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一、方法原理样品用磷酸溶解,然后加入盐酸破坏磷酸和铁生成的络合物,而使铁转为氯化物。加KI还原Fe~(3+)为Fc~(2+),并析出等当量的I_2,以淀离粉为指示剂,以Na_2S_2O_3标液滴定析出的游碘。 2Fe~(3+)+21~-=2Fe~(2+)+Ⅰ_2 I_2+2Na_2S_2O_3=2NaI+Na_2S_4O_6 二、试剂 1.硫磷混酸(或浓H_3PO_4单独溶样):1∶5。 2.浓HNO_3。 3.1∶1盐酸。 4.固体KI。 5.0.5％淀粉:称取0.5克淀粉,碘化钾0.5克置于少量水中溶解后,将此液注入100毫升沸水,煮沸至澄清冷却,滴几滴盐酸。有条件可加0.2克碘化汞(长期有效)。 6.0.02N硫代硫酸钠标准溶液:称取5克Na_2S_2O_3·5H_2O于1升刚煮沸后的冷却的水中,加0.1克Na_2CO_3,溶解后稀释至1升,摇匀。标定:准确称KIO_3 0.015～0.02克于250
First, the principle of the method Dissolved with phosphoric acid, and then add hydrochloric acid to destroy the complex formed by phosphoric acid and iron, leaving the iron to chloride. Adding KI to reduce Fe ~ (3 +) to Fc ~ (2+) and precipitating an equivalent amount of I_2, using amylose powder as an indicator, iodine ionized by Na 2 S 2 O 3 standard titration. 2Fe ~ (3 +) + 21 ~ - = 2Fe ~ (2 +) + I_2 I_2 +2Na_2S_2O_3 = 2NaI + Na_2S_4O_6 II. Reagents 1. Sulfuric acid mixed acid (or concentrated H_3PO_4 solute): 1:5. 2. Concentrated HNO_3. 3.1: 1 hydrochloric acid. 4. Solid KI. 5.0.5% Starch: Weigh 0.5g of starch and 0.5g of potassium iodide. After dissolving in a small amount of water, pour the solution into 100ml boiling water, boil it until it is clear and cool and drop a few drops of hydrochloric acid. Conditions can add 0.2 grams of mercury iodide (long-term effective). 6.0.02N sodium thiosulfate standard solution: Weigh 5g Na_2S_2O_3 · 5H_2O in 1 liter just boiled cooled water, add 0.1g Na_2CO_3, dissolved and diluted to 1 liter, shake. Calibration: Accurately weigh KIO_3 0.015 ~ 0.02 g at 250