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The Swimming Pool Operators

and Owners Resources Pages

Resource Page 4 - Basic Water Chemistry

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Common Chemicals found in Swimming Pools & Spa's

The common chemicals in pool and spa water can be sorted in to three distinct groups

(1) Chemicals used to disinfect the water

(2) Chemicals that affect the water "balance"

(3) Residual chemicals

Group No 1 includes Chlorine, Bromine, or any other disinfecting chemical used.

Group No 2 includes pH, Total Alkalinity and Calcium Hardness

Group No 3 includes Cyanuric Acid, Sulpahates, and Chlorides.

Groups No's 1 and 2 are essential to maintaining a correctly operated pool or spa. Group 3, we can do without! - they are undesirables, although in an outdoor pool, a little cyanuric acid IS useful.

Most of the chemicals used in swimming pools are compounds of a variety of different elemental chemicals. They are produced as compounds for a number of reasons - usually safety or convenience. We use the principle chemical to alter something in the pool water, and that chemical becomes used up. However, the other chemicals in the compound are not often used, and remain in the pool water. These are the residuals.

To understand this concept better, consider doing some "home improvements" - say fixing some new hinges and locks to that jewelry case (pool water). Ok - first we go down to the store to buy our "fittings" (our chemicals). When we get our fittings we find not only do we have our hinges and screws which we NEED (our chlorine or acid), we find that our fittings are packed up with cardboard and plastic (the residuals) - we DON'T need or even want the cardboard and plastic, but the fittings are PACKAGED in this so we HAVE to have it! - In other word, our chlorine and acids, etc are "packaged" with these residuals.

Ok, so now we go to our workbench (pool ) and fit the hinges and screws to the jewelry case (add the chemicals to the water). In order to allow our hinges to work we have to tear of the cardboard and paper. This wrapping material (the residuals) is left lying on the bench (in the pool). If we were to do a lot of this sort of work, very quickly our workbench (pool water) would get so cluttered with the useless wrappings (residuals) that we would be unable to find our tools to do any more work - the wrappings have halted (or slowed down) our work, and in the same way, excessive chemical residuals will slow down or even stop the action of the important chemicals.

The only way we can start working more efficiently is to have a good clear up - and be can start by "clearing" some space on the bench by getting rid of some of these wrapping - in our pool we clear up by backwashing the filter! Backwashing the filter achieves two aims - (1) dirt and debris is removed from the circulation system, and (2) some water (with a high residual content) is removed from the pool to make space for fresh water (with a low residual content) to be added - dilution of the pool water.

Some residuals in excess cause other problems too. On our work bench the cardboard and plastic may be a fire hazard - but in our pool, some residuals will attack the very structure of our pool, - the grout, cement and concrete. Others attack any metal found in the system - ie heat exchangers, cast iron circulation pumps, and in older pools - iron piping.

Acceptable Chemical levels

  • pH value . . . . . . . . . . . . . Ideal 7.4 pH . . min 7.2 pH - max 7.6 pH
  • Free Chlorine . . . . . . . . . 1.5 ppm (mg/l) to 3 ppm (mg/l) . . .Max 5 ppm (mg/l)
  • Combined Chlorine . . . . . normal max 0.5 ppm (mg/l) (or less) . . absolute Max 1.0 ppm (mg/l)
  • Total Alkalinity . . . . . . . . . Ideal 100 ppm (mg/l) . . min 80 ppm (mg/l) - max 140 ppm (mg/l)
  • Calcium Hardness . . . . . . Ideal 300 ppm (mg/l) . . min 250 ppm (mg/l) - max approx 600 ppm (mg/l)
  • Cyanuric Acid . . . . . . . . .30 ppm (mg/l) (If used) . . max approx 60 ppm (mg/l)
  • Sulphates . . . . . . . . . . . . .New pools max 200 ppm (mg/l) . . older pools max 300 ppm (mg/l)
  • Chlorides . . . . . . . . . . . . . Max 600 ppm (mg/l)

pH Values

The pH test is probably the most important test regularly carried out on pool or spa water, and the pH value needs to be accurately maintained whatever disinfectant is used. However, correct pH values are essential when water is disinfected with chlorine.

pH is a measurement of the degree of acidity or alkalinity of water. It is not a quantative measurement - like 2 ppm, 10 apples, or 5 years. It is a relative measurement - like twice a long as, half the weight of, or three times more than.

The pH scale ranges from 0 - extremely acid, to 14 - extremely alkaline. Neutral - neither acidic or alkaline is 7.0 pH.

Pure distilled water has a pH value of 7.0

Adding an alkali to the water will cause the pH value to rise. Adding an acid to the water will cause the pH value to fall. It must be remembered that most chemicals used in swimming pools, even those not used for pH control, will be either acid or alkaline.

For example, liquid chlorine (sodium hypochlorite or bleach) is used only for disinfecting purposes, but it is very highly alkaline. So, every time that it is added to the pool to increase the chlorine level, the pH value of the pool water also happens to be increased.

Sodium hypochlorite is probably the most alkaline disinfectant with a value of about pH 13. Calcium Hypochlorite has a pH value of around 8.5. The "Dichlor" type stabilized chlorine granules have fairly neutral pH value, and the "Trichlor" chlorine tablets are reasonably acidic. Chlorine gas, when dissolved in water is highly acidic.

As a general rule, the pH value of swimming pool water usually tends to rise, requiring regular inputs of acid. However, in softwater areas, low pH values can sometimes be a problem, often needing alkali to be added to the water.

There are three acids available for swimming pool use :--

  • Dry Acid ( Sodium Bisulphate or Sodium Hydrogen Sulphate ) is a coarse white granular powder and is mixed with water to produce a weak compound of sulphuric acid. Dry Acid leaves a residual of sulphates in the water.
  • Hydrochloric Acid (Muriatic Acid) is an almost colourless liquid which is diluted in ordinary tap water before adding to the pool water. The ideal dosing solution is around 8 - 10%. This acid leaves a residual of chlorides in the water
  • Carbon Dioxide, when bubbled into water, dissolves readily and produces Carbonic Acid. CO2 dosing systems inject CO2 gas directly into the water within the circulation pipework. This system is probably the safest acid dosing system, since there is no liquid to mix, carry, splash or spill. CO2 is not poisonous, although in a confined space it will displace oxygen. Carbonic Acid leaves a residual of Carbonates

ALL ACIDS, when mixed directly with chlorine, produce a highly poisonous CHLORINE GAS!

The pH of the human eyes is 7.4 pH, so the pH of swimming pool water needs the be very close to 7.4 - in order to prevent eye irritation through incorrect pH levels. If the pool water is maintained at 7.4 pH, and eye irritation is still experienced - you can be certain that it is not due to the pH levels!

The Effect of Chlorine is Controlled by the pH value of the Water

The effectiveness of free chlorine in water is dependant on the pH value. Chlorine ( and most other disinfectants too ) work far more effectively in low pH values than at high pH values. It is also far more unstable at low pH values, and can be rapidly lost from the water.

When chlorine is introduced into water, it dissociates in to two parts, - Hypochlorous Acid (HOCL), which is the strong, fast, oxidizing disinfectant, and the Hypochlorite Ion (OCL), which is a very slow, weak disinfectant. The HOCL will destroy most organisms in less than 2 seconds, - usually less than half a second. The OCL can take up to 30 minutes to achieve the same results.

The ratio of HOCL to OCL is determined by the pH value of the water. ((See the graph on page 8))

Unfortunately, the usual test for Free Chlorine records both HOCL and OCL components as Free Chlorine, so unless the pH value is also known it is impossible to tell the percentage of HOCL present in the chlorinated pool water,

At a value of 7.0 pH, 75 % of the free chlorine exists as HOCL

At a value of 7.4 pH, 52 % of the free chlorine exists as HOCL

At a value of 8 pH, only 22 % of the free chlorine in the water exists as HOCL.

The free chlorine level determined by the DPD test kit will measure both HOCL and OCL together. There is no easy field test for just HOCL measurement

Consequently, although a test kit may show that there is a significant amount of chlorine in your pool water, if the pH value was, for example 8.2 - only 16 % of that chlorine would actually be working quickly to oxidize and disinfect your pool water.

In a commercial pool, this could cause a catastrophic failure of oxidation and disinfection, allowing bacteria to breed quickly in large quantities, and the water to rapidly become cloudy with dirt and minute skin cells.

TDS - Total Disolved Solids

Total Dissolved Solids refers to the amount of chemicals dissolved in the water. The TDS value is usually taken using a compact electronic meter.

Pure Distilled water has a TDS value of Zero - there are no chemicals dissolved it - it is pure H2O. If, say, 300 ppm of calcium was to be introduced into this sample of distilled water, the TDS value would now become 300 ppm.

If say 200 ppm of chlorides was now added to this solution, the TDS value would become 500 ppm ..... and so on.

Tap water can have a TDS value of anything between about 50 ppm (or less) and 500 ppm, (or more) depending on the location of the source, and the amount of treatment it requires before being piped to the consumer.

Swimming pools, due to the chemical input have a much higher TDS value, - usually in the region of 1,000 ppm to 1,500 ppm.

It will be seen that the TDS value of swimming pool water is a measurment of the TOTAL amount of various chemicals dissolved in that water.

The main contributers to the TDS value are - Calcium, Sulphates, Chlorides, and when used, Cyanurates.

Although historically, a great deal of faith is put into measuring and maintaining TDS, we consider this a very inacurate and outdated method of water quality control. Consider a hypothetical pool with a calcium hardness level of, say, 300 ppm, a sulphates level of say 200 ppm, a chlorides level of say 450 ppm, and a cyanuric acid level of say 50 ppm. All these figures are within acceptable operating levels, and total 1,000 ppm TDS. But, however, if the chlorides level was 50 ppm, and the cyanauric acid level was 450 ppm, there would be a very serious disinfection failure, requiring the pool to be completely drained and refilled with fresh water, despite a TDS reading of 1,000 ppm which appears to indicate that all was well!

The above example indicates that it is very important to know the individual residual levels, and not simply rely on the overall figure.

Most TDS meters are relativily expensive (in comparison to other pool test equipment). They also often lose their accuruacy as batteries deteriorate, or electrodes become dirty, and they should be calibrated frequently in a known value calibration solution.

Chlorine Stabilizer - Cyanuric Acid

Chlorine can be easily destroyed by sunlight. On a hot sunny day, 2 ppm of chlorine can be lost from a pool in as many hours through decomposition by sunlight. Some years ago, it was found that if the weak organic acid - Cyanuric Acid was introduced to form a compound of chlorine in swimming pool water, the action of sunlight destruction of chlorine was greatly reduced.

Since then, the demand for chlorinated isocyanurates has multiplied, and provides the bulk of the residential chlorine sales. The "Chlorine Granules" or "Dichlor Granules" popularly used for disinfecting residential pools contain cyanuric acid.

The proportion of cyanuric acid in stabilized chlorine granules is fixed chemicaly, and cannot be altered.

Whilst the available chlorine gets used, will dissappear, and will need replacing regularly, - the cyanuric acid residual does not get used up. Its job is solely to reduce the effect of sunlight decomposition on chlorine, and although the chlorine will eventually be lost, - the cyanuric acid remains in the water.

Adding stabilized chlorine to the pool water will add the the cyanuric acid residual already there. The problems start when there is too much stabilizer (cyanuric acid) in the water, - the chlorine becomes Over-Stabilized.

Normally, if the pool owner looks after their pool correctly, they will be backwashing the filter regularly - so losing water which is subsequently replaced with fresh water. It is this Dilution with fresh water which keeps the stabilizer at a satisfactory level. But if the pool owner neglects the pool, and backwashes infrequently, or for only a very short time, there is not enough dilution to control the stabilizer residual level - and of course, the cyanuric acid level will rise.

The action of cyanuric acid can be illustrated with a Boxer in a Boxing Ring. The boxer representing chlorine. You can easily "test" the presence of the boxer! - And if you entered the ring (pool) he could spoil your day very quickly! - But, - he could very easily jump over the ropes and be gone! (chlorine decomposition by sunlight)

To reduce the possibility of our boxer jumping out of the ring, we can stretch a net over the top of the ring (stabilizer). If you were to get in the ring, the boxer could still spoil your day, he may just take a little longer doing so because of the net - but he is still very effective! (Our chlorine is stabilized correctly)

But, if we were to add another net to the ring on top of the first, - and another, - and another, - and another, ..... and so on (too much stabilizer), our boxer would be weighed down by all the nets, and his effectiveness would be greatly reduced. You could still accurately "test" that he was there - but he would no longer be an effective boxer!

If the number of nets (stabilizer level) was to continually increase, eventually our boxer would be incapable of moving (Chlorine Lock). We could still determine that he was present in the ring - but our test would not show that he was, in fact, completely restricted by these nets. (We would need to "test" the number of nets to decide if our boxer could cause any harm to anyone)

This is a fairly good explanation of the effect of Cyanuric Acid on chlorine, in pool water. Probably the biggest problem we encounter is that of Chlorine Lock - too much cyanuric acid in the water, because the owner has not carried out sufficient dilution.

At this point we need to determine the correct level of stabilizer. We have seen a wide range of "Maximum Levels" suggested. Originally, 200 ppm was quoted as a safe limit, since this level is considered the maximum safe Health level for bathers. We still sometimes see this figure suggested as the "maximum level" allowed.

However, the proven reduction in disinfection takes place at a much lower level. Nowadays, as knowledge of the product increases, it is generally accepted by professional and experienced pool operators and engineers that the ideal level should be about 30 ppm, and a maximum acceptable level should be about 40 - 50 ppm. In an outdoor pool, if the stabilizer level is below about 20 ppm, chlorine is being wasted unnecessarily.

It has sometimes been suggested (by a stabilized chlorine manufacturer) that chlorinated water with Cyanuric levels greater than 400 ppm will actually experience an increase in disinfection activity, although how much killing power is due the very high and toxic level of Cyanurates themselves is not mentioned, and anyway, since the maximum level determined for health reasons is 200 ppm the subject is of little interest (except perhaps to manufactures and distributors of stabilized chlorine).

Another reason that high levels of Cyanurates should not be allowed or encouraged, is that Cyanuric Acid attacks copper! Sustained high levels of Cyanuric Acid will frequently reduce the life span of a copper heat exchanger considerably. And, of course, no cyanurates should be present if a Silver/Copper ionic purefier is used in a pool.

The presence of significant levels of Cyanuric Acid can also "interfere" with the Total Alkalinity test and cause the reagent to indicate a higher total alkalinity than is actually in existance. We have found that a "Corrected Total Alkalinity" level can be calculated by subtracting 1/3rd of the Cyanurate level from the Total Alkalinity Level.

i.e. Indicated Total Alkalinity 130 ppm, Cyanuric Acid level 90 ppm, - so, 1/3rd of 90 is 30, - 130 minus 30 = 100. Therefore, Corrected Total Alkalinity is 100 ppm

Stabilized chlorine is usually not necessary in an indoor pool, since its purpose is to protect chlorine from sunlight.

Automatic chemical control systems also suffer with the presence of cyanuric acid in the pool water. An increasing number of manufacturers will not consider installing their equipment if stabilized chlorine is present. The presence of cyanuric acid may "poison" the delicate and sensitive junctions in both chlorine and pH sensor probes, causeing the automatic dosing equipment to become unreliable and/or inaccurate.

Although the "trichlor" chlorine tablets can be, and often are used for commercial pools, the Stabilized Chlorine granules should never be used commercially, and are essentially designed for the home pool owner to use as the regular disinfectant. Some pool owners make the mistake of using these stabilized chlorine granules for "shock" treatment! This is incorrect - a non-stabilized chlorine (such as calcium hypochlorite or sodium hypochlorite) should always be used for any shock treatment.

What Happens if I don't Have Enough Chlorine

in my Pool Water?

Chlorine is not only a disinfectant, it is also an OXIDIZER. This means that it "Burns Up" small particles of debris. If you leave a leaf in chlorinated water, after a few days it will turn from green or brown to a whiteish color. The chlorine will have bleached all the pigment in the leaf. If you leave the leaf in the water for longer, - after several days the fleshy part of the leaf will have disappeared, leaving only the stalk and the veins.The chlorine has oxidised, - or burnt up - the fine material between the leaf veins. However, if the leaf is left in the water for even longer (and the chlorine level is maintained), the leaf will eventually disappear.

When people use a swimming pool, large amounts of fine debris are washed of into the water from their bodies. This debris is mostly dead skin cells! We are constantly losing skin cells all of our life. Most of the dust found in our houses is dead skin cells. Usually, a lot of these skin cells in the water are too small to be trapped by the filter and remain in the pool. The only way to get rid of them is by "Burning" them out with an oxidizer, such as chlorine.

If the skin cells etc. in the pool water are not removed or destroyed quickly, two things will happen!

(1) The water will start to look dull and flat, and maybe even hazey or cloudy

(2) Bacteria will start to feed on the decomposing skin cells etc., causing a massive increase in bacteria levels in the pool water.

The reaction of chlorine with organic debris is very similar to the action of a bonfire!

If we were to have a bonfire in our backyard, we would normally use dry materials - dry grass, paper, dead dry wood etc. - and we would soon have a roaring bonfire!! - There would probably be only a little smoke, and the dry material would burn very quickly - probably, once it gets going, about as fast as we can feed the fire. We would be able to stand around the fire quite easily because there would be very little smoke, and the material would burn away to nothing rapidly, leaving only a little ash. This example equates to a pool which is correctly chlorinated, - and does not have too high a bather load.

If, with our bonfire, we were to suddenly dump a whole barrow load of wet grass, wet leaves, and green wood on it, the situation would change dramaticaly. Suddenly we would be enveloped in dense smoke! This smoke would cause intense irritation to our eyes, amd make us cough and splutter! The fire would slow down and start to smoulder! The material we are burning will not burn fast enough, now, and we would have to stop bringing more stuff to burn! This is exactly what happens if too many bathers use a pool - or there is not enough chlorine in the pool water. - The oxidation and disinfection process is overloaded, and a highly irrirtant gas - Nitrogen Trichloride ( a type of tear gas) - is given off from the pool surface. This gas has a harsh chlorinious smell - it hurts the eyes and and irritates the respiratory system. The bathers say "There is too much chlorine", when - in fact - there is TOO LITTLE chlorine present.

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