Best Practices When Testing Water
Wednesday, May 3, 2023
Start the swim season with fresh reagents, and replace missing instructions and any broken, faded, or stained equipment.
Reagents are perishables. Just like a head of lettuce or the pills in your medicine cabinet, the chemicals in a test kit (or on a test strip pad) will degrade over time, even under optimum conditions. The process of deterioration speeds up though when storage conditions are not ideal. Extremes of heat and cold, as well as prolonged exposure to air/sunlight/humidity/moisture/treatment chemicals, diminish their useful life. You won't get accurate readings with deteriorated reagents or with stained test cells or faded color standards. And no matter how many times you've performed a test, it's always best to have the manufacturer's instructions handy to double-check your testing procedure.
When you do replace reagents or labware, don't interchange products from different manufacturers or even within one manufacturer's product line.
Each manufacturer makes its reagents in different concentrations. Their color standards are developed for specific reagent concentrations and the view depths of test cells are also highly specific. (For instance, Taylor produces several different phenol red solutions for pH testing and a different type of color comparator is used with each.) The exception is that DPD liquids and tablets can be interchanged in most visual color-matching chlorine/bromine tests.
Read the manufacturer’s instructions carefully before beginning a test.
Not only are each manufacturer's tests different, sometimes a manufacturer changes a procedure. Take test strips. To get the proper exposure of reagent to water chemistry, you can be instructed to dip quickly, swirl the pads for a specified number of times, or swish the strip back and forth. Dip when you should swish, and color development will be compromised. If the wait times to observe between readings have changed and you're unaware, you will also get unreliable results.
Work with the specified sample volume. Add the specified amount of reagent(s). Mix as directed.
If a recipe calls for a teaspoon of salt and you add a tablespoon by mistake, the result will be inedible. Water tests are recipes too. When a chemist develops a test procedure, everything is carefully calibrated to work together to give an accurate reading, from the sample size, to the amount of reagent dispensed, to the way in which the chemicals are combined with the water sample, to the timing between each step of the procedure. If the instruction says to crush the tablet then stir until it's completely dissolved, you must do this or the reaction will be incomplete. If you're supposed to swirl after each drop and you decide to shake the sample instead, you could unwittingly alter the pH and alkalinity. Bottom line: failure to follow the recipe exactly will result in an inaccurate reading. An inaccurate reading will result in under- or overdosing the chemical treatment…or maybe not being aware any treatment is needed!
Work with a water sample that's representative of conditions in the pool as a whole.
If a pollster wants to learn if Americans support longer school days and he only asks students, his results will be misleading. If he only asks teachers—another special interest group—his results also will be misleading. In fact, the pollster will follow a strict protocol to make certain his report reflects the opinions of a wide swath of the public. Water sampling is like polling in that you don't want to base your assessment on atypical answers. For instance, water chemistry at the surface of the pool is atypical—it's interacting with the air's chemistry, evaporation is taking place, and it's where oils and debris float. Water chemistry at a return line, makeup water inlet, or chemical feeders is atypical—treatment chemical concentrations are different in these locations than in the pool at large. Water drawn from corner locations may not have experienced the mixing action open areas have. To get a representative sample, draw the water from mid-pool…or test at both the shallow and the deep end then average the results. Grab your sample from at least elbow depth. Then test the sample immediately so that its characteristics do not have time to change.
Conduct color-matching test in natural daylight or with the aid of a daylight simulator.
Anyone who has ever walked out of the house wearing two pieces of clothing that looked liked they matched until they got in the daylight knows about this problem. The term for the phenomenon is metamerism. Different light sources make for different colors. If a test is designed to be read in daylight (after all, pools are mostly outdoors!), artificial lighting (i.e., incandescent and fluorescent) will affect your color perception. Sunglasses do too, so take them off.
Practice good housekeeping: keep chemicals away from kids; don’t use a glass container that could shatter to collect sample water; to prevent contamination, keep reagent bottles capped tightly and never interchange caps; to prevent stains from setting in as well as cross-contamination of tests, wash out any residue from the test cell and wipe up spills promptly.
WHY? Because you’ll be sorry if you don't.