Ask Greg McMillan

Greg's Response:

A common misconception is that the slope of the titration curve and, therefore, the system sensitivity can be decreased by reagent dilution. Reagent dilution has a negligible effect on the shape of the titration curve: the curve slope will appear larger if the same abscissa is used because only a portion of the original curve is displayed. The numbers along the abscissa must be multiplied by the ratio of the old to the new reagent concentration to show the entire original titration curve. For example, the abscissa values would have to be doubled if the reagent concentration were cut in half. 

Properly designed dilution systems offer a variety of performance benefits. Most chemists use diluted reagents to make titration easier and more accurate. Because this is true for the lab, you can imagine how much more important it is in the field fraught with nonideal conditions. Dilution can reduce reagent valve plugging, reagent transportation delay, and reagent viscosity. It can prevent laminar flow and partially filled pipes and dramatically improve dispersion in a mixture. It also decreases the freezing point and winterization problem and corrosion for sodium hydroxide and plugging tendency. However, sulfuric acid becomes more corrosive when diluted. 

If reagent dilution is used, the system must be carefully designed to prevent creating reagent concentration upsets and delivery delays. The pH controller should throttle the diluted reagent. The mass flow of water should be ratioed to the mass flow of reagent, and a density controller should trim the ratio. Coriolis flow meters should be used to improve the mass flow measurement reproducibility and provide an accurate density measurement for concentration control. In addition, one could utilize the temperature from the Coriolis meter to compensate for the density, particularly if the feed temperature is not constant. For steep titration curves and in-line pH control, a storage tank for a diluted reagent with a high recirculation flow should be installed to smooth out the fast reagent concentration disturbances from the dilution.

Dynamic simulations with piping and control system dynamics are needed to determine the best liquid reagent dilution design.

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).