Measurement of pH is one of the most important and frequently used tests in water/ wastewater analysis. Practically every phase of water supply and wastewater treatment is pH dependent. At a given temperature, pH indicates the intensity of the acidic or basic character of a solution. A solution’s pH is the outcome of the balance between hydrogen ions (H+) and hydroxide (OH ¯ ) molecules, as well as its temperature. What is actually measured is the activity of hydrogen ions (H+). This measurement is then translated onto a scale that spans from 0 to 14 as the reciprocal of the logarithm of the hydrogen ion activity; – log [H+]. In general, pH values are reported to the tenth decimal in the standard units (S.U.) of the 0 to 14 scale.
Many laboratory procedures are pH dependent. Metabolic rates, organism reproduction rates, various chemical reactions and chlorine toxicity are all influenced by pH. Because of its effect upon so many aspects of water analysis, ACCURATELY MEASURING PH IS ONE OF THE CORNERSTONES THAT LABORATORY QUALITY ASSURANCE/ QUALITY CONTROL IS BUILT UPON.
Two methods for measuring pH values are approved by EPA for permit compliance reporting purposes. These are:
EPA Methods For Chemical Analysis Of Water And Wastes, Method 150.1 (Electrometric Method) Standard Methods for the Examination of Water and Wastewater, 18th, 19th or 20th editions, Method 4500-H+ B. (Electrometric Method)
Both methods are electrometric, which involves the use of a pH probe from which the signal is read out upon a millivolt meter. Although both methods are acceptable (and very similar), this text specifically discusses Method 4500-H+ B from Standard Methods.
Samples to be analyzed for pH should be collected in 100 – 500 mL polypropylene sample bottles. There are no methods of preservation, therefore, all pH samples are grab samples and must be analyzed within 15 minutes of sample collection. The quicker the samples can be analyzed, the more accurate the result will be. If fact, for many process control applications, pH is measured in-situ, such as in wastewater treatment lagoons. Effluent samples that are analyzed for permit reporting purposes should clearly indicate the sample time as well as the time of analysis to verify that the 15-minute holding time was not exceeded.
Measurement of pH is made with a pH meter that consists of:
* A potentiometer (millivolt meter)
* A glass sensing electrode
* A reference electrode (half cell)
* A temperature compensating device
For routine work, the pH meter must be accurate and reproducible to the nearest 0.1 pH unit. Before use, the meter must be calibrated to at least two standards and then checked against a third standard. Most labs use calibration standards (known as buffers) that have pH values of 4.0, 7.0 and 10.0 at 25º C. Buffers with these values can be purchased that are color coded so that the buffer with pH value 4.0 is red, 7.0 is yellow and 10.0 is blue. The colorcoding makes the buffers easier to identify while calibrating the pH meter.