GB719995A - Improvements in or relating to titration method and apparatus - Google Patents

Abstract

Apparatus for automatically determining the end point of a titration process by measuring the change in polarization voltage between two electrodes immersed in the liquid being titrated makes use of the fact that as the end point is approached the polarization voltage exhibits transient excursions of successively decreasing amplitude and increasing duration, and the process is assumed to be complete and automatically stopped when the duration of a transient excursion exceeds a predetermined duration sensed by a timing device controlling the supply of reagent. The titration vessel 10 (Fig. 2), includes a magnetic stirring device 11, 12 and supports through a stopper 13 a delivery tube 15 containing an armature 24 attached to a spherical-shaped plug 26 surrounded by a solenoid 28, so that energization of the solenoid raises the armature to allow liquid from a graduated <PICT:0719995/III/1> <PICT:0719995/III/2> burette 21 to flow into the titration vessel 10. The condition of the liquid 29 in the vessel 10 is determined electrically by two electrodes 30, 31. The titration control circuit (Fig. 1) comprises potentiometer arrangements for feeding the electrodes 30, 31 with DC from an AC source via a rectifier 56, in series with a sensitive relay winding 66 controlling change-over contacts 70-72, in series with one section S2 of a 3-postion 2-bank switch 75, wherein positions 2 and 3 allow energization of a relay 63 when a high current in winding 66 joins contacts 70, 72, or a low current joins contacts 71, 72. With switch 75 in position 1, the potentiometers 61, 64, 74 are adjusted to establish the required sensitivity. With switch 75 in, e.g. position 2 for forward titration, the low current in winding 66 makes contacts 71, 72 relay 63 is unenergized, contacts 85, 87 are made, and solenoid 28 is energized to raise the armature 24 and allow reagent to flow into the titration vessel 10. As the end point is approached the electrode current rises on a transient excursion, relay winding 66 is energized, contacts 70, 72 make, relay 63 is energized, contacts 85, 86 make, to break the circuit through the solenoid 28 thereby obstructing the flow of reagent, and to make the circuit of a motor 38 driving a timing mechanism 35. After the transient excursion, timer 35 returns to zero and relays 66, 63 cause reversal of switch contacts 85, 87 to repeat the process, the duration of the transients successively increasing. The timer 35 has a preset hand 40 and a hand 37 driven by motor 38, and when the duration of the transient allows the hands 37, 40 to contact, a cut-off switch 42 is operated to terminate the process. Lamps 90, 92, 93 indicate the conditions of switch 42 and contacts 85-87. p