Salt (Volhard Method)

SALT (Volhard Method) PRINCIPLE

To determine salt, an analysis is made for total chlorides and the result is expressed as sodium chloride. The method involves the digestion of the sample with concentrated nitric acid and oxidation of any remaining organic matter with potassium permanganate. The dissolved chlorides in the sample are precipitated with excess silver nitrate as silver chloride, and the excess silver ions backtitrated with thiocyanate using ferric alum as the indicator. When all the excess silver ions have reacted with thiocyanate, any slight excess of thiocycnate will react with ferric alum to form a red-coloured complex (ferric thiocyanate) at the end-point. n excess of thiocyanate may react with the precipitated silver chloride, since the solubility product of silver thiocyanate is !.!" that of silver chloride. The addition of nitroben#ene, diethyl ether or acetone overcomes this difficulty by coating the precipitated silver chloride and thereby withdrawing it from reaction with the thiocyanate solution. $itroben#ene is toxic and its use is best avoided.  APPARATUS

". %ipettes and burettes. &. 'aboratory glassware. REAGENTS

". erric alum indicator saturated a*ueous solution of reagent grade ferric ammonium sulphate (+&&). &. ilver nitrate solution, !.&! $ - dissolve "/.!0 g of silver nitrate, (previously dried at ""!12) in distilled water and dilute to " litre. tandardi#e (using excess silver nitrate solution) against !."! $ sodium chloride (3.403 g per litre). 5. %otassium thiocyanate solution (!."!!$) - dissolve 6./& g of reagent grade potassium thiocyanate in distilled water, and dilute to one litre. 0. 3. 8. /.

2oncentrated nitric acid. $itric acid ("7"). $itroben#ene. %otassium permanganate solution (39).

PROCEDURE

ccurately weigh appropriate amount (about &-5 g) of sample into a 5!! ml :rlenmeyer flask. (or shrimp paste or other highly salted foods, weigh about 3 g into &3! ml volumetric flask and digest for about one hour in water bath to dissolve the salt. ilter through $o. " filter paper. Take appropriate amount of this filtrate) (e.g. for shrimp paste, take "! ml). dd &3 ml of !."!!$ silver nitrate solution, swirl flask until sample and solution are in intimate contact. dd "3 ml of concentrated nitric acid. (The silver nitrate solution must be added first, followed by the concentrated nitric acid. The order of addition ensures complete precipitation of the chlorides. +f nitric acid is added first, loss of chloride by volatili#ation as hydrogen chloride could occur, since hydrogen chloride has a far greater vapour pressure than nitric acid.) ;oil in a fume hood until dissolved and add potassium permanganate solution (39) until colour disappears and the solution becomes colourless. (hould too much potassium permanganate be accidently added, colour removal can be effected by the addition of small *uantities of sugar). dd about "3 to &3 ml of distilled water, boil for 3 mins. 2ool and dilute to about "3! ml with distilled water. dd about " ml nitroben#ene, & ml of ferric alum indicator and shake vigorously to coagulate the precipitated silver chloride. Titrate the excess silver nitrate with !."!! $ potassium thiocyanate solution to a permanent light brown end-point. 2arry out a blank titration containing &3.! ml of !."!!$ silver nitrate solution, & ml ferric alum indicator and "3 ml of ("7") nitric acid with !."!!$ potassium thiocyanate.

or highly salted foods re*uiring dilution, multiply the above

calculation by &3!