Muriatic Acid v. Dry Acid

Adjusting your pH is very important to make sure that your pool will stay clean.  Chlorine works much better at a lower pH, but too low of a pH can cause swimmer irritation and ruin equipment.  So, keeping pH at its right level for pool water chemistry will save you headaches and trouble.

When it comes to how to adjust pH there are a few different ways.  You can add Muriatic Acid, Dry Acid, or adjust it through CO2.  Most residential pool owners end up using Muriatic or Dry Acid.  There are a few differences in each product; this article highlights some of the things you should know before deciding which one to use.

Muriatic Acid

Muriatic Acid can be found in hardware stores and pool stores and its correct name is Hydrochloric Acid.  It used to be called 'spirits of salt' because it was an acid that came from salts. If you are buying it for a pool, it usually goes by the name Muriatic Acid.

Muriatic Acid appears to be cheaper when you buy it at a hardware store, but there are different concentrations of it, and pool stores will sell the higher concentration (30%) while hardware stores are selling the lower concentration (15%).  It is relatively easy to use, but you need to be careful.  You don't want children handling it and it should never be stored near liquid chlorine.  You never want to mix Hydrochloric Acid with bleach because it will create the very toxic chlorine gas. Wearing gloves and goggles is suggested when using this product.

Dry Acid

Dry Acid is also known as Sodium Bisulfate.  It is much safer to use than Muriatic Acid and comes in a granular form.  It tends to be more expensive than Muriatic Acid, but for some, the ease in which you can use it, and the fact that it isn't quite so dangerous, is reason enough to justify the extra cost.  At pool stores, it is usually called "pH Down" or "pH minus."

Which One Should You Use?

Sodium Bisulfate (Dry Acid) adds sulfates to the pool water.  It also adds to the TDS (Total Dissovled Solids) of the pool water.  Neither are good for the pool.

According to the Pool Water Treatment Advisory Group, "An increasing number of reports have been received about problems being experienced in swimming pools due to the use of sulphate-based chemicals. In most cases the initial effects observed are some erosion of the cement grout with a white suspension forming in the pool water when the grout surface is agitated. In one case a hotel pool had a water supply with just over 6mg/l sulphate with balanced pool water but the use of sodium bisulphate (dry acid) and alum (sulphate-based salt) had raised the sulphate level to 1,582mg/l with erosion of the grouted joints quite evident."

"The total dissolved solids (TDS) need not be high for sulphate attack to occur if sodium bisulphate is used in pool water.  Sulphate attack occurs under balanced water conditions, even at concentrations below the maximum recommended concentrations, but more slowly since the rate of attack is concentration dependent.  Note also that sulphate ions can migrate into the cement mortars, renders, screeds or concrete and begin to react. However the effects will not be immediately evident as they manifest on the surface of mortars in contact with the pool water.  Where sulphate attack occurs on Portland cement mortars and concrete, the effects will not be as readily apparent, as in the initial stages the expanded reaction products fill up any pores or fine cracks within the mortar.  Once all the voids are filled the further growth of expanded reaction products starts to disrupt the cement mortar or concrete. A classic example of this was where the first sign of a problem was when the tiles in the overflow channel of a freeboard pool tented due to expansion from sulphate attack and the adhesive beneath were found to have disintegrated.

Note that using sodium bisulphate to lower the pH of the pool water effectively adds an equimolar mixture of sodium sulphate and sulphuric acid to the pool water and for the same hydrogen ion concentration you are adding nearly 3.3 times more ‘dissolved solids’ to the pool water than if you added the equivalent amount of hydrochloric acid.  This means that using dilute hydrochloric acid has far less effect on the TDS of the pool water and, since the sodium bisulphate has to be dissolved in water, the amount of ‘liquid’ added to the pool should be less using dilute hydrochloric acid to control pH.

Note that dilute solutions of either hydrochloric acid or sodium bisulphate with the same hydrogen ion concentration are hazardous; however there is no need to use concentrated hydrochloric acid as hydrochloric acid is available in a range of concentrations for use in swimming pools.

As a result of the long-term experience with the effects of using sulphate-based chemicals in treating swimming pool water, it is strongly recommended that the use of sulphate-based chemicals should be avoided in swimming pools of concrete construction, and where cement based renders, screeds, tile adhesives and grouts are present." (Sulfate Attack, PWTAG)

Saltwater Generated Pools

Pools with Saltwater Generators (SWG) should use Muriatic Acid because the sulfates can ruin the cell.  This information comes from most Saltwater Generator manufacturers.

Conclusion

If you have a salt water generator it is probably smarter to use the Muriatic Acid.  It won't add corrosive sulfates.  Also, if you have a gunite pool, you might be better off with Muriatic Acid, that way you won't have to worry about the sulfates ruining your concrete.  As a side benefit, if you are using a salt water generator, the Muriatic Acid turns into a salt or chloride just like the used up chlorine does.