I am a pool-maintenance and service worker. I’m a little new to the field; however, none of the technicians I work with seem to have a definite answer for me. When raising the alkalinity in pools, we add sodium bicarbonate, which also affects the pH by raising it as well. My quarrel is trying to calculate how much the sodium bicarbonate will raise the pH by. For example, I have a pool with a pH of 7.1, and the alkalinity is 30 ppm (parts per million) on a chlorine pool. I need to raise the alkalinity by 70 ppm, so what would my pH be at that point?


1 Answer 1


There is more to changing pH than just Sodium Bicarbonate(at least for pools). It is important to consider factors such as Calcium Hardness, Total Dissolved Solids, and temperature before making an estimate for the pH and Alkalinity. Regardless, once you are able to gather this data, you can use the Langelier Saturation Index to make estimates. The Langelier Saturation Index (LSI) is a tool used to gauge how "balanced" a pool's water is. This establishes whether and what kind of impact the pool water may have on the equipment. Although it makes swimming more comfortable, its main purpose is to prolong the equipment's life. Scaling is the process of determining whether or not the water will corrode the equipment.

VI = pH + TF + CF + AF - TDSF

pHF – pH (acid value) factor

CF – calcium hardness factor

AF – total alkalinity factor

TF – temperature factor of the water

TDSF – total dissolved solids factor

In order to calculate these factors, you can compare the SI unit(Note: Temp in Celsius not Kelvin) values from the following table. enter image description here

Source: https://www.melpool.com/en/water-balance-langelierindex-alkalinity/

Remember that pH and ppm are complicated matters and simple mathematical calculations can neglect these other factors. Hence, when starting out with pool management, consider reading guides/studies such as this and this.

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    $\begingroup$ As stated it's complicated: e.g., adding $\ce{NaHCO3}$ increases alkalinity, but it is not 1:1, because some of the calcium might precipitate as $\ce{CaCO3}$. $\endgroup$ Commented May 24 at 14:50
  • $\begingroup$ Yes. That is why the factor is for "dissolved solids". Furthermore, there are also dissolved minerals, salts, organics, chlorides, metals, and contaminants in water, which have an impact. $\endgroup$
    – Ronith
    Commented May 24 at 14:57
  • $\begingroup$ Basically (ha), it depends on why the pH is at its current level, i.e. specifically what else is already dissolved in the water. $\endgroup$ Commented May 25 at 7:46

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