- pH is a measure of the acidity or alkalinity of a solution.   If you eat pickles or drink grapefruit juice - both of which are acidic - you will lower the pH in your stomach.  You would then need to eat or drink something alkaline - like bicarbonate of soda or Milk of Magnesia - to neutralize the acid, and thus raise the pH within your stomach.  It is much the same with the water in your pool.  

A high pH figure is reduced by the addition of an acid; a low pH figure is raised by the addition of an alkali 

It is called the pH scale because it measures the power of a solution to yield Hydrogen ions. Isn’t Chemistry interesting?

The pH Scale

runs from 0 to 14, with a pH neutral solution having a mid-point value of 7

Acid 0-1-2-3-4-5-6- 7 -8-9-10-11-12-13-14 Alkali

  • pH-0 is highly acidic and pH-14 is highly alkaline
  • The scale is logarithmic, so each increase (or decrease) in value is a multiple of 10
  • Thus pH-9 is 10 times as alkaline as pH-8, and pH-4 is 100 times as acidic as pH-6

pH Scale diagram

The vital importance of correct pH

The pH affects the amount of Hypochlorous Acid (Chlorine sanitizer) and Cyanuric Acid (CYA, AKA Chlorine stabilizer) that is formed when you add Chlorine to the pool water, and so determines the effectiveness of the added Chlorine

  • At pH 6.5, 90% of the Chlorine will be Hypochlorous Acid and just 10% will be stabilizer; the Chlorine is under-stabilized and will break down swiftly in the sun
  • At pH 7.4 the figure for Hypochlorous Acid is 65% and for and CYA  it’s 35% - this is ideal
  • At pH 8 only 20% is Chlorine Sanitizer; the rest will be CYA - this results in over-stabilization of the Chlorine sanitizer so it is ineffective
  • The ideal range is between pH 7.2 – 7.6, matching the pH of your eyes, and ensuring the correct proportions of sanitizer and stabilizer
  • If you allow the pH to vary from this figure, you will need to add more chlorine to ensure adequate sanitizing of the water, or alternatively you will have unacceptably high levels of free chlorine in your pool
  • Low pH will corrode metals and leave behind metal oxides, which stain pool surfaces
  • Low pH will also dissolve pool plaster, grout and pool copings. It is uncomfortable to swim in a pool with low pH
  • Do not be tempted to run the water with a low pH to 'get more out of your Trichloro tablets' - the result will be corroded metal fittings and grout, stinging eyes, sore throat and a much higher chlorine bill
  • High pH will result in lime-scale formation on the pool surfaces, pipes, fittings and, most importantly, every grain of media in the Sand-filter, on the grids of a DE filter or on the membranes of a Cartridge filter
  • All of these scenarios result in reduced filtration capacity and further expense for the poor, beleaguered pool-owner

Raising or lowering pH

Sand-filter Calcification

  • At pH 8.6 and above the Calcium dissolved in the water combines with the carbonates in the water, resulting in Calcium Carbonate deposits (lime-scale) just like in a kettle or central heating boiler
  • Quite quickly the sand-filter will become a solid block of stone with just a couple of passages filled with loose sand, through which the water can circulate - a condition known as 'channeling'
  • This reduces the surface area of the filter and so the filter clogs rapidly; vacuumed dirt is forced through the filter and back into the pool
  • Calcium Carbonate also forms into microscopic particles that float suspended in the water, giving it a milky appearance
  • The sand-filter removes the particles but it will need back-washing more often; which costs money for the water and those expensive pool chemicals that are flushed away during a backwash

Probably the best nvestment you can make to your pool is to install an automatic pH Regulator as this device will keep the pH under control perfectly



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Ken Walker - MyPoolGuru©