A BRIEF DESCRIPTION OF THEIR MEANING AND PURPOSE
Expression of Test Results - PPM (parts per million)
In water treatment, results are most commonly expressed in parts per million (ppm). Other terms commonly encountered are milligrams per liter (mg/l) and grains per gallon (gpg).
For practical purposes, 1 ppm = 1 mg/l and 1 gpg = 17.1 ppm.
The term “ppm” is unitless; that is, as long as the same units are used on both sides of the relationship, any units can be used. For example, 1 ppm can be used to express all of the following; one ounce per million ounces, one pound per million pounds or one ton per million tons. However, one ppm is not one pound per million gallons because the units are not the same on both sides of the relationship.
1 pound per 1000 gallons = 120 ppm
An item that is 99.9% pure contains 1000 ppm of impurities. A boiler that makes up 4000 gal/day of water having 100 ppm of hardness, has a potential of accumulating over 1000 pounds of scale per year.
In natural waters, alkalinity is most commonly the result of bicarbonate and carbonate ions; in treated waters, alkalinity may also be contributed by hydroxide, phosphate, silicate, and other treatment ions.
The color changes of phenolphthalein indicator, which occurs at pH of 8.3 (P Alkalinity) and bromcresol green indicator, which occurs at pH 4.2 (Total Alkalinity) are the standard reference points for expressing alkalinity.
For boilers operating up to 300 psig, the accepted alkalinity range is 200 ppm to 700 ppm, with P being 60% to 80% of Total Alkalinity.
Balances alkalinities are no guarantee of clean, trouble-free boilers. However, this is one of the important factors, together with others, that must be properly controlled if the boiler is to be kept clean.
Chloride ions, unlike other ions that enter the boiler, are extremely soluble and do not precipitate or decompose when subjected to boiler conditions. Therefore, chlorides are