Lessons · Pharmacy · parts per million
Parts per million
One part per million is 1 gram in 1,000,000 mL, which is the same as 1 milligram in every litre.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Very dilute solutions, water quality figures and some trace ingredients are quoted this way, and the number is meaningless until it is turned into something a balance or a cylinder can measure.
How to think about it
Take the 1 gram in a million millilitres and reduce it once: a gram is 1,000 mg and a million millilitres is 1,000 litres, so one part per million is one milligram per litre. From there multiply by the litres for the milligrams. For a percent, divide the ppm by 10,000.
Worked example
1 ppm = 1 g in 1,000,000 mLOne in a million, by definition.
1 g = 1,000 mg and 1,000,000 mL = 1,000 L, so 1 ppm = 1 mg per litreThe same statement in units a pharmacy can weigh and pour.
8 ppm = 8 mg per litre = 0.008 mg/mLMultiply straight through.
1 ppm = 0.0001%A percent is parts per hundred, so a millionth is ten thousand times smaller.
Your turn
Write 25 ppm as a percent.
25 / 10,000 = %
Solve one, graded on the server
The trap
Handling ppm as though it were a percent. They are four decimal places apart, and a number that is ten thousand times off still fits in the box on the worksheet.