Guest Column - September 2005
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Shedding Some Light on Water Testing

Choosing the right light source for color-matching tests

By Michael Gardner

If you're responsible for maintaining water quality in a regulated pool or spa, then you're familiar with the color-matching tests used to monitor parameters such as pH, chlorine, bromine, copper and iron. In fact, you probably use these "colorimetric" tests several times each day to measure pH and sanitizer levels in accordance with health department requirements. Tests based on color comparison are popular because they're cost-effective, easy to use, durable and accurate. However, it's important to perform these tests under the right lighting conditions because the light source can affect your color perception. Read on to learn how understanding this concept can help you to maximize the accuracy of your test results.

Matchmaker, matchmaker

Colorimetric tests are all about making a match. With liquid test kits, you add reagents to a water sample where they react with the analyte of interest (for example, free or total chlorine, bromine, pH, etc.) to produce a color proportional to its concentration. You then compare the color of this treated sample with a set of printed-, liquid- or plastic-color standards until a match is found. The numerical value assigned to the standard is your reading. Working with test strips is similar: You compare the color of the reacted pad(s) to the color(s) provided in the chart on the label. All of this seems pretty straightforward and foolproof. Or is it?

An Alternative to Color Matching

Do you have trouble differentiating between color standards—such as the pink ones used in a standard DPD chlorine test—even though you're careful to use proper lighting? If so, you may be among the 6 percent to 8 percent of people (mainly men) estimated to have deficiencies in their red-green color vision. Fortunately, there are test choices available that completely eliminate the need to match colors. For example, some chlorine test kits employ what's referred to as an FAS-DPD titration. This drop-count variation on the colorimetric DPD method is rapidly gaining in popularity for several reasons:

  • It features an endpoint color change from pink to colorless, eliminating the need to match colors (see photo). All you do is count the number of drops dispensed until the color changes.
  • It reads chlorine from zero to 20 ppm.
  • It offers excellent sensitivity (your choice of 0.2 or 0.5 ppm increments).
  • It can determine a low concentration of combined chlorine even in the presence of a high concentration of total chlorine.
  • You don't have to perform subtraction to get your combined chlorine level—it's read directly.

NOTE: While FAS-DPD can be used for troubleshooting problem water without restriction, be sure to check with health authorities to verify it is approved for your mandated testing (that is, meets reporting requirements).

Seeing is believing?

Imagine this. One morning you're in the bedroom getting dressed for work, and you put on a pair of blue socks under incandescent lighting. You then go to the kitchen to grab some breakfast and notice, under fluorescent lighting, that your socks don't match after all. One of them is blue, but the other is actually black. What's going on? It's an optical phenomenon called metamerism, which explains why two objects can match in one kind of light but not in another. You may have experienced a similar situation if you've ever purchased a suit jacket and pants—or shoes and a purse—that appeared to be the same color in the store but were obviously of different hues when viewed outside in daylight. Two colors that match under one light source but not another are called metamers or a metameric pair.

To go back poolside, suppose you're performing a test for free chlorine. You add reagents to a water sample, which will turn pink if chlorine is present. Next, you contrast the color of the treated sample to the color standards—ranging from light to dark pink—provided in your test kit to determine the free chlorine concentration. If you compare colors in the artificial lighting of the guard station, it's possible the sample will appear to match one color standard yet another standard when viewed outside in daylight. Which is the correct match?

Read the user's guide

Manufacturers of quality colorimetric tests take lighting into account when designing their products, so be sure to follow their instructions to avoid a false color match. For example, one brand might be designed so its color-matching tests work best when viewed in daylight. Practically speaking, while northern light is preferable, daylight from any exposure will give you a good color match as long as you are not holding the comparator directly up to the sun and looking through it. Another manufacturer specifies testing in a "shaded area, viewing test results against a lighted background." Still another says, "Always read test results against a white background" and suggests holding a piece of white plastic or paper behind the comparator. (Sunglasses are a no-no with anyone's kits.) Note: Turbidity in a water sample as well as color from dissolved metals or algae also can interfere with matchmaking. If the water you're monitoring is turbid or colored, be sure to choose a test method that can compensate for this.

But what if you must test indoors, away from the daylight you need for reliable results? When testing under these conditions, it is advisable to use a special photographer's lamp—not standard fluorescent or incandescent room lighting—to simulate daylight for a proper color match. Pricing depends on their quality and uniformity of illumination, size and portability, but you can expect to pay in the range of $100 to $150 retail for a reliable "lightbox."

Michael Gardner is communications coordinator at Taylor Technologies, Inc., the Sparks, Md.-based manufacturer of water-testing products. For more information on color-matching tests, titration tests or pool/spa water chemistry, visit