迁移管形状对希托夫法测定离子迁移数结果的影响

在希托夫法测定Cu~(2+)离子迁移数的物理化学实验教学中,目前常用直形和U形两种类型的迁移管,其中使用U形迁移管从阴极区溶液浓度变化算出的迁移数误差较大。以CuSO_4为电解质,Cu电极为电解电极,采用分光光度法分别测定了通电后直形、U形和n形三种迁移管中CuSO_4溶液浓度随高度的分布情况。研究发现,迁移管的形状对中部区溶液的浓度有较大影响。对于U形和n形迁移管,由于阴极位于迁移管底部,主要受重力作用引起的对流因素的影响,通电后中部区浓度会发生明显改变,并影响到阴极区溶液浓度,导致误差较大;使用直形迁移管受该因素的影响很小,能得到更为准确的结果。

lnfluence of the Shape of Transfer Tube on Determining lon Transference Number by Hittorf Method

In physical chemistry experiment, straight and U-shaped transfer tubes are often used for determining Cu²⁺ ion transference number by Hittorf method. When a U-shaped transfer tube is used, the result calculated from the concentration change of cathode chamber solution usually has a significant error. The concentration distributions of CuSO₄ solution with height in the straight, U and nshaped transfer tubes after electrolysis were investigated via spectrophotometry, using CuSO₄ and Cu hoops as electrolyte and electrodes, respectively. It is found that the shape of transfer tube has a great effect on the concentration of central chamber solution. Because the cathode locates at the bottom of the U-shaped and n-shaped transfer tubes, the concentration of central chamber solution changes a lot after electrolysis due to convection caused by the gravity effect, and the concentration of cathode chamber solution is also affected. It would bring a significant error to the measurement of ion transference number. Contrarily, this effect is ignorable if a straight-shaped transfer tube is used, and thus more accurate result can be obtained.