Read chapter 21 in Simplified Procedures for Water Examination.
pH affects most of the chemical reactions which take place in water and wastewater treatment plants, so it is important to test pH of the source and treated water frequently. Changes in pH of the source water may require the operator to change the feed rate of various chemicals. In contrast, changes in pH of the treated water may indicate that chemicals are not being fed properly, perhaps due to a chemical feeder running out of chemicals. The pH test explained in this lab exercise gives the knowledgeable operator one of the best indicators of the chemical balance of the plant's water.
pH is typically measured using a pH meter, so this is one of the simplest tests performed in the water and wastewater laboratory. Basically, measuring pH consists of calibrating the instrument, placing the electrodes in a well-mixed sample, and then reading the pH directly from the pH meter.
Despite the utility of this lab procedure, you should be aware of its shortcomings. Measuring the pH of a solution does not tell us which acids and bases are present, nor does it tell us their concentrations. Instead, pH is merely an indicator of the overall balance of hydrogen and hydroxide ions in the solution.
You should also note that the precision and accuracy of pH measurement depends on the type and condition of the instrument and on the care used in calibration and operation. Under normal conditions, +0.1 pH unit represents the limit of accuracy of the pH meter, so you should report pH values to the nearest 0.1 pH unit.
Rinsing the electrode
The electrode is the portion of the pH meter which senses the pH. It is a very sensitive and fragile instrument which is susceptible to scratches. If scratched, the electrode will result in erroneous pH readings, so always blot the electrode dry carefully when cleaning it. Light and temperature can also harm the electrode, so it should be stored in a buffer solution at 10-25°C and protected from light.
The electrode is relatively free from interferences due to color, turbidity, colloidal matter, oxidants, reductants, or high salinity, except for a sodium error at high pH. This error at a pH above 10 may be reduced by using special "low sodium error" electrodes if your water is high in sodium and your pH is also high.
While using the pH meter, you should watch for erratic results arising from cracked electrodes, faulty connections, or fouling of electrodes with grease and/or solids residue. The two-point calibration explained in the procedure can help detect a faulty electrode. If a reasonably accurate value for the second buffer cannot be obtained after the meter has been standardized against the first buffer, then it is likely that one, if not both, of the electrodes is faulty and must be replaced.
The only reagents you will use in this lab are the buffer solutions used to calibrate and store the meter. The laboratory you visit will probably have these solutions on hand since they can be stored for at least a week and can often be bought commercially. If commercial solutions are available, the best practice is to use a fresh beaker of buffer solution for each calibration. If the lab does not have buffer solutions prepared, you can prepare them using the procedures outlined in Standard Methods.
- Turn the pH meter on and allow the instrument to warm up.
Every brand of pH meter operates slightly differently, so we will not explain which buttons to push for every stage of the procedure. Instead, you can refer to the instrument's manual, if necessary.
- Compensate for the temperature.
pH is slightly temperature-dependent, so we use the following steps to ensure that changes in temperature do not cause errors in readings. You will also notice that we record the temperature of the sample in a later step so that we will know at what temperature the pH was taken.
- Bring all of the buffer solutions, the sample, and the pH meter to the same temperature. (The simplest method is to bring all to room temperature.)
- If the pH meter does not have automatic temperature compensation, measure the temperature of the buffer solutions and adjust the temperature compensation dial as necessary.
- Use buffers to calibrate the meter.
To get the most accurate pH reading of our sample, we calibrate the meter before each use. During calibration, we place the electrodes in a series of buffer solutions with known pH values and set the meter to those values. Typically, we will use a buffer solution with a pH of 7.0, a buffer solution with a pH of 4.0, and a buffer solution with a pH of 10.0 for calibration, though we can use other buffers if necessary. In all cases, the second and third buffers should have pH values at least two pH units higher and lower than the first buffer solution. The following procedure is used to calibrate the meter to each buffer solution.
- Remove the electrode(s) from storage solution, rinse them with distilled water and carefully blot the electrode(s) dry with a high quality laboratory tissue.
- Place the electrode(s) in a buffer solution and measure the pH of the buffer while gently stirring the solution.
- Adjust the pH meter reading to the known pH value of the buffer solution, if necessary. Then place the instrument in standby mode.
- Repeat this procedure with the next buffer solution.
- Prepare the sample.
In order to get a good pH reading, you should ensure that the sample is not contaminated and that it is well-mixed. You can mix the sample using either a magnetic stirrer or a stir bar. In either case, the goal of stirring is to ensure that the sample is well mixed so that the pH reading is representative of the entire sample. Do not stir too vigorously or carbon dioxide will enter the water, forming carbonic acid and lowering the pH.
- Record the source of your sample water in the Data section.
- To prevent contamination, rinse out a sample cup several times with sample water and fill it with the sample water to be tested.
- If a magnetic stirrer is available, place a clean stir bar in the sample water and turn on the stirrer. If not, you will need to mix the sample with a stir bar at intervals.
- Measure the pH of the sample.
- Determine the sample's temperature and record the temperature in the Data section. Adjust the meter's temperature compensation dial if necessary.
- Remove the electrode(s) from the storage solution, rinse the with distilled water and carefully blot the electrode(s) dry with a high quality laboratory tissue.
- Place the electrode(s) in the sample. Continue stirring the sample as the pH is measured by the meter. Record the sample's pH in the Data section.
- Place the instrument in standby mode and remove the electrode(s) from the sample. Rinse the electrode(s) thoroughly with distilled water and carefully blot dry.
- When all samples have been tested, place the electrode(s) in tap water or pH 7.0 buffer solution for storage and shut off the meter.
Source of Sample
American Public Health Association, American Water Works Association, and Water Environment Federation. 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.
The procedure outlined above can also be viewed in the virtual lab. There is an assignment that needs to be completed concerning the virtual lab, so please print the assignment first and then answer the questions as you perform the lab. Once you have the assignment completed, log in and complete the assignment online to be entered directly into the database.