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定電流電解回路と紫外線発光ダイオード(UV LED)を光源とする比色計を組み合わせた小型の電量/光度滴定装置を開発した.定電流電解回路は,電源電圧の+5Vから約+30Vを発生する昇圧部と,ジャンクション型電界効果トランジスター(JFET)を用いる定電流回路から成り,電解電流は0〜2mAの範囲で調節できる.比色計部分は,光源に370nmの紫外線LEDを,検出器にフォトダイオードを用いた.発生極槽と比色セルは兼用で,光路長2cmのガラスセルを利用した.このセルには,発生極である白金板電極(表面積30mm^2),及び塩化カリウム塩橋をセットした.また,セルホルダーの下部にマグネチックスターラーを内蔵し,電解中に溶液をかくはんできるようにした.対極槽にはガラス製サンプル瓶を用い,白金線の対極をセットした.組み込み用コントローラーにより,吸光度,電解の積算時間及び電解電流を測定し,液晶ディスプレイに表示した.また,吸光度及び積算時間は,コントローラーに内蔵のシリアルインターフェースを通じてPCに転送して時間-吸光度グラフを作成し,変曲点から滴定の終点を求めた.本装置をまずチオ硫酸ナトリウムの標走に応用し,ヨウ素滴定法と比較した.食用油の過酸化物価の定量にも応用し,ヨウ素滴定法と比較した.いずれも,分析値はヨウ素滴定法の結果とよく一致し,本装置が実用的な性能を有することが分かった.
A palmtop-size coulometric titrator based on the photometric detection principle was developed for determining the peroxide value (POV) in samples of edible oil. The circuit for electrolysis was composed of a voltage booster that could generate +30V from +5V of a power source, while a circuit for constant-current control was fabricated using a junction-type field-effect transistor (JFET), which could supply a current of 0 to 2 mA. In this titrator, a photometric cell with a 2-cm optical path length served as an electrolysis compartment containing a generator electrode. The end point of titration was determined photometrically by monitoring the absorbance of the electrolytic solution during the electrolysis with a home-made colorimeter using an ultra-violet light-emitting diode (UV-LED, emission wavelength 370 nm) as a light source and two photodiodes (PDs) as detectors. The data corresponding to the absorbance, elapsed electrolysis time, and the electrolysis current were collected by an external PC via a serial interface built in the controller. It was demonstrated that the titrator could provide good accuracy and precision in the standardization of a sodium thiosulfate solution. Finally, the device was applied to the determination of POVs in samples of edible oil; the analytical results were in good agreement with those determined by iodometry. Moreover, the titrator showed a capability for the determination of some colored samples whose end point was hardly determined by conventional iodometry. The advantages of versatility, accuracy as well as sensitivity in end point detection make the device attractive for the determination of POVs in many practical samples.
