.
• Calculate the moles of potassium thiocyanate used.