Understanding Water Alkalinity

hot tub water quality

In the pool or spa environment, water quality is the most important factor for personal health and enjoyment.

Acidic water can hurt the swimmers’ eyes and possibly irritate mucous membranes or skin. The low pH can also affect the mechanical equipment and could corrode some plastics or liners.

For alkaline environments, and higher pH, the water could become opaque and form white, crusty scales on the fixtures. Ideal water pH is be between 7.2 and 7.8.

Acid and base chemistry represent distinct quantities of two ions in a liquid solution. An acid mostly contains an H+ ion, (or cation/proton) in a given molecule. That hydrogen ion can be easily donated or removed when added to a liquid solution.

That dissociation creates the pH scale. pH is the potential of hydrogen in a given concentration. The pOH is the potential of hydroxide OH (anions) in a particular concentration.

The numerical chart of the pH scale is typically 1-14. The colors here may remind you of the water litmus test where a plain colored strip is inserted in a liquid solution.

The resulting paper turns red, pink, blue or dark blue. The base or alkaline side will be blue and the acidic range will turn red/pink. Any non-changing test will be the neutral narrow margin.

Additional testing of water samples from the pool or spa will be hand-held. A reagent will be released as droplets. Then the sample will slowly turn a shade of pink if testing for pH or blue if testing for pOH.

Phenol Red indicates for acids. Phenolphthalein can be found in many alkalinity tests. Either method is the fastest visual representation of the overall pH or pOH of water. The side of the compartment or bottle will show the color shades for contrast. The dramatic color change in the sample will follow the pH scale and quickly distinguish whether the water is acid or alkaline.

The PH Scale

Alkalinity is an important measurement to continuously assess. Depending on water quality readings, this will determine which chemicals need to be added or altered for the crystal clear recreational experience.

High alkalinity will show cloudy water, algal blooms or may disguise quality. Any sanitizer or chlorine disinfector works best during neutral pH. Frequent analysis helps predict the ebb and flow of the water environment. Rainfall, water fill-up and pool usage affect water rapidly.

In some geographic areas, acidic or alkaline water is the norm. Local water companies regularly check the water and monitor it for consistency.

When either type of water is added to a pool or spa, any chemical maintenance will need to adjust the pH to a neutral state. Check the filters and circulators first. The mechanical failure could cause a disruption in water chemistry and circulation.

By utilizing both the pH testing and Total Alkalinity (TA) test, the baseline water chemistry can be established. In the TA kit, the reagent Thiosulfate (S2O2−3) acts as the Chlorine neutralizer and is always added first. If this is not added initially, the results will be skewed.

Depending on the kit, Bromcresol Green-Methyl Red (C36H29Br4N3O7S ) Indicator Solution comes in liquid or powder. Add the measured drops to the sample and swish around. This will appear dark green like algae.

Next, drop by drop of Sulfuric Acid (H2SO4) will be added and swirl the sample. Be sure to count each drop that you added. Slowly, the green mixture will turn clear then lilac and then purple. Finally, go very exact and add a single drop methodically until it turns bright fuchsia. That is the final resting color to distinguish Total Alkalinity.

Total Alkalinity = # of Sulfuric Acid Drops x 10

Ex: 5 Drops x 10 = 50ppm (Low)

Ex: 15 drops x 10 = 150ppm (High)

If a rusty, orange color appears, too much chlorine remained in the sample and you will need to repeat the test. The recommended range of Total Alkalinity in pools or spas is 80-120 ppm.3

Depending on the readings, the alkalinity might need to be raised. Sodium bicarbonate (NaHCO3) is suggested for raising the alkalinity to the desired value because it is a fairly weak base and nontoxic for humans.

You may see this substance called Alkalinity Up, Alkalinity Plus or Alkalinity Increaser on the packaging. This will take time and frequent pH checks to ensure the TA is increasing. There is always a chance to over-add the compound which would create a high alkaline environment.

Ironically high alkalinity can be lowered by adding acid compounds to the water. Liquid HCL or hydrochloric acid can be poured directly into the pool.

Domestic muriatic acid is 20% strength of HCL and will be easier to find in stores. Be warned that either chemical is highly pungent and will burn skin and membranes. Use in a well-ventilated area and handle with care.  Do not allow anybody to swim or use the facilities until water is retested.

Sodium bisulphate or NaHSO4 is the dry powdered equivalent to HCL. That can be poured directly into the water and may need to be stirred to dissolve. Avoid touching the powder without gloves and exposed skin can be burnt.  Wait until the pH is lowered to near neutral.

Some of these chemicals may be called pH reducer, pH decreaser, pH lowerer, and pH buffer. Those common names are describing what the chemical is doing to the water. For some instances the pH reducers can work too well in the residential pool or spa.

In conclusion, the next time water chemistry is checked, the pH could be too low and results in an acidic environment. Until the water balance is achieved, it is recommended not to allow bathers inside. The goal then is to raise the pH to neutral or between the 7-range.

This delicate equilibrium between acidity and alkalinity shows how important regular water monitoring for pools or spas actually is.


  1. https://commons.wikimedia.org/wiki/File:PH_scale_2.png
  2. https://archive.epa.gov/water/archive/web/html/vms510.html
  3. http://sarasota.floridahealth.gov/programs-and-services/environmental-health/pools-and-spas/_documents/pools-how-to-use-your-test-kit.pdf

Image credit: donmarcyp on Flickr


About the Author:

Natalie Abram

Natalie is new to freelancing, but has worked as an aquatic content editor for the past 7 years. She teaches biology online at multiple schools. Her background and Master’s education is in Biology, Aquatic Ecology and Education.

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