Continuous on line true titrations by feedback-based flow ratiometry. The principle of compensating errors

We introduce a new concept for continuous on-line titrations based on feedb ack-controlled now ratiometry and the principle of compensating errors. The system has been thoroughly tested by applying it to acid-base neutralizati on titrations with indicator-based end point detection. In a typical case, the total flow (F-T, consisting of the sample and the titrant flows) is hel d constant while the titrant (e.g., a standard base containing an indicator ) now F-A varies linearly in response to a controller output voltage. The s ample (e.g., an acidic solution to be titrated) now F-A constitutes the mak eup and thus also varies (F-A = F-T - F-B) The status of the indicator colo r in the mixed stream is monitored by an optical detector and used either f or governing the controller output or for interpreting the results of the t itration. Three methods (PID based control, fixed triangular wave control, and feedback-based triangular wave control implemented on a PC) were examin ed. In the last and the most successful approach, the titrant now is initia lly ramped upward linearly. At the instant a change in the color is sensed by the detector, the tirtrant now rate F-H is higher than the true equivale nce flow rate F-E because of the lag time between the first compositional c hange and its detection. The sensing of the change in color causes the syst em output to immediately reverse its ramp direction such that the titrant n ow now goes down linearly at the same rate. At the instant a change in colo r, in the opposite direction this time, is again sensed, the titrant now ra te F-L is lower than F-E by exactly the same amount that F-H was higher tha n F-E. This principle of compensating errors (F-E = (F-H + F-I)/2) allows t rue titrations with excellent reproducibility and speed (0.6% RSD at 3 s/ti tration and 0.2% RSD at 10 s/titration) and titrant volume consumption as l ittle as 12 mu L/titration and solves an old conceptual problem in now base d titrations.