Inline pH Measurement in Hot Undiluted Ammonium Nitrate

Problems arise when measuring pH at AN storage temperatures, typically 130 °C, which are normally too high for in-line pH sensors. At such temperatures, pH sensors experience rapid evaporation of internal electrolyte. Sensor lifetimes of less than a week are very common.

Cooling down the concentrated solution for measurement is not an option, as crystallization will occur, as mentioned above. Therefore, it is common practice to dilute the AN with hot condensate to a 10 wt % solution in a slip stream in which pH is measured after cooling the flow to around 25 °C – 60 °C. With a change in temperature and concentration, the pH also changes and compensation and set points must be observed. The pH of a neutral 10 wt % AN solution will be in the region of 4.5 at 25 °C.

Before it is returned to storage, the diluted AN used for measurement must be re-concentrated again through evaporating the dilution water. The required energy and infrastructure for sampling and concentrating make this a costly but necessary operation if traditional pH sensors are being used.