取新鲜蒜瓣试样若干,捣成糊状.称取约50.00 g,加浓硝酸20 mL处理使其中所含亚砜基氧化成SO2-4,加水,过滤并定容.分取此滤液一份(约十分之一),置于滴定杯中,用氢氧化钠溶液调酸度至pH 5-6,加入一倍体积的乙醇,以铅离子选择电极指示终点,用铅标准滴定溶液进行电位滴定.终点电位预先用二次微商法求得.方法所测得结果的RSD为0.44%,且与重量法测定结果相比无显著差异.

Sample of fresh garlic was ground into slurry and a definite amount (about 50.00 g) of the slurry was taken and treated with 20 mL of conc.HNO3 to oxidize the sulfoxide in the sample into sulfate ion.The suspension was diluted with water,filtered and made up to a definite volume.An aliquot of the filtrate (ca.1/10) was taken and it acidity adjusted to pH 5-6 with sodium hydroxide solution,and an equal volume of ethanol was added to the solution.Titrate the SO2-4 in the solution with standard lead nitrate titrating solution,using Pb(Ⅱ)-ion selective electrode to indicate the end point,the potential of which was determined by the method of 2nd derivation.RSD′s of this method was shown as 0.44%,and no significant difference were found between the results obtained by this method and by the gravimetric method.