在吸附法中滴定中必須保持接近中性與摩

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在吸附法中,滴定中必須保持接近中性,與摩爾法的原理相同,因為 fluorescein 陰離子為鹼性,在酸性下會導致螢光黃酸根離子無法形成, 在鹼性下會形成 Ag 2 O 沉澱產生。
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3. Volhard 法中,若不將 AgCl 沉澱過濾,將會造成什麼誤差?理由何在? 且為何只有 AgCl 沉澱才須過慮,是否有其他的方法可達到同樣目的? AgCl Ksp=1.82×10 -10 AgBr Ksp=5.2×10 -13 AgI Ksp=8.2×10 -17 AgSCN Ksp=1.0×10 -12 Ag 2 CrO 4 Ksp=1.0×10 -12 使用 Volhard 法時若不將 AgCl 沉澱過濾除去會造成實驗誤差。因為在滴入 KSCN 時,由於銀離子減少使得 AgCl 解離出銀離子,由上表 K sp 可知,在滴 定終點時: AgCl Ag + +Cl - K sp =1.82×10 -10 AgSCN Ag + +SCN - K sp =1.0×10 -12 Mass balance [Ag + ] = [Cl - ] [SCN - ] [Cl - ] 2 [Cl - ][SCN - ] 1.82×10 -10 [SCN - ] 2 [Cl - ][SCN - ] 1.0×10 -12 可計算得 [Ag + ] 1.3528×10 -5 [SCN - ] 7.3921×10 -8 [Cl - ] 1.3454×10 -5 沒有 AgCl 固體存在時,很容易就達到足夠的 [SCN - ] 以產生 Fe(SCN) 2+ ;若 AgCl 溶回溶液之中會造成多餘的銀離子消耗 SCN - ,要達到足夠的量與 Fe 3+ 反應成 Fe(SCN) 2+ ,就必須多加入 SCN - ,其結果便會造成 KSCN 滴入過量。如果是 AgBr AgI 則不需要過濾的步驟: AgBr Ag + +Br - Ksp=5.2×10 -13 AgI Ag + +I - Ksp=8.2×10 -17 由於 AgBr AgI 之溶解度比 AgSCN 低,因此不用擔心回溶的可能性。
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  • Fall '14

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