GB1427795A - Endpoint detection in colourimetric titrations - Google Patents

Abstract

1427795 Titration apparatus; determination of bicarbonate in blood serum H DAHMS 6 Feb 1973 5748/73 Headings G1A and G1B In apparatus for colorimetric titration of a liquid containing an indicator substance having different light absorption characteristics on different sides of a colour change interval indicative of a predetermined end-point, light is passed through the liquid while a titrant ion is introduced thereto, first and second electrical signals are generated respectively corresponding to the intensity of light passing through the liquid at a first and a second wavelength respectively corresponding to the light absorption characteristics of the liquid at one side and at the other side of the colour change interval, and a third signal is generated corresponding to the ratio of the logarithms or the logarithm of the ratios of the first and second signals so that the titration is stopped when the third signal reaches a pre-set level. As shown, the liquid 12 contained in a transparent container 13 is a mixture of human blood serum, aqueous chloride-free titration solution and a small amount of a pH indicator e.g. phenol red. Titrant ion is introduced by electrolysis using electrodes 14, 16. As the titration proceeds, the end-point of the titration of chloride with electrolytically generated silver ion at the silver anode 14 is determined by amperometric electroder 17, 18. However, the electrolysis is continued to generate hydroxyl ions for titrating the remaining acid in the liquid. This end-point is determined colorimetrically by detecting the light passed. through the liquid at the two different wavelengths, determined by filters 205, 206, using photo-cell detectors 215, 216 which provide the first and second signals. The logarithmic ratiometer 210, Fig. 2 (not shown), provides the third signal, corresponding to the ratio of the logarithms of the first and second signals, to a meter relay 47 which acts to stop the electrolysis and a timer 49 when the pre-set level is reached. As an alternative, a single light detector can be provided with the two filters arranged in a rotating chopper. A synchronisation signal can then be provided with appropriate gating circuits and smoothing capacitors to separate the two signals before they are fed to the ratiometer. Further, the two filters can be provided in front of the container. As further alternatives, the titration can be performed using titrant liquid either manually or automatically in which a motor driven piston gradually forces the titrant liquid from a burette. Digital read-out may be obtained by a circuit in which the motor of the timer (or of the burette piston) drives the movable contact of a potentiometer to continuously change its output voltage which is amplified and displayed on a digital voltmeter.