A trace result labelled ppm needs the underlying ratio. Milligrams per kilogram and milligrams per litre coincide numerically only under an additional density condition.
Write the ratio in explicit units
A parts-per-million concentration can be expressed on a stated mass basis. For that convention, 1 mg/kg represents a mass ratio of one part in a million. A result in mg/L instead divides mass by solution volume, so density is required between the two. Primary reference.
Convert a fictional trace record
Take an invented trace concentration of 5.0 mg/kg in a solution whose specified density is 1.10 kg/L. One litre has 1.10 kg of solution, so it contains 5.0 × 1.10 = 5.5 mg of the trace constituent. The resulting volume-based value is 5.5 mg/L.
With an expressly assumed density of 1.00 kg/L, the same conversion would give 5.0 mg/L. The familiar equal numbers therefore follow that assumption, not the letters ppm themselves. A mole-fraction ppm convention would require a different calculation and entity definition.
Preserve what the laboratory reported
For an original traceability record, retain the laboratory’s ratio definition, constituent identity and density used for any derived value. Preserve the original result alongside the conversion so the receiving team can tell which number was actually measured and which was calculated.
This guide defines no residue limit or acceptable water quality. The equipment-review question is narrower: does the concentration record retain enough information to compare its mass-based and volume-based versions? A rounded match between numbers should not substitute for that dimensional check.
Solution review worksheet
This blank worksheet is for your own project. It contains no H M machine trial result.
| Question | Record to retain |
|---|---|
| Which ppm convention applies? | Explicit mass, amount or other ratio |
| What is the original result? | Value, units and constituent identity |
| What supports the volume conversion? | Solution density and applicable conditions |
Customer Questions
What is the fictional volume-based result?
5.5 mg/L.
When do 5 mg/kg and 5 mg/L coincide numerically?
For the stated conversion when density is 1 kg/L.
Does every ppm label mean mg/kg?
No. Its ratio basis must be specified.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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