Exercise

Practical 11 - Estimation of Salt (Mohr Method)

Aim: To estimate the salt contents in given samples by using Mohr method.

Theory and Principle: Common salt is used during preparation of pickles, sauces, ketchup, fruit chutney and appetizers. Besides, imparting taste, the salt in appropriate concentration also acts as a preservative. In different products, it is estimated by using Mohr method. The sample is titrated with standard solution of silver nitrate for estimation of salt by using potassium chromate as an indicator. From the titre the concentration of salt is estimated by using a standard expression.

Reagents/ apparatus/ glassware required
  1. 0.1N Silver nitrate (AgNO3): Dissolve 17g silver nitrate in 1 litre water.
  2. Phenolphthalein: Dissolve 1.0 g of phenolphthalein in 100ml of 95% alcohol.
  3. 0.1 N NaOH: Dissolve 40.005g NaOH pellets in water and dilute to one litre in a volumetric flask. Then dissolve 10 ml of the above solution to 100 ml with water to make 0.1N NaOH.
  4. 5% Potassium Chromate: Dissolve 5g Potassium chromate in 100ml water.
  5. Glasswares: Conical flask, beakers, volumetric flasks, pipette and burette.
Procedure for estimation of salt
  1. Weigh accurately a known sample (5-10 g paste/pickle or 25g puree). Dilute with distilled water.
  2. Neutralize with 0.1N NaOH using Phenolphthalein as indicator.
  3. Transfer it to 250 ml volumetric flask, make to volume with distilled water and filter.
  4. Take an aliquot (10 to 25ml) and titrate with 0.1N Silver Nitrate using 1ml 5% Potassium Chromate as an indicator.
  5. Note the volume of silver nitrate used to produce red brown end point colour.
  6. Carry out blank titration using distilled water and 1.0 ml of the indicator (excluding sample).
Observations

Sample No.

Wt. of sample

Volume made up

ml of sample taken

Titre value (sample) – titre value for blank

% NaCl

1.






2.






3.








Calculation:
Titre value* x N of AgNO3 x Volume made x 58.45 x 100
Sodium chloride (%) = ------------------------------------------------------------------
ml of sample taken x weight of sample x 1000

*Titre value = Sample titre - blank titre

Problem:
10 g of tomato ketchup was neutralized with 0.1 N NaOH and volume made to 250ml and filter. 25ml of the aliquot was titrated against 0.1N AgNO3 to give a titre value of 5.0ml. Calculate the quantity of NaCl present in the tomato ketchup.
*The blank titre was found to be 1.0 ml

Solution:
Wt of sample taken= 10g
Volume made up = 250ml
ml of aliquot taken for estimation= 25ml
Titre=5.0 ml
Blank titre=1.0 ml
(Sample titre- Blank titre)× Normality of AgNO3 ×volume made up × 58.45 ×100
% Sodium Chloride = -----------------------------------------------------------------------------------------
Wt of sample taken× ml of aliquot ×1000

(5-1) ×0.1× 250× 58.45 ×100
= --------------------------------------
10× 25×1000

4×0.1×250×58.45×100
= --------------------------------
10×25×1000

= 2.34%

Inference: The give sample of tomato ketchup contained 2.34% sodium chloride.
Last modified: Wednesday, 7 March 2012, 10:42 AM