GB1421223A - Potentiometric titration method and apparatus therefor - Google Patents

Abstract

1421223 Automatic titration E I DU PONT DE NEMOURS & CO 16 April 1973 18292/73 Heading G1N The end-point of a potentiometric titration is determined automatically using an electronic computer which generates a representation of the actual end-point of the titration. In the determination of amine end groups in a nylon polymer by titration with a perchloric acid titrant a solution of the specimen is brought into contact with an electrode connected to a digital pH meter and an automatic burette. After the pH reading has become stable the stepping motor of the burette is controlled d digital computer via an interface to deliver 250 Ál of titrant. After waiting 10 seconds the pH meter is read by the computer and the initial slope value of the titration (#m V/250 Ál) is calculated by the computer. Further additions of 250 Ál of titrant are added and the same procedure carried out by the computer until the slope value reaches 0À06. At this point the size of the increments is reduced to 80 Ál and the titration monitored until the slope-value reaches 0.12 when once again the increments are reduced to 27À3 Ál. During this part of the titration the computer is programmed to count the number of seconds required until the meter output is stable and then wait for an equal length of time before reading the meter. When the slope-value reaches 0À35 the increments are finally reduced to 9À74 Ál and titration continued on through the end-point, indicated by a decrease in the change in meter output per increment. The computer orders five more incremental additions after the end-point before completing the titration. The last thirteen values of meter output (mVs) and corresponding values of total titrant volume added are stored in the computer which then makes a least squares analysis to fit the stored data to a cubic equation of the form Y = A + BX + CX<SP>2</SP> + DX<SP>3</SP> where Y is the millivolt reading and X is the volume of titrant. The end-point X end (point of maximum slope) is then determined from the equation Xend = -C/3D. Alternatively instead of the cubic equation one of the form Y = C Cot<SP>-1</SP> [(x-a)/b] may be used using the method of successive approximations. The computer is also fed with the specimen weight and moisture content so as to calculate the concentration of amine end-points.