Precipitation Titration
Titration is a process by which the concentration of an unknown substance in solution is determined by adding measured amounts of a standard solution that reacts with the unknown. Then the concentration of the unknown can be calculated using the stoichiometry of the reaction and the number of moles of standard solution needed to reach the so called end point.
Precipitation titration is a type of titration which involves the formation of precipitate during the titration technique. In precipitation titration, the titrants react with analyte and forms an insoluble substance referred to as precipitate. It continues till the last amount of analyte is consumed. When the titrant is excess, it reacts with the indicator and signals to terminate the titration process. It is used to determine chloride by using silver ions.
Precipitation titrations are based upon reactions that yield ionic compounds of limited solubility. The most important precipitating reagent is silver nitrate. Titrimetric methods based upon silver nitrate are sometimes termed argentometric methods. Potassium chromate can serve as an end point indicator for the argentometric determination of chloride, bromide and cyanide ions by reacting with silver ions to form a brick-red silver chromate precipitate in the equivalence point region.
Types of precipitation reactions include:
- Volhard method
- Fajan’s method
- Mohr method
Mohr’s Method
The volhard method was first described by the Jacob volhard a german chemist in 1874 . This is an indirect titration procedure used for determining the anions that precipitate with silver. In other words, this technique determines halide ions (F, Cl, Br, I) and anions such as phosphates and chromates in acidic environments using silver ions.
In the Volhard method silver ions are titrated with a standard solution of thiocyanate ions. 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 amount of silver that is precipitated with chloride in the sample solution is calculated by subtracting the excess silver from the original silver content.
What You Need To Know About Mohr Method
- In this method, silver nitrate is used as a titrant and chloride ion solution as analyte. Potassium chromate is used as indicator resulting in reddish brown complex.
- It is a direct method of titration.
- In the Mohr method, potassium chromate is used as an indicator.
- Mohr method is used to determine the concentration of chlorides in neutral medium.
- A red precipitate of silver chromate forms at the end point in Mohr method.
- Neutral or alkaline conditions (pH 6.5-9.0)are required in the Mohr method because chromate ion is the conjugate base of the weak chromic acid.
- In the Mohr method, titration of iodine and cyanate is not possible.
- In the Mohr method, the titration is carried out at room temperature because solubility of silver chromate increases with rising temperature.
Volhard’s Method
Mohr method of determination of chlorides by titration with silver nitrate is one of the oldest titration methods still in use – it was researched and published by Karl Friedrich Mohr in 1856. The idea behind is very simple – chlorides are titrated with the silver nitrate solution in the presence of chromate anions. End point is signalled by the appearance of the red silver chromate.
The Mohr titration is sensitive to the presence of both chloride and bromide ions in solution and will not be too accurate when there is a significant concentration of bromide present as well as the chloride. However, in most cases, such as sample, the bromide concentration will be negligible. For this reason, the method can also be used to determine either the total concentration of chloride and bromide in solution, or the concentration of bromide when the chloride concentration is known to be negligible.
Mohr’s method is used to detect the concentration of chloride ions in water samples from a variety of sources, including river water, stream water, and a variety of pharmaceuticals and chemicals.
What You Need To Know About Volhard Method
- In the Volhard method, chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocynate, creating distinct wine red complex.
- It is an indirect method of titration.
- In the Volhard method, ferric ammonium sulphate is used as an indicator.
- Volhard method is sensitive to low PH.
- A red soluble ferric thiocyanate forms at the end point in volhard method.
- Acidic conditions are required in the Volhard method.
- Thr Volhard method can be used to determine iodide, bromide and chloride.
- In the Volhard method, titration is carried out below 20 degrees Celsius to prevent the color of ferric thiocyanate complex from fading.
Volhard Method vs Mohr Method: Key Differences
| Basis | Volhard Method | Mohr Method |
|---|---|---|
| Principle | Precipitation titration using silver ions | Precipitation titration using silver ions |
| Indicator | Potassium chromate (K2CrO4) | Potassium chromate (K2CrO4) |
| Analyte | Chloride ions (Cl-) | Chloride ions (Cl-) |
| Titrant | Silver nitrate (AgNO3) | Silver nitrate (AgNO3) |
| Formation of Precipitate | AgCl (Silver chloride) | AgCl (Silver chloride) |
| Endpoint Detection | Formation of a red-brown Ag2CrO4 | Formation of a white AgCl precipitate |
| pH Conditions | Neutral to slightly acidic (pH 6-7) | Slightly acidic (pH 5-6) |
| Additional Reagents | Ferric ammonium sulfate (Fe(NH4)2(SO4)2) | None |
| Interference from Other Ions | Complexing agents can help mitigate | Less interference from other ions |
| Applications | Used for determining chloride concentrations | Widely used for chloride and bromide assays |
| Accuracy and Precision | Generally, Volhard is more precise | Mohr may have lower precision |
Key Takeaways
- In the Mohr method, potassium chromate is used as an indicator while in the Volhard method, ferric ammonium sulfate is used as an indicator.
- In the Mohr method, a red precipitate of silver chromate forms at the end point while in the Volhard method, red soluble ferric thiocyanate forms at the end point.
- Neutral or alkaline conditions (pH 6.5-9.0) are required in the Mohr method while acidic conditions are required in the Volhard method.
- In the Mohr method, titration of iodine and cyanate is not possible while the Volhard method can be used to determine iodide, bromide, and chloride.
- In the Mohr method, the titration is carried out at room temperature because solubility of silver chromate increases with rising tempereture. In the Volhard method, titration is carried out below 20 degrees Celsius to prevent the color of ferric thiocyanate complex from fading.
Applications of Precipitation Titration
- It is used for the determination of halides ion in the solution.
- It is used to measure salt content in food, beverages and water.
- It is used for Sulphur, thiocyanate, dichromate etc.
- Many drugs such as carbromal KCl infusion, NaCl infusion etc can be analysed by precipitation titration.
- It can be used for the determination of anions in the analyte.