為何於將達當量點之前才加入澱粉指示劑

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為何於將達當量點之前,才加入澱粉指示劑?比較若於開始滴定前就加入會如何? 澱粉是網狀聚合物,其可用來指示 I 2 之存在是因為碘分子會進入其網狀結構中而呈現深 紫色;如果滴定前就加入澱粉溶液,大部分的碘分子都會被澱粉包覆,而被包覆者將不易 接觸到 S 2 O 3 2- 並與之反應,造成變色速度下降並可能影響到滴定終點之判別。若在接近終 點前才加入澱粉指示劑,受到包覆的碘分子相對少量,引起的判斷誤差較輕微。 3. 分析未知物 Cu 中的含量時,加入 KSCN 的目的為何? 由於 Cu 沉澱表面會吸附一些 I 2 使滴定終點不明顯,並影響準確度,故在接近化學劑量時, 加入少量 KSCN Cu 沉澱成 CuSCN CuI+ SCN => CuSCN +I CuSCN 溶解度比 CuI 小得多,所以較不易吸附 I 2 ,而使結果比較準確。
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  • Fall '14

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