Water Quality and Chemistry Glossary of Terms


The amount of calcium and magnesium dissolved in the water. “Water” or “total” hardness refers to the total magnesium and calcium dissolved in the water. Calcium hardness refers to just the calcium. Measured by a test kit and expressed as ppm. The proper range is 200 to 400 ppm.


Also called muriatic acid – A very strong acid used in pools to lower the pH and total alkalinity. It can also be used for various cleaning needs. Used in “acid washing” a pool. Use extreme care in handling.


The lightest chemical element. A component of water, and a frequent product of many chemical reactions. pH is a measure of hydrogen in its ionic form in water.


The positively charged nucleus of hydrogen atom. The relative degree of acid or base of a solution (called pH) is a measure of hydrogen ions.


This is sometimes called “tap” or “refill” water. It is the water used to replace water lost to evaporation, splash-out, leaks or swimmer drag-out in the pool.


See also PPM
MGL is an abbreviation for Milligrams per Litre. Milligrams per litre and Parts per million are numerically the same for water and therefore can be used and interchanged with pool water chemistry. Both MGL and PPM are used to measure the amount of a substance that is present in a liquid. In the case of swimming pools it refers to the chemical amount within the pool water.


Any substance that is neither animal nor vegetable. It is any class of substances occurring in nature, usually comprising of inorganic substances, such as quartz or feldspar, of definite chemical composition and definite crystal structure. It sometimes includes rocks formed by these substances. Ground water dissolves these rock substances, and the dissolved minerals are present in tap water. Depending on the kinds of rocks the water comes in contact with,the minerals dissolved in the water may be just a few or they may be many. Water handness is mostly comprised of these minerals.


A chemical used to make chlorine or bromine harmless. Used in test kits to counteract the bleaching effect of the chlorine or bromine in order to increase the accuracy of pool water tests. Sold as chlorine and bromine neutralizer, it is used to destroy excessive amounts of chlorine or bromine, so the high levels will not affect swimmers.


Water that has a very low calcium and magnesium content (water hardness) – usually means less than 100 ppm or 6 grains. Also water that has gone through a water softerer. Pools and spas should never be filled with soft water from a softener. Water with less than 100 ppm of hardness should be increased to a minimum of 150 to 200 ppm using calcium chloride.


Also called “tap” water – It is the water used to fill or refill the pool or spa.


A family of chlorine pool sanitizers that contain conditioner (cyanuric acid or isocyanuric acid) to protect the chlorine from the degrading UV rays in sunlight. Most common types are sodium dichlor and trichlor. The granular form is dichlor which is fast-dissolving and can be used for regular chlorination or superchlorination by broadcasting into the pool or spa. Tablet or stick form is trichlor (which is usually used in a chlorine feeder – either the floating type or in-line erosion type) used for regular chlorination only.


Insoluble solid particles that either float on the surface of or are in suspension in the water, causing turbidity. They may be held in suspension by agitation or flow. They may be removed by filtration, but if the particles are too small, they may not be trapped by the filter. In these cases, a clarifier or alum may be needed to remove them.


The total amount of alkaline materials present in the water. Also called the buffering capacity of the water. It is the water’s resistance to change in pH. Low total alkalinity causes metal corrosion, plaster etching and eye irritation. High total alkalinity causes scale formation, poor chlorine efficiency and eye irritation.


The amount of chlorine, both free and combined in the pool water that is available to sanitize or disinfect the water.


Total Dissolved Solids – Also called TDS – A measure of the total amount of dissolved material in the water. It is comprised of the spent or carrier chemicals added every time chemicals are added, as well as the hardness, alkalinity, chlorides, chlorides, sodium, magnesium, calcium, etc. Maximum amount in pools is 2500 ppm. Maximum in spas is 1500 over starting TDS. The only way to effectively lower TDS is to drain part or all of the water and replace it.


The cloudy condition of the water due to the presence of extremely fine particles in suspension that cannot be trapped by the filter because they are too small. Adding a clarifier, such as an organic polymer or alum, will coagulate the particles and make the filter more efficient.


Potential Hydrogen – Indicates the level of acidity or alkalinity of water on a scale ranging from 0-15. A low pH causes etched plaster, metal corrosion and eye irritation. A high pH causes scale formation, chlorine inefficiency and eye irritation. The ideal range for pH in swimming pools is 7.4 to 7.6.


An abbreviation for parts per million. It is a weight-to-weight expression. It means 1 part in 1 million parts, such as 1 lb. of chlorine in 1 million lbs. of water. Many of the common pool water tests, as well as acceptable ranges, are stated as ppm. For example, free available chlorine should be kept between 1.0 and 3.0 ppm; total alkalinity should be between 80 and 120 ppm; and and water hardness should be between 200 and 400 ppm. See Also MGL.