カール・フィッシャー法による水分の自動電量滴定装置
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概要
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The titrator was consist of a cell assembly and an instrument which was constructed of an electrolytic current source, a counter circuit with a digital readout, a control circuit of titration and an operation circuit (Fig. 1). Water is determined with the Karl Fischer reagent generated at the anode by electrolysis of exclusive electrolytes.<BR>The electrolytic current source supplies generally a controlled current of 107.1 mA with which 1 μg of water may be titrated by electrolysis for 0.1 second. The counter circuit measures the net time consumed for electrolysis by counting a clock pulse of 100 Hz synchronously by electrolysis. The measured counts are displayed numerically at a unit of 0.1 sec on four "Nixie" tubes, showing 1 μg of water per 1 count.<BR>The controlled current can also be varied within the range of ± 15% by using a three turn potentiometer which serves as a "compensator". When the compensator dial is set at 10.00, the current is held constant at 107.1 mA, and it may varies the current from 8.50 to 11.50. The stability and reproducibility are within 0.1%. The compensator makes it possible to exhibit figures at a unit of ppm for any weight of sample within 1.000±0.150 g.<BR>Whenever the power switch is turned ON, the instrument is kept a condition at which electrolysis for cancellation of a blank is performed without counting time. The operation switch, SW<SUB>2</SUB>, (Fig. 1) is kept usually at the position of BLANK, by pushing the SW<SUB>2</SUB> to SAMPLE, relay E is turned ON and the control circuit is suspended the function, and then by pushing the SW<SUB>2</SUB> to TITN relay E is turned OFF and relay D is turned ON, and titration starts. Even if an operator pushes the SW<SUB>2</SUB> to TITN prior to pushing to SAMPLE, no change will occour in the operation circuit.<BR>The titration was followed up by detecting the potentials with a pair of polarized platinum electrodes. Signal E<SUB>s</SUB> from the detector is compared with a reference (E<SUB>b<SUB>1</SUB></SUB>=-260 mV or E<SUB>b<SUB>2</SUB></SUB>=-150 mV) in the control circuit(Fig. 3). The value of E<SUB>s</SUB> varies from about -400 mV at an excess of water to about 0 mV at an excess of reagent. From the start of titration (|E<SUB>s</SUB>|>|E<SUB>b<SUB>1</SUB></SUB>|) to the point of |E<SUB>s</SUB>|=|E<SUB>b<SUB>1</SUB></SUB>|, the relay A is turned ON and electrolysis and counting time are advanced. At |E<SUB>s</SUB>|≤|E<SUB>b<SUB>1</SUB></SUB>|, the relay A is turned OFF and electrolysis and counting of time are suspended. Such an operation will be repaeted for a few minutes as the first stage of the control. During |E<SUB>b<SUB>1</SUB></SUB>|>|E<SUB>s</SUB>|>|E<SUB>b<SUB>2</SUB></SUB>|, the reference is changed periodically (by a period of t<SUB>1</SUB>) from E<SUB>b<SUB>1</SUB></SUB> to E<SUB>b<SUB>2</SUB></SUB>, for a short time (t<SUB>2</SUB>) by action of relay B, and then relay A is turned ON over the same time interval. The periods of t<SUB>1</SUB>and t<SUB>2</SUB> are set at the timing circuit (Fig. 3). Such an operation will be repeated until |E<SUB>s</SUB>|≤|E<SUB>b<SUB>2</SUB></SUB>| as the second stage of control. After the condition of |E<SUB>s</SUB>|<|E<SUB>b<SUB>2</SUB></SUB>| has been continued over the time of (t<SUB>1</SUB>+t<SUB>2</SUB>+α) where α is a time lag produced by the charge of the capacitor C<SUB>1</SUB> (Fig. 1), relay D is turned OFF and the titration is completed. Smaller than about 3 mg of water may be determined with the titrator within the precision of 5 μg H<SUB>2</SUB>O.<BR>The titrator may also be permitted to use for the determination of "Bromine Index" by switching the controlled current to 120.75 mA, E<SUB>b<SUB>1</SUB></SUB> to -550 mV and E<SUB>b<SUB>2</SUB></SUB> to -280 mV by one touch operation. The "Bromine Index" may be displayed at a unit of 0.01 mg Br<SUB>2</SUB> per 1 count.
- 社団法人 日本分析化学会の論文
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