Coagulation Control with a Recording Potentiometer

For the past decade, hydrogen-ion concentration, commonly known as pH, has been used to obtain information relating to coagulation control, corrosion correction, and other problems. Efficient utilization of this information demands maintenance of the proper pH value under every condition of service. With normal operating conditions in the average water plant, maintenance of hydrogen-ion concentration between fairly wide limits produces satisfactory coagulation control, particularly if the water is well buffered. However, with slightly buffered waters, or under conditions of flood, with resultant rapid change in turbidities and pH value, many plants find it impossible to obtain optimum floe precipitation without the use of alkali. If this extreme condition is not reached the increased dose of coagulant, together with the reduced buffer characteristics, caused by dilution, often produces rapid changes in pH values so that the usual "spot" colorimetrie test cannot adequately control the plant, even when samples are collected at frequent intervals. Since the indicator color is also masked by suspended material, even though tested with the block comparator or similar device, erratic results will be obtained giving inefficient control. Many purification plants are forced to obtain water from a "flashing" stream under these adverse conditions. At the time of greatest need the plant operator is faced with a difficult problem to maintain the proper pH value for good floe precipitation. Utilization of frequent colorimetrie determinations, having a more or less limited value, particularly if very rapid buffer concentration and turbidity changes are occurring, is not satisfactory. The color tint produced by Brom Phenol Blue or Brom Cresol Purple will cause serious error, and Thymol Blue in artificial light