How To Estimate Sodium Chloride Using Mohr Method Principle Formula And Calculation
Analytical chemistry is the branch of chemistry that deals with the identification and analysis of the quantity or quality of chemicals or elements in a sample under consideration. Argentometry is a quantitative analytical method. It is a titration involving silver (I) ions. Argentometry is mainly used for the estimation of chloride ions in a sample. Because chloride ions can easily react with a silver(I) and form silver chloride. Silver chloride is insoluble and remains a white colour precipitate. Volhard method is a method in argentometry that helps in the estimation of sodium chloride in a sample.
Volhard Process
The Volhard process is a method used in argentometry for the estimation of chloride ion in a sample. The Volhard process is named after the scientist Jacob Volhard and is an example of back titration. In this method, excess silver nitrate is added to the sample containing chloride ions. Silver nitrate and sodium chloride are inorganic compounds. The precipitated silver chloride is filtered off then the remaining sodium nitrate is titrated against ammonium thiocyanate. Ferric ammonium sulphate is added as an indicator. Endpoint is the appearance of blood red colour due to the formation of a complex.
Theory of Volhard Method
Volhard's method is famous for the estimation of sodium chloride in a sample. Excess silver nitrate is added to the sample which reacts with the chloride ion of sodium chloride. The reaction of sodium chloride and silver nitrate is an example of a precipitation reaction where a white coloured silver chloride is precipitated.
The reaction of sodium chloride and silver nitrate is as follows
NaCl + AgNO3 →AgCl + NaNO3
The precipitated AgCl is filtered off. The reaction is carried out under acidic conditions. Addition of AgNO3 should be followed by addition of concentrated HNO3. Excess AgNO3 is added to ensure complete precipitation of AgCl.
A concentrated solution of KMnO4 is added to oxidize any organic matter that is not disposed of by HNO3. Following boiling cooling and dilution the excess AgNO3 is back-titrated against KSCN. And add ferric ammonium sulphate as an indicator. Ferric ammonium sulphate reacts with excess thiocyanate and forms a red blood-coloured complex.
SCN- can react with AgCl ppt. To prevent this diethyl ether or nitrobenzene is added to the solution.
Applicability
This method is useful for the estimation of salt-processed meat products at a level ≥ 0.08%.
Procedure of Volhard Method
Excess silver nitrate followed by concentrated HNO3 is added to the analyte. The precipitated AgCl is filtered off. Concentrated KMnO4 is added to oxidize the organic component which could not disposed of by con. HNO3. The mixture is boiled, cooled and diluted. It is then titrated against potassium thiocyanate. Ferric ammonium sulfate is added as the indicator. Nitrobenzene or diethyl ether is added to prevent the reaction of AgCl remaining in the solution with SCN-. The endpoint is determined by the appearance of red blood colour.
Result Calculation In Volhard Method
Volhard method is the back titration for the estimation of chloride ions. Here excess standard sodium nitrate solution is added to the sample. The excess silver is then back titrated against potassium thiocyanate. The amount of silver that is precipitated with chloride ions is calculated by subtracting the excess silver from the original silver content.
Ag+ + Cl– → AgCl
Ag+ + SCN– → AgSCN
SCN– + Fe3+ → [FeSCN]2+
Assay of Sodium Chloride
Assay in chemistry means the process of determining the composition or quality of a substance. For the assay of sodium chloride Volhard, the process is used.
The Volhard method of assay of sodium chloride is described below Weigh accurately 0.1 g of sample and dissolve it in 50 mL water. To this add 50 mL of 0.1 M silver nitrate, 5mL of 2M nitric acid and 2mL of concentrated KMnO4 . Shake well and titrate with 0.1 M ammonium thiocyanate taken in the burette. 2 ml of ferric ammonium sulphate solution is added as an indicator. Titrate until the colour become reddish brown.
Mohr method is also used for the assay of sodium chloride.
Interesting Facts
The reaction could not be carried out in a medium containing HCl. Because chloride ions will volatilize to HCl. The vapor pressure of HCl is higher than HNO3.
Geoforry in 1729 gives the first true description of a titration.
Key Features
An assay of sodium chloride is done by the Volhard titration method
Volhard titration is a back titration with excess silver nitrate against thiocyanate
Endpoint is indicated by a red blood colour by adding ferrous ammonium sulfate indicator
The amount of silver precipitated is calculated by subtracting the amount of excess silver from the original amount of silver.
FAQs on Estimation Of Sodium Chloride By Argentometric Titration
1. What is the estimation of sodium chloride in chemistry?
The estimation of sodium chloride is the quantitative determination of the amount or concentration of NaCl in a sample, usually by titration with a standard silver nitrate solution. It is most commonly carried out using the Mohr’s method, where chloride ions react with silver ions to form a white precipitate of AgCl.
Key reaction involved:
• AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
This method is widely used in analytical chemistry to determine chloride content in salt, food samples, and water.
2. What is Mohr’s method for estimation of sodium chloride?
The Mohr’s method is a precipitation titration used to estimate sodium chloride using standard AgNO3 and potassium chromate as an indicator. In this method, chloride ions are titrated with silver nitrate until a reddish-brown precipitate appears.
Steps involved:
- Chloride ions react with silver ions to form white AgCl(s).
- After all Cl- ions are consumed, excess Ag+ reacts with chromate ions to form reddish-brown Ag2CrO4(s).
• 2AgNO3(aq) + K2CrO4(aq) → Ag2CrO4(s) + 2KNO3(aq)
The appearance of reddish-brown color indicates the end point.
3. What is the principle behind the estimation of sodium chloride?
The principle of sodium chloride estimation is based on a precipitation reaction between chloride ions and silver ions forming insoluble silver chloride. The reaction follows the stoichiometric ratio of 1:1 between Ag+ and Cl-.
Main reaction:
• Ag+(aq) + Cl-(aq) → AgCl(s)
Because one mole of AgNO3 reacts with one mole of NaCl, the concentration of NaCl can be calculated using the titration data and mole concept.
4. What is the chemical reaction involved in the estimation of sodium chloride?
The main chemical reaction in the estimation of sodium chloride is the formation of insoluble silver chloride (AgCl) from silver nitrate and sodium chloride. This is a precipitation reaction.
Balanced equation:
• AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Net ionic equation:
• Ag+(aq) + Cl-(aq) → AgCl(s)
This reaction is the basis for calculating the amount of NaCl present in the sample.
5. Why is potassium chromate used as an indicator in Mohr’s method?
Potassium chromate is used as an indicator because it forms a reddish-brown precipitate of Ag2CrO4 only after all chloride ions have reacted. It helps in detecting the end point of the titration.
Sequence of reactions:
- First, Ag+ reacts with Cl- to form white AgCl(s).
- After complete precipitation of Cl-, excess Ag+ reacts with CrO42-.
• 2Ag+(aq) + CrO42-(aq) → Ag2CrO4(s)
The first permanent reddish-brown color indicates the end point.
6. How do you calculate the concentration of sodium chloride from titration data?
The concentration of sodium chloride is calculated using the formula M1V1 = M2V2 based on the 1:1 stoichiometry between AgNO3 and NaCl. Since one mole of AgNO3 reacts with one mole of NaCl, their moles are equal at equivalence point.
Steps:
- Calculate moles of AgNO3 used = M × V (in liters).
- Moles of NaCl = moles of AgNO3.
- Find molarity of NaCl = moles / volume of NaCl solution.
7. What are the conditions required for accurate estimation of sodium chloride by Mohr’s method?
The estimation of sodium chloride by Mohr’s method must be carried out in a neutral or slightly alkaline medium for accurate results. Acidic or highly alkaline conditions interfere with the indicator and precipitate formation.
Important conditions:
- pH should be around 7–10.
- Solution should be free from other halide ions like Br- or I-.
- Indicator (K2CrO4) should be added in small amount.
8. What is the difference between Mohr’s method and Volhard’s method for chloride estimation?
The main difference is that Mohr’s method is a direct titration in neutral medium, while Volhard’s method is a back titration performed in acidic medium. Both are used for chloride estimation but differ in procedure and indicator.
Key differences:
- Mohr’s method uses K2CrO4 indicator.
- Volhard’s method uses Fe3+ as indicator.
- Mohr’s method forms AgCl directly.
- Volhard’s method involves excess AgNO3 and back titration with KSCN.
9. What is the molar mass of sodium chloride used in calculations?
The molar mass of sodium chloride (NaCl) is 58.5 g mol-1. It is calculated by adding the atomic masses of sodium and chlorine.
Calculation:
- Na = 23.0 g mol-1
- Cl = 35.5 g mol-1
- Total = 23.0 + 35.5 = 58.5 g mol-1
10. What are common sources of error in the estimation of sodium chloride?
Common errors in sodium chloride estimation arise from incorrect end point detection, improper pH, or presence of interfering ions. These factors affect the accuracy of precipitation titration.
Main sources of error:
- Adding excess indicator (K2CrO4).
- Performing titration in acidic medium.
- Presence of other halides like Br- or I-.
- Incomplete mixing or loss of precipitate.
