Swimming Pool Testing Terminology

Testkit and Testing Glossary

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A B C D O P S

Acid: 1. Any chemical which lowers pH. Common acids include lemon juice (citric acid), vinegar (acetic acid), Vitamin C (ascorbic acid), and muriatic acid (hydrochloric acid). Sodium bisulfate is a granular acid sold in the pool trade as 'PH Down' or other similar names.

2. The condition of being acid. A pool with a pH level below 7.0 is referred to as 'acid'.
Acid Demand: A measure of the amount of acid required to return (pool) water to the desired pH. This is an important measurement for pool service guys, who try to make the entire adjustment at one time, since they may not return to the pool for a week or more. It is much less important for homeowners.
Alkalinity: A measure of the amount of acid required to reduce the pH to a particular level. In pool testing, this level is pH 4.6. In pool chemistry, any number of chemicals can contribute to alkalinity, including carbonates/bicarbonates, borates (borax), and cyanuric acid (stabilizer).

In all pools, a certain amount of alkalinity is important to help stabilize the pH. In concrete pools (plaster, Marcite, DiamondBrite, etc.) alkalinity is even more critical, to avoid corrosion of the pool's surface. Excessive alkalinity is one of the most important factors leading to scale at the water line and in heaters, and to metal stains.

Contrary to virtually all pool literature, alkalinity can NOT effectively be lowered simply by adding acid to your pool. Adding acid will lower BOTH the pH AND the alkalinity, but as soon as you restore pH levels the alkalinity will reappear as well.

A combination of aeration AND lowered pH CAN lower alkalinity. Also, in pools with high alkalinity, the common 'PH UP' based on soda ash (sodium carbonate or washing soda) should NOT be used. Use borax to raise pH, instead.

-- see "pH" for more information --
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Balanced Water: A fairly general term that is used in several different ways in the pool industry.; The most specific meaning refers to pool water in which 5 factors (pH, carbonate alkalinity, calcium hardness, temperature and total dissolved solids) are in a particular relationship which is thought to avoid both scale formation AND corrosion. A variety of calculated index values are used to determine whether water is 'balanced' or not, including the Langlier, Ryznar, and Hamilton index. The PS230 kit I sell includes a sort of a slide rule which can be used to calculate the Langlier index.
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Calcium Hardness: A measurement of how much calcium is dissolved in the pool water. Too much, and you'll tend to deposit scale at the water line, around fountains, and inside the heater (if you have one). Too little, and you'll tend to erode plaster, concrete, tile grout, and the like.

There's no straight answer to how much is just right: it depends on pH, alkalinity and so on.

500 PPM is more than you'd ever want to put in your pool water on purpose.
Below 100 PPM is too low for any type of concrete pool, but not for vinyl or fiberglass pools.
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Drop Count (titration): A titration is a chemical test method, in which you put a special dye (indicator) into your sample, and then drip bits of a reagent in the sample, till the dye changes color. The point at which the color change occurs is called the 'end point' for that titration. In a lab, you'd use a very accurate 'dripper', called a buret. But, for pool purposes, a dropper bottle with a specially calibrated tip, like the ones in the PS230 is accurate enough.

When you use the dropper bottle, you count the number of drops it takes change the indicator's color, and then multiply the number of drops by the factor for that test.

One very important point, for all titrations, whether you use a 'dripper', a dropper, or a buret, is to SWIRL the sample, between each drop, especially when you get close to the end point.

A second point is that end points can be 'fuzzy'. That is, the color changes, and then it changes back. With the PS230, you want to keep dripping till the color stays changed for 10 seconds. So, if you drop a drip, and the color changes, and then, after 5 seconds, it changes back, drop another drip. Hopefully, that drop will keep the color changed for 10 seconds. If not, drip a drop one more time.
DPD: DPD is an 'indicator' used with to test for halogens, including chlorine, bromine, and iodine. It has a very long chemical name, which I'm not going to reproduce here, and it's not good for you, so don't eat it or put it in your eyes.

In the PS230 kit, DPD is present in two forms: as a powder (R-0870) used with the chlorine drop count method, and in drops, used the the color match chlorine test.

When you put DPD in a water sample that contains a little chlorine (0.1 - 15 PPM) it turns pinkish, or red. When there is more chlorine than that, it may bleach out, and remain clear. The powder is more resistant to bleaching than the drops, but if the chlorine level is really high, you need to be sure to use 2 scoops of DPD powder, instead of just one.
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OTO: OTO is an 'indicator' used with to test for halogens, including chlorine, bromine, and iodine. It's chemical name is orthotolidine, and neither it, nor the hydrochloric acid it's dissolved in is not good for you, so don't eat it or put it in your eyes, either.

OTO is in the chlorine test drops found in the PS200 kit, and many other OTO kits.

When you add OTO to a sample of water that contains 0.2 PPM of chlorine, or more, it turns light yellow, to brown. In the usual range, from 1 PPM to 3 PPM, it's yellow. Unlike DPD, OTO does not bleach out at high chlorine levels. At higher levels, it's dark yellow. Don't swim in the pool, if the sample turns brown.
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pH: pH is a somewhat strange measurement that answers the question, "How acid is it?"

Commercial muriatic acid has a pH of about 0; dissolved crystal drain cleaner (sodium hydroxide) has a pH of about 14. The chemical neutral point is 7, but that's normally too 'acid' for most pools. Instead you'll want your pool in the slightly basic range ('basic' is the opposite of 'acidic') range from 7.4 to 8.0.

The relationship between pH and alkalinity is confusing. If you understand the relationship between temperature (measure in degrees) and heat (measured in calories or BTUH) Just as temperature answers the questions, "How hot is it?", but NOT the questions, "How hard is it to change it's temperature?", so pH answers the question, "How acid is it?", but not, "How hard is it to change it's pH?". This is sometimes measured as "acid demand" or "base demand", and sometimes as "total alkalinity". The difference between these is confusing and for most people it's best to avoid acid/base demand measurements.

You'll see all sorts of things in print about the normal pH range of this or that part of your body . . . and you should ignore it. Different parts of your body have different pH levels. For example, your ear canal is preferably rather acidic. When it's not, you tend to get ear infections, which is why vinegar (containing the weak organic acid, acetic acid) can prevent them.

Also, don't panic just because something is acid.

You eat acids ALL the time: carbonated drinks contain carbonic acid, to which the bottler adds phosphoric acid (also used to clean filters) in order to keep the drink acid enough to prevent the fizz (carbonic acid) from leaving. Vinegar contains acetic acid (sometimes, quite a lot of it!); lemon juice contains citric acid; vitamin C is ascorbic acid, and . . . you contain muriatic acid (hydrochloric acid) in your stomach, where it helps dissolve your food.
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Stabilizer: Stabilizer is cyanuric or isocyanuric acid. It's also called conditioner or sunshield in some parts of the country. It combines with chlorine in pool water (but NOT bromine) to prevent sunlight from destroying the chlorine (UV photolysis). But, when it combines with chlorine, it also weakens it. This isn't always bad, but it's always something you should keep in mind.

Without stabilizer, the water at the center of your pool, on a bright sunny day, might have no chlorine in it at all. In fact, a study that Monsanto did a number of years ago suggests that at noon on a sunny day, in the top foot of water in a clear pool, the half-life of unstabilized free chlorine is as little as 15 minutes.

But, with stabilizer, it will take a somewhat higher level of chlorine to kill germs and particularly, to kill algae.

Stabilizer reduces photolysis, reduces chlorine corrosion of metal components, reduces the irritating effects of chlorine . . . but it also reduces chlorine's ability to oxidize and sanitize.

The more stabilizer, the greater all these effects, good and bad, become.

Generally, you want stabilizer levels in an outdoor pool between 30 and 70 PPM.
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