Parts per million, or ppm, is often used to express the concentration of an impurity, or the tolerable concentration of it.<\/p>\n
For example, the EPA<\/a> has set the maximum allowable concentration of chlorine in drinking water as 4 ppm (long-term; variations may be allowed over short intervals).<\/p>\n Parts per million is calculated by mass: specifically, Often you read that ppm is equal to mg\/L. The reasoning uses the idea that milli<\/em> means 0.001 and, at the same time, 1L of water has mass 1000g (or 1 million mg):<\/p>\n mg\/L = 0.001g\/1000g = 1mg\/1000000mg = 1\/(1 million)<\/p>\n Depending on how strictly you interpret the definition of ppm, mg\/L is a potential oversimplification. Let’s imagine you add 30.0 mg table salt to a litre of water. Now, the total mass has also increased by 30.0 mg. Stricly speaking, your ppm might be seen as<\/p>\n ppm = 30.0\/1000030. = 29.999ppm<\/p>\n Well, even to four significant digits (and we only have three, anyway), that’s still 30.00ppm.<\/p>\n Since ppm is normally used to describe very small concentrations, mg\/L is a practical definition.<\/p>\n Sources:<\/p>\n www.nesc.wvu.edu<\/a><\/p>\n US EPA<\/a><\/p>\n
\nppm=(grams substance of interest)\/(million grams)<\/p>\n