Is it true that PPB and mg/L are interchangeable?

The statement that PPB (parts per billion) and mg/L (milligrams per liter) are interchangeable is false. While both units are commonly used to express concentrations of substances in water, they are not universally interchangeable because they measure different quantities, depending on the density of the substance in question.

PPB is a measure commonly used in analytical chemistry to quantify very low concentrations and is defined as one part of a substance per billion parts of the total solution or mixture. Conversely, mg/L as a concentration unit indicates how many milligrams of a substance are present in one liter of solution.

The equivalence between these units can only occur under specific conditions, such as when dealing with substances that have a density of 1 g/mL, like water at standard conditions. In that case, 1 µg/L equals 1 PPB and equally 1 mg/L equals 1,000 PPB. But this is not true for all substances, and therefore it’s crucial to understand the specific context and substance involved when interpreting concentration data to avoid erroneous assumptions.