The acids and bases of extreme concentrations (c > 0.1 mol
dm3) are unsuitable for amperometric determination due to the
difficulty to keep constant ionic strength in a given supporting
electrolyte solution, as well as due to an inappropriately high limiting
current densities, accompanied by extensive hydrogen deliberation
in acids and extensive anodic dissolution of tungsten in
alkalis. To avoid these problems, one may use the method of standard
additive. Namely, once the calibration curve jL vs.c(OH) is
determined for a given narrow range of ionic strengths, it may be
further used with high reliability and reproducibility, by measuring
the change in jL upon addition of small volumes of analite to a
controlled volume of standard alkaline solution (1 103 mol
dm3–0.1 mol dm3). As demonstrated below (Fig. 6), an excellent
linearity of jL vs. c(OH) was evidenced upon injection of small portions
of 1 mol dm3 H2SO4 in 0.1 mol dm3 KOH solution. The time
needed for current to take up stationary value was shorter than 1 s,
with further relative variation lower than 1%.