Just how much do swimming pool chemicals add to the total dissolved solids (TDS) content of the water? Do all chemicals of equal amounts (by weight) add identical amounts of TDS to swimming pool water?
When one pound of a chemical is added to 20,000 gallons of water, the usual amount of TDS being added to the water is 6 parts per million (ppm) of TDS. While this is true with most chemicals, it is not true with all chemicals. After being added, some chemicals will escape from the water and not contribute to TDS. Another possibility is that portions of some chemicals will convert into water and thus will no longer be part of TDS.
We will first look at adding a pure chemical that provides the full increase of its weight to the total dissolved solids content of the water. Chlorine gas is a good example. At levels of 1000 parts per million or below, chlorine gas is nearly 100 percent soluble at a pH of 5.0 or above. Therefore, if one pound of pure chlorine is dissolved into 20,000 gallons of water, the result will be 6 ppm of chloride (which is TDS).
One way of calculating this is as follows: 20,000 gallons of water weighs about 166,700 pounds, which is one-sixth of a million. Multiply the one pound of chlorine by six to get the 6 ppm of TDS.
As with all acidic sanitizers, after chlorine gas is added, it is usually necessary to add sodium carbonate (soda ash) or sodium bicarbonate (bicarb) to adjust the pH and alkalinity upward. Interestingly however, adding these alkaline chemicals does not ultimately add the full amount of TDS per pound added.
When soda ash and bicarb are added to pools, these chemicals will react with acid that has been added to form carbon dioxide, which, over a period of time, will release into the atmosphere. Only the sodium element from these two chemicals remains in the water and increases the TDS.
The following equations will help illustrate:
Na2CO3 + 2HCl = 2NaCl + H2CO3
H2CO3 = CO2 + H2O
In the first reaction, soda ash (Na2CO3) reacts with acid (HCl) from acidic sanitizers, muriatic acid, or dry acid, to form sodium chloride (NaCl) and carbonic acid (H2CO3). Sodium chloride remains in the water as salt, which does contribute to TDS, while carbonic acid will dissociate into carbon dioxide (CO2) and water (H2O).
The pure water that is formed does not contribute to TDS, and carbon dioxide will eventually escape into the atmosphere – thereby not contributing to the TDS content. Sodium bicarbonate also reacts with acid and forms the same compounds – salt and carbonic acid.
To determine the actual amount of TDS that is being added by these chemicals, a look at their molecular weight is needed. The (rounded off) molecular weight of soda ash is 106, and sodium bicarbonate is 84. Sodium by itself has a molecular weight of 23, and there are two sodium molecules in soda ash, only one in sodium bicarbonate. The sodium element (which adds to the TDS content) comprises about 43.4 percent of the soda ash, and about 27.4 percent of the sodium bicarbonate. Therefore, when adding one pound of soda ash, only 43.4 percent would contribute to TDS, and with sodium bicarbonate, 27.4 percent contributes to TDS.
The calculations are as follows: if one full pound of a material in 20,000 gallons of water would normally equate to 6 ppm of TDS, then one pound of soda ash, with only 43.4 percent of its total weight (sodium) remains as TDS, contributes about 2.6 ppm of TDS. Likewise, one pound of sodium bicarbonate (with 27.4 percent as sodium) adds about 1.65 ppm. (Note: Until these chemicals actually react with an acid, the entire contents of the various compounds would be present as TDS).
Another type of chemical which does not introduce the full TDS of its weight are compounds which adds hydroxide or which forms hydroxide when added to water. Hydroxide (OH) will likely combine with a hydrogen ion to form pure water (H2O) and no longer be TDS. Sodium hypochlorite (often referred to as liquid chlorine or bleach) is an example of this type of compound.
In every gallon of sodium hypochlorite (in this example, 15% trade or 12.5% by weight), there is about 1.25 pounds of chlorine and about 1.6 pounds of sodium hydroxide, for a total of 2.85 pounds of potential TDS. This would normally equate to about 17 ppm of TDS for every gallon added to a 20,000 gallon swimming pool. However, since hydroxide accounts for about 42.5 percent by weight of sodium hydroxide (and since hydroxide will eventually react with hydrogen and convert to water), this component is subtracted from the compound, so that, together with the chlorine (contained in bleach), about 2.2 pounds in a gallon of sodium hypochlorite adds to the TDS content of a solution. Thus one gallon of sodium hypochlorite adds about 13 ppm of TDS (rather than 17 ppm) in 20,000 gallons of water. (Bleach also adds a small amount of alkalinity to the water).
With 10 % (weight) bleach, about 10.5 ppm is added, and with 5.5% (weight) household bleach, about 5 ppm TDS is added, per gallon to 20,000 gallons of water.
Also, chemicals that include oxygen or nitrogen will also likely lose portion of their weight, thus a lower overall TDS contribution. Calcium hypochlorite (65%) is an example of a compound that adds oxygen. The percentage of the calcium hypochlorite contribution that ends up as oxygen is about 22 percent. Cal Hypo also contains a small amount of water. Therefore, its contribution to TDS is only about 75 percent of its total weight. Lower strength Cal Hypo will result in a slightly lower percentage of TDS contribution per pound. (Of course, part of the TDS contributed by calcium hypochlorite is calcium, which is added to the water and increases the hardness).
Trichlor, sodium dichlor, cyanuric acid, and regular salt, added to swimming pool water will result in the full increase of their weight to TDS.
Muriatic acid (hydrochloric acid) is product that, when added to pool water, will contribute significantly to the TDS content. Muriatic Acid at 31.45 percent strength adds about 3 pounds of TDS per gallon. Although the hydrogen from this acid will react with hydroxide to form water, it is such a small percentage of the total weight of the acid that it will not significantly effect the contribution of this acid to TDS. Therefore, one gallon of muriatic acid in 20,000 gallons of water will increase the TDS 18 ppm.
It is recognized that this information on TDS isn’t critical to the understanding and maintenance of water balance, but perhaps some will find it interesting.