Linear flow gradients for automatic titrations

A manifold to perform automatic continuous flow titrations is presented. The system is based on the use of a variable-speed peristaltic pump which delivers the titrant at a linearly increasing flow rate and another pump working at a constant and higher speed, placed after the detector, the difference between both flow rates being compensated by the aspiration of the titrand through a T-piece. A solution containing the indicator is also propelled by the fixed speed pump. With such an arrangement, the flow rate through the spectrophotometric cell is constant and there is no drift of the baseline. The equations describing the behaviour of such a system are first obtained and then experimentally checked by using both acid-base and complexometric titrations. The experiments prove that the approach is highly reproducible (relative standard deviations (R.S.D.) below ± 1.5%), but inaccurate, since the time necessary to reach the equivalence point is longer than that predicted by theory. To avoid the need of a previous calibration, a second gradient can be performed by decreasing the variable-pump speed to zero. In this way, two equivalence points are obtained. Since both gradients are diametrically opposite, the difference in time between the two equivalence points agrees with that predicted by theory, as the errors can only cancel each other out. For acid-base and complexometric titrations the errors are below ± 2%.