Precipitation Titration – Definition, Principle Indicators, Curve, Methods, Applications, Difference

Precipitation Titration – Definition, Indicators, Curve, Types, Methods, Applications, Difference

Precipitation Titration Definition

  • It’s a type of volumetric analysis that relies on a precipitate’s development.
  • Silver nitrate is the most common precipitating agent used in precipitation titration (AgNO3).
  • Argentometric titration is a type of titration that uses silver nitrate.
  • The term Argentometry comes from the Latin word Argentum, which meaning “silver.”
  • The majority of argentometric titrations are used to determine halides (Cl, Br & I). These are also used to determine SCN, CN, and other chemicals that react with Silver nitrate to generate insoluble products (precipitate).

Principle of Precipitation Titration

The amount of precipitating reagent added equals the amount of material precipitated, according to the principle of precipitation titration.

Precipitation Titration Indicators

For endpoint identification in Argentometric titrations, three conventional approaches based on indicator colour might be used:

  • Mohr’s method– It works on a principle of formation of coloured precipitate at the end point.
  • Volhard’s method– This method results in the creation of a soluble, coloured complex at the end point.
  • Fajan’s method– There is an adsorption of a coloured indicator on the precipitate at the end point.

Precipitation Titration Curve

The amount of precipitating reagent added equals the amount of material precipitated is the basic premise of precipitation titration.

Precipitation_titration_curve

Precipitation Titration Methods

Mohr’s Method

  • For the determination of chlorides and bromides, the Mohr’s method uses a silver nitrate solution as the titrant.
    • Silver chloride is produced when a chloride-containing solution combines with a standard solution of silver nitrate.
    • Once all of the chloride in the solution has been precipitated, the following excess drop of the titrant causes a reaction between silver and the indicator ions.
    • The indicator in this approach is potassium chromate (K2CrO4). In neutral and alkaline circumstances, chromate is yellow. The chromate ions interact with Silver ions at the terminus to generate red-colored Silver chromate.
    • Only when all of the chloride ions (from NaCl) have precipitated will silver chromate form. The following extra drop of Silver nitrate causes Silver chromate to form (red coloured).

AgCl + AgNO3 →   AgCl (white) + NaNO3

2AgNO3 + K2CrO4→    AgCrO4 (red) + 2KNO3

  • Errors occur as a result of the requirement to apply more titrant before the endpoint colour is evident. To avoid this error, conduct out blank titrations in which just the indicator is present.
  • It’s also critical to utilise the same amount of indicator (1 ml) during all titrations.
  • Mohr’s method only works in the pH range of 6 to 9.
  • Silver will precipitate as Silver oxide above this pH.

2AgNO3 + 2NaOH → Ag2O↓ + 2NaNO3 + H2O

  • The endpoint is delayed because chromate changes to dichromate, a brilliant orange colour, below this pH. To lower the acidity of the solution, calcium carbonate is frequently added.

2CrO42- + 2H+ → 2HCrO4 → Cr2O72- + H2O

Volhard’s Method

  • In Volhard’s method, chlorides, bromides, and iodides are titrated in an acidic medium.
  • The sample is titrated with standard thiocyanate solution after a known excess of silver nitrate solution is added.
  • The titration employs iron (III) alum as an indicator and uses back titration with potassium thiocyanate. With an excess of ammonium thiocyanate, this indicator forms a coloured complex.
  • To create a stable compound, the solution must be acidic, with a concentration of roughly 1 M Nitric acid.
  • Because an alkaline pH can lead to the formation of Fe(OH)3 precipitate.
  • The approach is suitable for determining silver directly.

NaCl + AgNO3(Excess) → AgCl (white) + NaNO3

AgNO3 + NH4SCN → AgSCN + NH4NO3
Fe3+ + NH4SCN → [FeSCN]2+(reddish-brown) + NH4+

Attempt: MCQ on Complexometry & Precipitation Titration

Fajan’s Method

  • The indicator reaction occurs on the surface of the precipitate with adsorption indicators. The indicator, which is a dye, is ionised in solution, usually as an anion (In).
  • In a precipitation titration, adsorption indicators are organic molecules that tend to be adsorbed onto the surface of the solid precipitate.
  • Consider a NaCl titration with AgNO3. Cl is in excess before the equivalence point, and Cl is the dominant adsorbed layer. This repels the indicator anion, and the secondary layer of adsorbed ions is made up of cations like Na+, which are more loosely bonded. [Figure a)]
  • Ag+ is in excess beyond the equivalence point, and the surface of the precipitate becomes positively charged, with Ag+ as the main layer. This attracts and adsorbs the indicator anion (which has a negative charge) in the counter layer.
image Precipitation Titration - Definition, Principle Indicators, Curve, Methods, Applications, Difference
  • One reason for the colour change is that the indicator forms a coloured complex with Ag+, which is aided by adsorption on the precipitate’s surface (it becomes “insoluble”).
  • Fluorescein, Dichlorofluorescein, Bromcresol green, Eosin, Methyl violet, Rhodamine 6 G, Orthochrome T, Bromphenol blue, Thorin are some examples of adsorption indicators.

Estimation of Sodium chloride: (Volhard’s Method)

  • Volhard’s method can be used to determine the concentration of sodium chloride.
  • In a glass-stoppered flask, weigh roughly 0.1 g sodium chloride and dissolve in 50 ml water.
  • Add 50.0 ml of 0.1M Silver nitrate, 5 ml of 2M Nitric acid, and 2 ml of Dibutyl phthalate, shake well, and titrate with 0.1M Ammonium thiocyanate using 2 ml of Ferric ammonium sulphate solution as an indicator.
  • 0.005844 g of NaCl is comparable to one millilitre of 0.1M Silver nitrate.

Difference between Mohr’s Methods and Volhard’s Method

Sr. No.Mohr’s MethodVolhard’s Method
1.    It is a direct titration of the sample with Silver nitrate.  This is an indirect method in which the sample is precipitated with the excess Silver nitrate and is back titrated with Ammonium thiocyanate.
2.  Potassium chromate is used as an indicator.Ferric ammonium sulphate is used as an indicator.
3.  At the endpoint red ppt of Silver chromate is formed.At the endpoint red solution of   Ferric thiocyanate is formed.
4.  This reaction is carried out in the pH range of 6-9.This reaction is carried out in the acidic pH.  
5.    This method works on the principle of the formation of a coloured precipitate.This method works on the principle of formation of a coloured solution of complex.  
6.Titration of I, CN is not possible with this method.Determination of Cl, Br, I is possible with this method.

Applications of Precipitation Titration

  1. It’s used to figure out how many halide ions are in a solution.
  2. It is used to determine the amount of salt in food, beverages, and water.
  3. Precipitation titration can be used to test a variety of medications, including carbromal, KCl infusion, and NaCl infusion.
  4. It can be used to figure out how many anions are present in an analyte.

Limitations of Precipitation Titration

  1. This approach can only titrate a small number of halide ions.
  2. Coprecipitation is also a possibility.
  3. Analyzing the endpoint is really challenging.

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