Cedergren A, Jonsson S
Department of Analytical Chemistry, Umeå University, S-901 87 Umeå, Sweden.
Anal Chem. 1997 Aug 1;69(15):3100-8. doi: 10.1021/ac970034y.
A new type of diaphragm-free coulometric cell for continuous coulometric Karl Fischer titrations of water in the range 0.1-1000 μg is described. The relative standard deviation obtained for titrations of 1 μg amounts of water was typically 1%. The background due to diffusion of water from the air was normally in the range 0.3-0.9 μg of water min(-1) depending on environmental humidity. The variation in the background was normally ±0.01 μg min(-1). The construction makes it possible, at any time in a sequence of titrations, to renew the catholyte by means of a Teflon plunger inside the cathode compartment. In this way, the interference effects caused by oxidizable reduction products of methyl sulfite which are formed at the cathode can be controlled in a very simple way. These products are rapidly eliminated by means of a normal titration before a new titration starts. The need for this draining step differs depending on the type of reagent used. The coulometric titration system makes use of true potentiometric end-point detection, and this principle makes it possible to control the iodine level at the end-point at much lower levels as compared with commercial instrumentation. The analytical advantages gained by this option are demonstrated for the determination of water in ethylenediamine, a task which was found to be impossible when using end-point concentrations in the range (3-7) × 10(-5) M, which is typical for the bipotentiometric indicating system used in commercial instruments. Recovery rates in the range 100-102% were obtained and are shown to be dependent on the type of reagent used. The most accurate results were obtained for an imidazole-buffered methanolic reagent in which the concentration of sulfur dioxide was kept relatively low (0.10 M). The diaphragm-free cell described was shown to be compatible with all of the commercial reagents (designed for coulometry) investigated, including the well-known Hydranal products Coulomat A, AK, AG, AG-H, and AD.
本文描述了一种新型无隔膜库仑池，用于在0.1 - 1000μg范围内对水进行连续库仑卡尔费休滴定。对1μg水进行滴定时，获得的相对标准偏差通常为1%。由于水从空气中扩散导致的背景值，通常在0.3 - 0.9μg水·min⁻¹范围内，具体取决于环境湿度。背景值的变化通常为±0.01μg·min⁻¹。这种构造使得在一系列滴定过程中的任何时候，都可以通过阴极室内的聚四氟乙烯柱塞更新阴极电解液。通过这种方式，可以以非常简单的方式控制在阴极形成的亚硫酸甲酯的可氧化还原产物所引起的干扰效应。在新的滴定开始之前，通过常规滴定可以迅速消除这些产物。进行此排放步骤的必要性因所使用试剂的类型而异。该库仑滴定系统采用真正的电位终点检测，与商业仪器相比，这一原理使得能够在更低的水平控制终点处的碘含量。通过该方法获得的分析优势在乙二胺中水的测定中得到了证明，当使用商业仪器中典型的双电位指示系统，终点浓度在(3 - 7)×10⁻⁵M范围内时，发现该任务无法完成。回收率在100 - 102%范围内，且表明其取决于所使用试剂的类型。对于二氧化硫浓度保持相对较低(0.10M)的咪唑缓冲甲醇试剂，获得了最准确的结果。所描述的无隔膜池与所有研究的商业试剂（专为库仑法设计）兼容，包括著名的Hydranal产品Coulomat A、AK、AG、AG - H和AD。
