by Sean Griffin, January 26, 2010
Muriatic Acid & pH in swimming pools
In medieval times, hydrogen chloride (HCl), known then as “alchemist’s spirit of salt” or “acid salis” was prepared by heating salt (sodium chloride) with iron sulfate. Later a German chemist by the name of Johann Glauber made HCl by combining salt with sulfuric acid which became the common method of preparing HCl. This strong acid is passed into water creating hydrochloric acid, which is further diluted to make the commercially available solution referred to as muriatic acid.
The word muriatic is derived from muria which is Latin for salt water or brine. During the Industrial Revolution in Europe, muriatic acid was produced as a waste product being released into the atmosphere in its gaseous state. Currently HCl is obtained by absorbing hydrogen chloride released during industrial production.
Muriatic Acid is an inorganic, corrosive, aqueous solution. The corrosiveness of HCl can be used in many ways for breaking down the cellulose structure of a substance, resulting in complete hydrolysis. This can be used for leather tanning, brick cleaning, treating and galvanizing metals, to refine and manufacture a wide variety of products, and (as it pertains to this article), pool maintenance.
Muriatic Acid can be used for several pool repairs and maintenance procedures including acid washing, paint and plaster prepping, brick and tile cleaning, filter and cartridge cleaning, chemistry adjustments and more.
It is important when working with hazardous material to take the proper precautions. Follow all safety procedures, wear proper protective gear and know your emergency contacts. Muriatic acid is most definitely a hazardous material and has the potential to be lethal. Sufficient breathing apparatuses, protective eyewear, and rubber gloves are a must when working with acid.
Gunite pools are are often finished with a plaster coating known as a “white coat” or "marcite", or simply "plaster". Plaster finishes are at risk of organic and mineral staining. These stains can be combated using several methods. Stain removing products, concentrated chlorine, non draining acid wash products, and ascorbic acid based products can help with treating these eyesores.
Over time staining can become so severe an acid wash is performed. The pool is drained and acid is poured down the wall of the pool in this procedure. Diluted acid etches away a thin layer of the plaster removing stains, cleaning grout joints, and killing any algae spores that may be embedded into the porous plaster finish. Acid washing can eliminate stains but cannot be performed to often. As the plaster is removed the texture of the pool finish becomes rougher as more sand in the plaster mix becomes exposed. Most plaster coats can withstand 3 or 4 carefully done acid washes over their lifetime. Acid washing a pool should be performed by a licensed professional, as there are many devils in the details. For a deeper look into swimming pool acid washing, see our pool info section.
Etching away the existing plaster is actually the goal when prepping to have a pool re-plastered. Known in the pool industry as an “acid burn” undiluted acid is applied to the pool to help create the rough texture needed for the proper bonding of the new plaster coat. Washing masonry or brickwork with acid neutralizes the alkalinity leaving it optimal to accept paint. For this reason acid rinsing is also part of prepping for painting a pool.
For this same reason acid plays a major role in balancing your water chemistry. Muriatic acid will lower the Ph and the total alkalinity of water. Acid will lower pH more than it will lower the total alkalinity. This can be a frustrating exercise, if you are trying to lower alkalinity, but keep having to raise the pH again after each treatment (which also raises the alkalinity a bit).
There is a trick to adding acid that will result in gaining a slightly greater alkalinity adjustment. The rule is - "To lower pH more than Alkalinity, Walk the Acid. To lower Alkalinity more than pH, Pool the Acid". Meaning, walk the acid around the pool as you pour it in, to lower pH more....or to lower alkalinity more, pool the acid. To properly pool your acid, shut off the pool pumps for 10 minutes to allow water movement to cease. Then, in one area, usually a quiet, deep corner of the pool, put the acid jug under water, and pour it out underwater. This "pooling" of the acid in one area, keeps a greater concentration in one area, resulting in a greater exchange of hydrocarbons and a resultant increase in bicarbonates, which make up your total alkalinity.
When we speak of pH in pools, what do we really mean? Well, some say pH stands for 'power of hydrogen'. It is really a scale of relative acidity or basicity of the water. The scale ranges from 0 to 14, with 7.0 being neutral, or neither acidic or basic. Below 7.0 is acidic, and above 7.0 is basic. The pH scale is a logarithmic scale, which means that each point of movement along the scale is a ten-fold change. For example, a pH of 7.2 is 10 times as acidic as 7.3 - and 10 times more basic than 7.1.
Everything around us has a pH level. Fruits such as citrus typically have a low pH, being rather acidic. Milk on the other hand is quite basic in it's pH level. The human eye has a pH level of around 7.3, which coincidentally, is the range we aim for in swimming pools.
Adjusting the pH level in your swimming pool to the range of 7.2-7.6 is easily accomplished by adding an acid if your pH is high, or adding a base if the pH is too low. (follow all package instructions).
Most pool owners find it safer to use a granular acid, in the form of sodium bisulfate, rather than the very caustic muriatic acid. For the base, soda ash is a common chemical used to raise your pH level - by adding this basic powder to the water.
Keep acid away from other chemicals, and never mix acid with any other chemicals. Mixed with Chlorine, and you can create a very dangerous mustard gas. A few years ago, a lifeguard poured acid into a vat of chlorine at the Ramada Inn in Tyson's Corner. Evacuation ~ what a mess. Fortunately, there were no fatalities. Muriatic acid is the most dangerous chemical in a pool service truck, especially if mixed with chlorine.
Keeping pH in range will give the pool owner several benefits, including swimmer comfort and greater chlorine efficacy. A pH level that is too low, is a "corrosive" water condition, which can etch plaster and steel pool components. A pH level that is too high creates a "scaling" water condition, which can cause scale or mineral deposits to form on pool surfaces. High pH levels also makes your chlorine very sluggish, and provides a more suitable environment for algae to flourish.