Ask Greg McMillan

Greg's Response:

The titration curve is the most important piece of information for designing, commissioning, and troubleshooting pH control systems. Lab titration curves are typically done by placing a known volume of the process sample in a Pyrex glass jar and logging the pH indication for each incremental volume of reagent added via a burette. The volume of reagent added between the data points must be reduced drastically near the equivalence point. If the sample or reagent contains a strong acid or base, it is difficult to generate data points near the equivalence point.

If the titration curve has a long, flat tail, relatively few data points are needed until the first bend. However, the starting point representing the influent pH for various operating conditions must be plotted accurately to estimate the valve rangeability and stick-slip and the mixing equipment size and agitation requirements.

If the sample volume is given and the concentrations of the reagent used in the lab and the control system are equal, the reagent volumetric flow can be calculated for a given influent flow and pH. The titration curve used for system design should have an abscissa that is the ratio of reagent to influent flow.

The lab temperature during titration is rarely equal to the process temperature. The sample pH will change with the temperature because the dissociation constants change with the temperature. This is a change in actual solution pH and is not to be confused with the change in millivolts generated by the glass electrode per the Nernst equation. Conventional temperature compensators use a temperature sensor embedded inside the electrode to correct for the Nernst effect.

Every sample must be time-stamped with the exact time sample was taken. If the titration curve changes with time, separate samples should be gathered over a representative period and titrated individually. The samples should not be combined for titration.

Use a charge balance for a solution at the same temperature as the process with all the acids, bases, and salts including the change in dissociation constants with temperature and with carbonic acid added to match lab titration curves.

Dynamic simulation can significantly enhance the design of optimal titration curves for pH system management by addressing several key challenges. These include simulating high-resolution data near the equivalence point with adjustable reagent volumes, incorporating temperature-dependent dissociation constants to reflect process conditions, and modeling the effects of variable influent pH levels on system dynamics. The simulation can also account for the complexities of mixing, reagent concentration variations, and temporal changes in sample composition. Using simulation technology, combined with rigorous validation against experimental data, this approach promises to refine and optimize titration processes, ensuring more accurate and effective pH control systems.

For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).

Top 20 Mistakes in Lab Titration Curves

1. An insufficient number of data points was generated near the equivalence point.
2. The starting pH (influent pH) data points were not plotted for all operating conditions.
3. The curve does not cover the whole operating range, including control system overshoot.
4. There is no separate curve zoom-in to show the curvature in the control region.
5. There is no separate curve for each different split-ranged reagent.
6. The effect of the sequence of the different split-ranged reagents was not analyzed. 
7. The effect of back mixing different split-ranged reagents was not considered.
8. The effect of overshoot and oscillation at the split-ranged point was not included.
9. The sample or reagent solids dissolution time effect on the abscissa was omitted. 
10. The gaseous reagent dissolution time and escape effect on the abscissa were omitted. 
11. The sample volume was not specified. 
12. The sample time was not specified.
13. The reagent concentration was not specified. 
14. The sample temperature during titration was different than the process temperature. 
15. The influent sample was contaminated by absorbing carbon dioxide from the air. 
16. The influent sample was contaminated by absorbing ions from the glass beaker.
17. The influent sample composition was altered by evaporation, reaction, or dissolution. 
18. The laboratory and field measurement electrodes had different types of glass. 
19. A composite sample was titrated instead of individual samples. 
20. The laboratory and field reagents used different compounds.