(一)仪器和用具
锥形瓶:250ml; 滴定管; 回流冷凝管; 恒温水浴锅;电炉;吸管:25ml; 天平:感量0.001g; 烧备、试剂瓶等。
(二)试剂
精馏乙醇:称取硝酸银2g,加水3ml;注入乙醇中,竭力震荡。另取氢氧化钠3g,溶于15ml热乙醇中,冷却后注入主液;充分摇动,静置1~2周,待澄清后吸取清液蒸馏;
中性乙醇; 0.5N氢氧化钾乙醇溶液;0.5N盐酸标准溶液;1%酚酞乙醇溶液 。
(三)操作方法
称取混匀试样2g(W,准确至0.001g)注入锥形瓶中,加入0.5NKOH乙醇溶液25ml,接上冷凝管,在水浴锅上煮沸约30min,煮至溶液清澈透明后,停止加热,取下锥形瓶,用10ml中性乙醇冲冷凝管下端,加5滴酚酞指示剂,趁热用0.5N盐酸溶液滴定至红色消失为止。同时,进行空白试验。
(四)结果计算
皂化价按公式3-31计算:
皂化价(mgKOH/g油)=(V2-V1)×N×56.1 / W
式中:V1—滴定试样用去的盐酸溶液体积,ml; V2 —滴定空白用去的盐酸溶液体积,ml;N —盐酸溶液的当量浓度; W
—试样重量,g; 56.1—氢氧化钾的毫克当量。
双试验结果允许差不超过1.0mgKOH/g油,求其平均数,即为测定结果。测定结果去小数点后第一位。
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