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Stain & Scale

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Stain, Scale and Clarification

There are two major kinds of contaminants in pool and spa water- "Organic and Inorganic".

Organic contaminants in a pool or spa are things like bacteria, algae and bather waste, which are objectionable because they can affect the health of swimmers and bathers. Organic contaminants are controlled by such things as chlorine, bromine, ozone, ionizers, ultra violet sterilizers, algaecides and non-chlorine shock treatments chemicals which are subjects of other articles in these web pages.

Inorganic contaminants or materials like copper, iron, calcium and magnesium are objectionable because they may cause stains and scale on the pool or spa walls or cause cloudiness or discoloration of the water. Inorganics contaminants are controlled by an entirely different type of chemical treatment, which is the subject of this page.

Scale, stains, discoloration and sometimes cloudiness in a pool or spa are caused by minerals coming out of solution, called precipitation. If the precipitated minerals in the water form hard crusty deposits on the pool walls , this is called scaling or scale. Scale and or cloudy water is often formed from incorrect water balance and or high calcium or magnesium levels in the water. When calcium hypochlorite chlorine is used for sanitizing or shocking pool or spa water, the calcium in this chemical often clouds the water temporarily as the calcium dissolves into solution. If the precipitated metals have a color are smoother and are deposited on the pool walls, this is called staining. Staining and or colored water is caused by heavy metals like copper, iron, manganese. Quite often when you first fill a spa for example the water is colored green, from the copper in the source water pipes (not as common in well water sources). This is the same thing that causes a bath tub to get green stains in the household bath tub. You will notice that this green in your bath tub occur quicker when you bath with soap and or bath salts as they raise the pH of the water causing the copper minerals to come out of solution depositing on the bath tub surface. If chlorine was present in any quantity then the copper ions would form copper oxide and become a grey black color.

All these minerals come from the source water or from the pool or spa equipment itself if the water is out of balance. The minerals when in water form oxides, hydroxides, carbonates, sulfates, silicates and phosphates. Depending on the chemicals you are using in your water treatment program, you may be adding some more of them each time you treat the water, as with copper or silver algaecides, ionizers electrodes, chlorine generators electrodes or evens forms of chlorine like calcium hypochlorite.

When metals are dissolved in water, they exist as electrically charged particles called ions. Ions can be either positively or negatively charged. Positive ions are called cations. Negative ions are called anions. All metal ions happen to be positively charged, so metal ions are all catatonic. Sometimes, different ions of the same metal have stronger or weaker charges. Iron, for example can form ferric ions (Fe+++) or ferrous ions (Fe++). The "+" signs indicate the strength of the electrical charge on each of the ions, therefore you can see that the ferric ion has a stronger charge than the ferrous ion. Now that we know that the dissolved minerals in the pool or spa water can be a problem, what can be done to eliminate them or fix it so they won't come out of the water.

De-ionizers

One way to remove all the metals would be to run all the water through an ion-exchange resin. Systems that remove metals this way are called de-ionizers. The special resins used to remove metal ions from the water are called zeolites. Zeolites exchange unwanted metal ions for ions that will not cause any problems. These resins are expensive, and you would need large amounts to remove metals in your pool or spa.

Water Softeners

Or if you wanted to remove calcium and magnesium from the water you could use a water softener. These units function by exchanging sodium ions (from salt in the water softener tank) for the calcium and magnesium ions in the water. The resulting water contains a larger amount of sodium, but sodium in pool water does little except to build up TDS (Total Dissolved Solids). You may be familiar with this method if you have a home water softener unit to soften your water source (i.e.. Well Water).

Precipitates

Or if you prefer, you could remove the calcium and magnesium by precipitating them and vacuuming the residue to waste. The chemical most commonly used for this purpose is tri sodium phosphate (TSP). When added to water, TSP combines with calcium or magnesium to form a very fine white, powdery precipitate (calcium phosphate or magnesium phosphate). For example, one pound of TSP will precipitate one pound of calcium phosphate or magnesium phosphate for a total of two pounds residue which will cloud the water and eventually settle on the bottom which can be vacuumed to waste. De-ionizers, water softeners and TSP are all capable of removing all of the calcium hardness from pool or spa water. However, if soft water or de-ionized water is used in a plaster swimming pool or spa, the water will be extremely aggressive and will begin to dissolve the calcium right out of the plaster walls or grout resulting in pools where large chunks of plaster have fallen right off the walls. Pool and spa water must have a minimum of 150 ppm of hardness. The ideal range for hardness is from 200 to 400 ppm.

Sequestriants and Chelates

Another option when controlling metals is to use a sequestering or chelating agent, also known in the industry as stain and scale inhibitors, stain and scale preventers, metal suspenders or metal removers.

Sequester

The word sequester comes from the Latin word sequestrare meaning to "stand apart". It is also defined as "to set aside; to separate and reject; to eliminate." In chemistry the term "sequester" as in sequester metal you must either remove it from solution or combine it with something else to prevent it from coming out of solution.

Chelate

The word chelate comes from the Greek work chelate, which means "Claw." In chemistry a chelated metal is one in which a central metallic ion is joined by two or more non-metallic atoms that bond with the metal and prevent if from combining with anything else. The substance that causes this reaction is called a chelating agent. It is a molecule, usually organic, that is soluble in water and can react (bond) with metal ions and "hold onto them tightly" to keep them from precipitating - hence the "claw" definition. The difference between the terms sequester and chelate is so minimal that you can pretty much us the two terms interchangeably. Use whichever term you are comfortable with. When metal containing compounds are dissolved in water the metallic portion of the compound exists as a positively charged, free moving ion in solution as mentioned earlier (remember the section on positively and negatively charged ions). Like magnets opposite charges attract and like charges repel. Therefore the free metal ions in water react to surround themselves with negative ions or the negative side of polarized molecules present in the solution. Like a magnet, polarized molecules have both a positive or negative side or pole as well. Sequestering and chelating agents use these properties to bind or complex with the metal ions to keep them in solution (because they have a negative charge compared to metal ions). Once they bind or complex with metal ions they become a different molecule altogether which no longer contain the metal staining properties. Some agents are so strong that they will literally react with the metal compounds that have already precipitated on the pool wall and pull the metal ion back into solution thereby removing the stain. Such stain control/removal products are readily available from many manufacturers. However, that's really asking quite a lot from a sequestriant or chelating agent but does occasionally work. The easiest way to remove a stain or scale from a pool wall is to prevent it from occurring in the first place and this is really what sequestering agents are best at - prevention. Unfortunately most people, service technicians included, don't become aware of a metal problem until they see a stain appear. Then they add a sequestering agent and expect it to pull the precipitated metal off the walls of the pool which, as mentioned earlier doesn't always work.

 

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