Several methods, both volumetric and gravimetric, for the determination of allyl isothiocyanate in mustard oils have been suggested in the literature.123 The most satisfactory one is a modification of the official procedure of "The United States Pharmacopoeia”124 This method is based upon the reaction of the isothiocyanate radical with silver nitrate. The excess silver nitrate solution is determined by titration with standardized ammonium thiocyanate solution in the presence of ferric ion ; the titration must be continued until the red color of ferric thiocyanate is first observed.
C3H6NCS + NH4OH + AgNO3 -> AgNCS + C3H6OH + NH4NO3
Mustard oil flask
DIAGRAM 4.11.  Mustard oil flask.
Procedure: Dilute about 4 cc. of the oil, accurately weighed, with sufficient alcohol to make exactly 100 cc. of solution. Pipette 5 cc. of this solution into a 100 cc. mustard oil flask (see Diagram 4.11) and add 50 cc. of 0.1 N silver nitrate solution and 5 cc. of 10% ammonia solution. Connect the flask to an air-cooled reflux condenser, 1 m. long, and heat on a steam bath for 1 hr. Allow the liquid to cool to room temperature and then add sufficient distilled water to fill the flask to the 100 cc. mark. Mix well and filter through a dry filter. Reject the first 10 cc. of filtrate. Transfer 50 cc. of the subsequent filtrate by means of a pipette into a 100 cc. saponification flask; add about 5 cc. of concentrated nitric acid and 2 cc. of an 8% solution of ferric ammonium sulfate. Titrate the excess of silver nitrate solution with standardized 0.1 N ammonium thiocyanate. Carry out a blank determination simultaneously, using 5 cc. of alcohol and the same quantities of reagents but omitting the oil. Calculate the percentage of allyl isothiocyanate; each cc. of 0.1 N silver nitrate is equivalent to 0.004958 g. of allyl isothiocyanate.
Percentage of allyl isothiocyanate 19.832 (b - a)/w
where: b = cc. of ammonium thiocyanate solution required for the blank;
           a = cc. of ammonium thiocyanate solution required for the determination.
           w = weight of oil used for the original dilution.
The blank should require about 24 to 25 cc. of ammonium thiocyanate; the actual determination, about 4 to 5 cc. if a 4 g. sample of oil is employed.
122 Ber. 35(1902), 24.
123 See "Suggested Additional Literature," p. 359.
124 Twelfth Revision, 46. (Now official in "The National Formulary," Eighth Ed., 29.)

1 Comment:

rostam on September 13, 2021 at 8:39 AM said...

Percentage of allyl isothiocyanate 19.832 (b - a)/w
why don't multiply Normality ?
please make me understand

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