Cheese salt determination using the Volhard procedure is described below. Other methods which have proven to give accurate results are:
- Automatic Chloride Titraters operate on the principle of coulometric silver ion generation to titrate chloride ions in the sample. When all chloride ions are titrated free silver ions cause a conductivity change which signals the end of titration.
- Quantab Chloride Titrater depends on the reaction of chloride ions with silver dichromate, which is brown, to form silver chloride chromate ion and silver chloride which is white. The reaction takes place on a calibrated strip which permits direct estimation of chloride content.
Volhard procedure for salt determination
Apparatus and Materials
- A torsion moisture balance
- 250 ml erlenmeyer flask and a 500 ml beaker
- Two graduated cylinders, one 50 ml and the other 100 ml
- A 10 ml pipette and a 5 ml graduated pipette
- Burette graduated in ml and 1/10 ml, and burette stand
- An electric or gas hot plate
- Chemically pure concentrated nitric acid
- Saturated potassium permanganate solution
- 0.1711 N potassium thiocyanate solution (contains 16.63 g per litre) in a brown glass bottle
- 0.1711 N silver nitrate solution (contains 29.07 g per litre) in a brown glass bottle
- A saturated solution of ferric ammonium sulphate
- Boiling chips such as carborundum granules or glass beads.
- Fume hood
- Prepare cheese sample as for cheese moisture test.
- Weigh about 3 g cheese into a clean dry 250 ml erlenmeyer flask.
- Add 10 ml of 0.1711 N silver nitrate solution as accurately as possible to the flask. If cheese contains more than 3% salt, add more silver nitrate.
- Add 15 ml of the chemically pure nitric acid.
- Add 50 ml of distilled water.
- Add a few boiling stones.
- Place flask on hot plate in fume hood and boil.
- When contents of flask are boiling uniformly, carefully add 5 ml of saturated potassium permanganate. Continue boiling until purple colour disappears, then add a second charge of 5 ml of potassium permanganate. When purple colour again disappears, add another 5 ml of potassium permanganate. Continue boiling until all cheese particles are digested. To ascertain when digestion is complete, remove flask from hot plate and allow to stand quietly for a few moments. Undigested cheese particles will float upon the surface, while the white precipitate of silver chloride will sink to the bottom of the clear liquid. When no more white particles are seen upon the surface, digestion is complete.
- Add sufficient distilled water to bring the volume up to approximately 100 ml. Allow precipitate to settle and very carefully pour off the liquid into a beaker. Be careful not to pour off any of the white precipitate of silver chloride.
- Add 100 ml of distilled water to flask and swirl contents to wash precipitate.
- Add 3 ml of saturated ferric ammonium sulphate as an indicator and titrate the excess silver nitrate with 0.1711 N potassium thiocyanate. A reddish colour denotes the end point.
- The number of ml of 0.1711 N silver nitrate originally added minus the titration value found in step 11, divided by the weight of the cheese in the sample equals the percentage of salt in the cheese.
3.00 g of cheese to which 10.00 ml of 0.1711 N silver nitrate had been added gave a reading of 4.00 ml in Step 11.
4.00 ml 0.1711 N potassium thiocyanate required to combine with excess silver nitrate.
6.00 ml 0.1711 N silver nitrate combined with salt in cheese.
Therefore per cent salt by weight = 6.00/3.00 = 2.00
Because the salt in the cheese is measured by its chloride content, it is necessary to test the reagents used for chloride, or related substances content. This is done by carrying out a test using sucrose instead of cheese. The titration value subtracted from the original amount of silver nitrate added is subtracted from the value found in Step 12 before dividing by weight to find the percentage salt in the cheese.
To check the strength of the 0.1711 N silver nitrate solution, dissolve 10 g chemically pure dry sodium chloride in sufficient water to make up one litre of solution. Each ml of this solution is equivalent to one ml of 0.1711 N silver nitrate. When the silver nitrate has been standardized, each ml of silver nitrate is equivalent to one ml 0.1711 N potassium thiocyanate.