Eric Trompier outlines how to achieve state-of-the-art pH-stat titration.
The primary function of a pH-stat analyser is to determine an optimum value for the pH of the studied reaction; keep the pH constant by adding a reagent to neutralise the OH- or H3O+ ions generated in situ by a chemical/biochemical reaction or dissolution process; display live, print and record the required amount of reagent and the variation of pH versus time; calculate the kinetics of the studied reaction based on the consumption of reagent required to keep the pH constant over time.
Common applications include the measurement of enzyme kinetics and the activity of antacid products. Other examples of chemical/physical processes versus time include gas analysis, dissolution rate and crystal growing.
The quality of a pH-stat system is judged on its ability to keep the pH value of a chemical reaction constant, despite its tendency to change due to the nature and the kinetics of the reactions involved, the buffer effect, the hydrodynamics of the titration cell (volume, stirring efficiency) and the position, response time and stability of the measuring sensors.
A simple proportional control of titrant addition based on the difference in pH/mV at the set value speed as used in end point titration is not sufficient and an offset will occur. This is why, in addition to a Proportional control of titrant/reagent addition speed, the following are required:
* An Integral function preventing the titrant speed reaching zero even though the pH/mV is close to the set point.
* A Derivative function allowing the system to react more quickly to a change in sample pH/mV, which gives the so-called PID (Proportional Integral Derivative) regulation loop.
To reach set point quickly with no risk of overshoot, one solution is to integrate two different regulation modes for titrant/reagent additions on the same workstation. Radiometer Analytical's TitraLab pH-stat systems offer both PID areactive' mode and the unique apredictive' Adaptive Addition Algorithm (AAA) which allows the regulation algorithm to automatically respond to any change in buffer capacity of the solution during the operation.
When choosing a pH-stat configuration, certain features are of particular advantage:
* Bidirectional regulation using both acid and base reagents. Whatever your initial pH, the system automatically selects the appropriate titrant/reagent to reach your set point.
* Biburette regulation using two burettes containing the same reagent and working alternately. Whatever your titrant volume requirement, this ensures there is no risk of losing control of the kinetics studied or compromising accuracy during burette refilling.
* Bi-solution delivery in which the first burette regulates at set point while the second delivers another reagent which is necessary to the reaction but does not affect the regulation process.
* A temperature mode to regulate against temperature rather than pH of the solution versus addition of a titrant.
* Method linking in the same beaker.
* Dedicated software to allow several workstations to be handled simultaneously with automatic and continuous data recording.
* Modular design allowing a sample changer, additional burettes and electrode inputs to be added according to your experimental requirements.
The single and biburette pH-stat titration workstations in Radiometer Analytical's TitraLab range offer all the above features to ensure fast and reliable regulation of your analyses.
Eric Trompier is TitraLab Product Manager with Radiometer Analytical SAS, Villeurbanne, France. www.radiometer-analytical.com
