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Swimming Pool Water Chemistry
And Water Testing

Testing Your Water

The subject of water testing may transport you back to the days when you were in school and final exams were approaching -- all too quickly for your comfort. Rest assured, pool- and spa-water tests are much less painful than you may imagine. What's more, regularly testing your pool and spa water is a critical part of any maintenance routine. Each element defined in the Keeping Your Water Balanced section has a specific test to give you an up-to-the-minute evaluation of your pool and spa water. Once again, your pool/spa supply shop is the best place to buy these test kits. To give you a brief outline of their use, we review all the various tests you -- or your service professional -- must perform on your pool and spa.

How Much of What Chemical?

This is the science of water treatment. The table below, Chemical Adjustments, is part of the formula. But each pool has its own personality and demands. Only through record keeping and experience can one determine and perform the correct tasks.

  Unit Minimum Ideal Maximum
Free Chlorine, ppm 1.0 1.0 - 3.0 3.0
Combined Chlorine ppm None None 0.2
Bromine ppm 2.0 2.0 - 4.0 4.0
pH No 7.2 7.4 - 7.6 7.8
Total Alkalinity ppm 60 80 - 100
(for liquid chlorine, Cal-Hypo and Lithium Hypo)

100 - 120
(for Gas Chlorine, Dichlor, Trichlor and Bromine Compounds

TDS ppm 300 1000 - 2000 3000
Calcium Hardness ppm 150 200 - 400 500 - 1000+
Cyanuric Acid ppm 10 30 - 50 150

Sanitizer/Oxidizer Tests

There are two well-known test kits available to test levels of chlorine and bromine, the DPD and OTO. Each has a long, scientific name, but all you really need to know are the initials. The DPD is the preferred of the two because it tests for free available chlorine -- the most powerful "killing form" of chlorine. Its chemical reagent reacts with the free available chlorine in the sample water to change its color. The ideal range of 1.0 to 3.0 ppm for pools or 1.5 to 3.0 ppm for spas will command a pink to red color. To get an accurate reading, just compare your test sample to the manufacturer's chart that should be included with the test kit.

Note: An extremely high level of chlorine tends to bleach out the test water, making it appear as if there isn't any chlorine present. This would normally prompt you to add more chlorine, and in most cases, that's correct. If you think you have received a false reading, conduct the test again, except this time, fill the test vial with 50 percent chlorine-free or tap water and multiply the results by two. This will help you determine whether you have a low level or very high level of chlorine in your water.

The other, less common, test is the OTO, which works on the same basic principle, but will turn the water yellow to deep orange when chlorine is in the proper range. One of the major differences between the OTO test and the DPD test is that OTO cannot distinguish between total chlorine and free available chlorine. You won't get an accurate estimate of how much killing chlorine is in your pool or spa water. A final note: Bromine can also be tested using either method, but the results need to be multiplied by 2.25 to yield a true reading - - unless, of course, you have one of the test kits that include a bromine color chart as a reference. So far so good? Let's move on to pH.

pH Readings

The most common pH test for pool and spa owners is the phenol-red test. Just like the DPD and the OTO, the phenol-red test has a reagent that mixes with the acid in the water to cause a color change in the sample. If your pH is in the correct range, the water sample should turn a shade of red ranging from pink to orange. If you get a yellow result, you have a low pH or acidic water. If the color is a deep purple, your pH is too high. Once again, you'll need to compare your results with the color chart provided with the test kit to get an accurate reading.


pH of water is determined by adding a reagent (containing an organic dye) to the sample at a measured rate. Reagents to cover the entire pH range of 1.0 to 14.0 are available. Because most pools are kept in a pH range of 7.2-7.6, the pH test that uses phenol red as a reagent is particularly effective (pH range of 6.8-8.2). Phenol red is available in either liquid or tablet form. To test with phenol red, add one tablet or a specified number of drops of liquid, to the water sample, and compare the resulting color to the test-kit standard. At the low end, the sample is yellow, developing an increasingly red color as the pH increases. At the high end, the color is almost purple.

Some reagents are subject to bleaching by chlorine; others react with the chlorine to form new compounds that can give false readings. Bleaching action should be neutralized by adding a 2-4 drops of chlorine neutralizer (sodium thiosulfate) before the test is performed. (Some indicator solutions sold specifically for pools already contain a chlorine neutralizer to combat this bleaching action.)

Solution Method for Testing for pH Value

  1. Rinse the sample cell in pool water.
  2. Fill the sample cell to the marked fill line with pool water.
  3. Add 2-4 drops of chlorine neutralizer to remove the chlorine and prevent bleaching. (As mentioned earlier, many test kit manufacturers market pH indicators containing chemicals that eliminate the bleaching effect of chlorine, making this step unnecessary.)
  4. Add the prescribed number of drops of phenol red to the sample cell.
  5. Cap the sample cell, and mix the contents by inverting several times, or swirl. Do not use a finger in contact with the solution.
  6. Compare the color of the sample with the test-kit color standards.

Look for a hue or color comparison, not a color intensity, as in the chlorine test. A shade between two standard color values indicates that the pH is the midpoint between the values assigned.

Tablet Method for Testing for pH Level

  1. Rinse the sample cell in pool water.
  2. Fill the sample cell to the marked fill line with pool water.
  3. Add one phenol red indicator tablet to the cell. Invert the sample cell to dissolve the tablet.

CAUTION: Do not shake the solution vigorously. If the tablet fails to dissolve with gentle agitation, crush it with a clean plastic rod.

After the tablet has dissolved, compare the color with the standards, as above.

Note: The spa's temperature makes a definite impact on the accuracy of this test. Therefore, the test should only be conducted with spa water when its non operating temperature is between 60 and 80 degrees Fahrenheit. Adjusting pH is a very simple task. If you have a low pH, add soda ash or sodium bicarbonate. If your water's pH is high, add liquid muriatic acid or dry acid. (Remember, both soda ash and muriatic acid are too strong for use in spas.) Before you attempt to change the pH, however, you need to perform a second test to figure out how much of which substance you'll need to add to the water. A base-demand test determines how much soda ash or bicarb is required and an acid-demand test calculates how much muriatic or dry acid is needed. These tests are called titration tests or end-point reactions. In this type, the number of drops of a special liquid it takes to cause a color change corresponds with the amount of soda ash or acid required. Consult the test kit manufacturer's recommendations for the exact amounts. One more caution: Before doing anything about your pH levels, you also need to test the total alkalinity of your pool or spa water, as described below.

Total Alkalinity Test

This test -- the last of the regularly performed tests -- measures total alkalinity. As noted above, this test should be done before you do your pH test and adjustments. The two factors are interrelated and must be dealt with together. Total alkalinity is determined by using a titration test with two reagents. The first changes the color of the test sample, the second triggers an end-point reaction. It's with the second test you count the number of drops of reagent added to the water -- swirling the sample after each drop -- until it changes color. Note: This reaction occurs very suddenly so don't add the second reagent too quickly.


Take a 100ml pool water sample in a stoppered bottle. Using Palintest Alkalinity M tablets, add one tablet at a time shaking the bottle until dissolved. Keep adding tablets until the color changes from yellow to bright pink. Count the number of tablets used and work out the alkalinity - Alkalinity = (number of tablets x 40) - 20 ppm.

Testing for total alkalinity is essential to make proper determinations of the saturation index as well as for bather comfort and ease of pH control.

A pool's total alkalinity may be determined through use of one of many commercially available test kits. A quantity of pool water is mixed with an indicator; a blue color reveals the presence of alkalinity. A reagent is added to this mixture from a dropper. The operator counts the number of drops necessary to neutralize the alkalinity and bring about a color change. Alkalinity can be determined by multiplying the number of drops used by a constant as provided by the test kit manufacturer (usually 1 drop = 10 ppm).

Tablet methods also are available for determining alkalinity. Although less accurate than the drop counting method, the process is easy to follow and is less susceptible to operator error. Tablets are added one at a time until the desired color change takes place. When sufficient tablets have been added to bring about the end-point color change, the number of tablets required is multiplied by the constant provided by the test-kit manufacturer.

It is recommended that the results of total alkalinity be considered before adjusting pH. The direction of pH change, or even the need for adding chemicals, is greatly influenced by the level of total alkalinity. Total alkalinity does not vary quickly and often is only tested once a week.

Calcium Hardness Levels

Tests for calcium hardness are performed at the beginning of the swim season, when you start up a new pool or after draining and refilling a pool or spa. After that, they only need to be done every three months or so. But don't forget about it entirely because when allowed to go too high or too low, the water's hardness levels can cause all kinds of problems. (For more on water hardness, see the section on Keeping Your Water Balanced.) Like total alkalinity, an end-point reaction best measures calcium hardness. The first reagent is a pH buffer to bring the pH level up to approximately 10. The second step adds a dye that, when reacting with calcium, turns the sample water a different color. Next, the titrant (EDTA) is added one drop at a time until the water changes color. The total number of drops required gives you the amount of hardness when compared with a manufacturer's chart.


Take a 100ml pool water sample in a stoppered bottle. Using Palintest Calcium Hardness tablets, add one tablet at a time shaking the bottle until dissolved. Keep adding tablets until the color changes from pink to bright violet. Count the number of tablets used and work out the hardness - Hardness = (number of tablets x 20) - 10 ppm.

Calcium hardness in a freshly filled pool can be approximated by taking 70% of the total hardness level. However, more accurate measurements can be made by using a test kit designed specifically to determine calcium hardness and by following the manufacturer's directions regarding color changes.

In these tests, a specific amount of water is taken from the pool, and an indicator is added to the sample. A reagent is added, causing a color change that indicates the calcium hardness level in ppm.

Total Hardness


To test for total hardness, an exact amount of pool water (usually 60 ml) is treated with a solution called a buffer. Then a dye is added. The reagent is added to the sample and mixed, one drop at a time. The number of drops necessary to change the water color, multiplied by a constant provided by the manufacturer, determines the hardness in ppm.

The tablet method for testing water hardness is equally simple. A tablet containing the pH buffer, the indicator dye, and the hardness reagent is added to a 100 ml sample of water. The color changes as additional tablets are added. The number of tablets required to bring about this change is recorded and multiplied by a constant provided by the test-kit manufacturer.

Total Dissolved Solids


Total dissolved solids (TDS) is the measurement of all materials dissolved in the water, i.e., calcium, carbonates, dissolved organic and inorganic materials, salts from chlorine residue, swimmer waste, soluble hair and body lotion or anything placed in the pool that can be dissolved.

TDS is measured by using a portable electronic analyzer. The equipment is specifically used to measure for dissolved solids and should be standard equipment for operation of spas and hot-water pools. A pool should be dumped and refilled when a TDS reading exceeds 1,500 ppm above the domestic water supply reading. This equipment is quite expensive and is not normally at the disposal of the private pool owner, so you have two choices:

  1. Take your sample of pool water to your pool center and ask them to measure TDS for you. They will normally be happy to do this for you free of charge if you usually buy your chemicals etc from them.
  2. As TDS plays a less important part in the calculation than the other factors you can estimate it, but do bear in mind that this will then only give you a rough idea of whether the water is scale forming or corrosive. As a guide, for pools which have been recently refilled, use a figure of 750ppm. For pools which have not been recently refilled, or where there has been little water replacement by backwashing, or where cyanuric acid levels are above 200ppm, use a figure of 1500ppm for TDS.

Super Chlorination or Shocking

Super chlorination is a term that describes an extra large dose (usually 8 to 10 ppm) of chlorine to oxidize organic compounds and kill and remove algae and other contaminants from the water. This is the same as using three to six times the normal dosage of a chlorinating agent. For example, a 50,000-gallon pool requires about four gallons of liquid pool chlorine (12% Available Chlorine) or six pounds of a granular chlorinating compound such as calcium bypochlorite (65% Available Chlorine).

As mentioned, HOCI is the form of chlorine that provides sanitation. Because it is an extremely active chemical, however, it also reacts with organic impurities. When there is enough HOCI present, the impurities are completely oxidized. Combined chlorine is formed when there is an insufficient supply of HOCI or when there is a very high level of organic impurities. Combined chlorine compounds can be oxidized by increasing the HOCI level in the water. The point at which all the organic impurities are oxidized is called the breakpoint. The addition of sufficient chlorine to reach this point is known as breakpoint chlorination.

Every once in a while, your pool or spa water may become a veritable hotel of unwelcome contaminants and bather waste products. You can often detect when this happens because a "chlorine" odor may begin emanating from your pool or spa, or you may notice that you're experiencing some skin and eye irritation. But the most reliable -- and better -- way to monitor your water quality is to take a second look at your sanitizer readings. If your chlorine test readings keep dropping hard and fast, you may be faced with the need to evict chloramines from the water. (For more on how chloramines form, see the section on Keeping Your Water Balanced.) This is done easily enough with a process called shocking. No, you don't electrocute the chloramines. What this process entails is adding extra high doses of sanitizers to the water. By adding a large dose of chlorine -- a process called super chlorination or shocking -- you bring up your residual of free available sanitizer in a relatively short period of time. The newly added sanitizer will promptly rid your pool and spa water of those annoying guests. To be safe, do not use your pool or spa for at least 24 hours after shocking with chlorine. You should also test for the proper sanitizer levels before getting back in the water.

Note: There are non chlorine shocking products available that lessen the amount of time swimmers need to stay out of the pool or spa. Check with your local pool/spa supply store or professional service technician for more information.

Adding Chemicals to Water

  1. Add large amounts gradually in thirds over a 2-hour period.

  2. Add directly into the pool or spa when no swimmers are present and time is sufficient to permit even distribution of the chemicals

  3. Add indirectly in small amounts slowly through skimmer or overflow to prevent corrosion of metals

  4. Add chemicals through feeders or feeder lines that follow pump and filters, especially DE filters.

  5. Add granular chlorine or soda ash solution directly to the pool, but separately. Always mix chemicals into plastic containers that have been filled with water first.

  6. Add chemicals evenly by walking the perimeter of the pool.

  7. Add chemicals to achieve maximum for Free Available Chlorine, FAC (3.0 ppm) and minimum pH (7.4) parameters in anticipation of a heavy bather load. This is an effort prevent falling below minimum standards in FAC during or following the loading period.

  8. Add chemicals frequently to prevent highs and lows in readings. Large reading fluctuations are hard on soft metals and produces a bounce effect on water treatment.

  9. Add additional chemicals only following an adequate time period that permits a second or third chemical reading.

  10. Add chemicals in sequence to adjust for (1) FAC, (2) Total Alkalinity, (3) pH, (4) Cyanuric Acid and (5) Total Hardness.

  11. An effort should be made to offer adequate time for chemical distribution and most importantly, a second reading before leaving the scene. Screen test for pH, FAC and combined chlorine first, and if pH is extreme, test for total alkalinity. Test for balance or saturation, TDS and metals can be decided based on the history of the pool.


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