DIFFUSIONAL MICROTITRATION OF 20-MU-L SAMPLES WITH OPTICAL INDICATION

Both the simplicity and robustness of diffusional microtitration have been taken advantage of to implement an extremely simple on-line optic al indication scheme for the analyses of 20 mu L samples, The system c onsisted mainly of an LED as the light source and a phototransistor as the detector, A green LED was used in acid/base titrations with a mix ture of bromocresol purple and bromothymol blue as the color indicator and 0.1 III KOH as the reagent. Correspondingly, a blue LED was imple mented in EDTA microtitrations of calcium with murexide as the indicat or and 0.1 M EDTA as the reagent. In the acid/base determinations, a r epeatability with 4.6% standard error was obtained for a series of 19 microtitrations of 40 nmol samples of HNO3. Further, a linear (r(2) = 0.998) five-point calibration was produced with three trials at each a mount in the range of 20-60 nmol samples. An equally linear (r(2) = 0. 997) six-point calibration was achieved in the determination of CaCl2 by EDTA, also with four trials at each amount, in the range of 10-40 n mol. As a real-life problem, tap water hardness was determined with a coefficient of variation of 0.64% (n = 5).