酸价指中和1 g油脂中的油脂中的游离脂肪酸所需氢氧化钾的毫克数。
(一)仪器和用具
滴定管;锥形瓶:250ml;试剂瓶;容量瓶、移液管、称量瓶等;天平:感量0.001g。
(二)试剂
0.1N氢氧化钾(或氢氧化钠)标准溶液;中性乙醚-乙醇(2:1)混合溶剂:临用前用0.1N碱液滴定至中性。指示剂:1%酚酞乙醇溶液。
(三)操作方法
称取均匀试样3~5g(W)注入锥形瓶中,加入混合溶剂50ml,摇动使试样溶解,再加三滴酚酞指示剂,用0.1N碱液滴定至出现微红色在30S不消失,记下消耗的碱液毫升数(V)。
(四)结果计算
油脂酸价按公式3-27计算:
酸价(mgKOH/g油)= V×N×56.1 / W ×100
式中:V—滴定消耗的氢氧化钾溶液体积,ml;
N—氢氧化钾溶液当量浓度;
56.1—氢氧化钾的毫克当量;
W—试样重量,g。
双试验结果允许差不超过0。2mgKOH/g油,求其平均数,即为测定结果,测定结果取小数点后第一位。
注:1)测定深色油的酸价,可减少试样用量,或适当增加混合溶剂的用量,以酚酞为指示剂,终点变色明显。
2)测定蓖麻油的酸价时,只用中性乙醇不用混合溶剂。
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