PART PER MILLION: One liter of water has a mass of 1kg. But 1kg is equivalent to 1000g or 1 million mg. therefore, if 1 mg of a substance is dissolved in 1 L of water, we can say that there is 1 mg of solute per million mg of water. In other words, there is one part per million (1 ppm)
Neglecting the small changes in the density of water as substances are dissolved in it, we can say that, in general, a concentration of 1 mg per liter is equivalent to one part per million: 1mg/L=1ppm. Conversion is very simple; for example, a concentration of 17.5 mg/L is identical to 17.5ppm
The expression mg/L is preferred over ppm just as the expression g/L is preferred over its equivalent of ppb. But both types of units are still used and the student should be familiar with each.