Pool water chemistry and water balance: pH, alkalinity and LSI

The table below gives the parameters that enter the operating specification of a pool project and the ranges that can be verified in the sources. For parameters where the source texts give no numerical range, the table does not invent one; it states in which direction the specification should limit them.

Values should be read as mg/L CaCO3 equivalent; in the pool sector ppm CaCO3 and mg/L CaCO3 are the same quantity, and dividing by 50 converts to meq/L. Calcium hardness enters the LSI calculation as the CaCO3 equivalent, which is 2.5 times the ion concentration, rather than as the ion concentration itself.

The two concepts most often confused on site are pH and total alkalinity. pH is a logarithmic scale showing the hydrogen ion concentration of the water at that moment. Total alkalinity measures how much the pH of the water will resist when acid is added, that is, its buffer capacity; by the source definition it is 'the capacity of water to resist acidification' and should not be confused with basicity. In pool water this capacity is provided by bicarbonate and carbonate ions.

The practical consequence is this: in a pool with 40 mg/L alkalinity the pH can jump from 7.2 to 7.9 within the day; in a pool taking the same load at 100 mg/L alkalinity the pH moves by a few hundredths. If the operator corrects the pH with acid every day and finds it high again the next day, the problem is alkalinity, not pH. The alkalinity article stresses that adding or removing carbon dioxide does not change alkalinity but does change pH; fixtures that drive off CO2, such as waterfalls, raise pH while leaving alkalinity constant. This is why, among pool chemicals, sodium bicarbonate is sold separately as an 'alkalinity increaser' and sodium carbonate as a 'pH increaser'.

To calculate LSI, pHs is found first. The source formula has four factors: A = (log10[TDS] - 1) / 10; B = -13.12 x log10(°C + 273) + 34.55; C = log10[Ca, mg/L CaCO3] - 0.4; D = log10[alkalinity, mg/L CaCO3]. Then pHs = (9.3 + A + B) - (C + D) and LSI = measured pH - pHs. The source gives factor B as 2.09 at 25 °C and 1.09 at 82 °C; the table shows the intermediate values at pool temperatures and the other factors for typical pool water.

Worked example: for pool water at 28 °C, TDS 1,000 mg/L, calcium hardness 250 mg/L, alkalinity 100 mg/L and measured pH 7.4, pHs = (9.3 + 0.20 + 2.03) - (2.00 + 2.00) = 7.53; LSI = 7.4 - 7.53 = -0.13. The source states that values between -0.5 and +0.5 show no marked dissolving or depositing behaviour; this water is considered balanced. If in the same pool the pH rises to 7.8, alkalinity to 150 mg/L, hardness to 400 mg/L, temperature to 30 °C and TDS to 1,500 mg/L, then pHs = (9.3 + 0.22 + 1.99) - (2.20 + 2.18) = 7.13 and LSI = +0.67: the water scales, and lime builds up in the exchanger and the grout. Conversely, the combination pH 7.0, alkalinity 60 mg/L, hardness 150 mg/L, 26 °C and TDS 800 mg/L gives pHs = 8.01 and LSI = -1.01: the water is corrosive and dissolves calcium out of the concrete pool plaster.

The three examples show that balance is built from the combination; in a heated pool project, the water going from 28 °C to 34 °C raises the LSI by about 0.1 without any chemical being added.

Whatever form chlorine is added to the water in, the result is hypochlorous acid (HOCl), and HOCl partly dissociates in water into the hypochlorite ion (OCl-): HOCl <=> H+ + OCl-. The pKa of the dissociation is 7.53; at pH 7.53 half of the chlorine is HOCl and half is OCl-. The source states that at neutral pH the composition is about 75 percent HOCl and 25 percent hypochlorite, while in basic solution effectively only OCl- remains. For pool water this means: free chlorine measured at pH 7.2 contains markedly more HOCl than the same reading at pH 7.8. The test kit shows the same figure in both cases; what reveals the difference is the pH.

The choice of pool chemicals affects this balance from two directions. Sodium and calcium hypochlorite push the pH up as they dissolve, while trichlor and dichlor tablets are acidic and pull pH and alkalinity down. A pool using trichlor needs regular alkalinity increaser, a pool using hypochlorite needs regular acid dosing; the choice of disinfectant also sets the balance side.

Because the parameters depend on each other, the correction order changes the result: total alkalinity first, then pH, then calcium hardness. A pH set before alkalinity is corrected drifts in the absence of a buffer; an LSI calculated before hardness is corrected gives the wrong target pH. Cyanuric acid and TDS are parameters that are monitored rather than adjusted: they only rise, and the only correction is water replacement. This order is the skeleton of the pool maintenance programme and is also applied as the column order on the pool water measurement record sheet.

The saturation index was developed by Wilfred Langelier in 1936 to predict whether a protective calcium carbonate film would form on the pipes of drinking water networks. Water with a slightly positive index leaves a thin lime layer on the inner pipe surface and isolates the metal from the water; water with a negative index dissolves that layer. The same logic was carried into the pool construction sector, because a pool also has surfaces to protect: cement-based plaster and grout, the mortar under the tiles, stainless steel ladders and nozzles, the heater exchanger.

The adaptation for pool construction brings two differences. The first is temperature: mains water is around 10-15 °C while pool water is between 26-34 °C, and factor B carries that difference. The second is TDS: pool water accumulates dissolved solids through evaporation and chemical addition, and factor A must be re-taken every season. The Swimming pool sanitation article notes that pool pH is kept at 7.2-7.8 in the LSI approach and at 7.8-8.2 in the alternative approach called the Hamilton index; the difference comes from which side is given more weight, the HOCl share of chlorine or the scaling tendency. A pool project should state in its specification which index it takes as the basis.