For reporting chemical analysis of water samples some time we report cations and anions in mg/lit and some time in ppm. Some people may find difficult to understand.It is better to use either use mg/lit or ppm.

Well that depends: ppm is only feasible if you know the density or density of the solution is 1.

Most chemical analysis on water samples are volume based. Therefore you will receive results in weight of analyte (e.g. your ions) per volume of solution (sample). If density is 1, than 1 litre is 1 kg and 1mg/L is 1ppm. For sea water 1ppm and 1mg/L are no longer the same.

Would be interesting: Does anyone know a procedure (chemical analytical method) that start by weighing in a water sample?

Guess mg/L is a better unit for expressing your results.

Thanks Tobias for your very valuable information and appreciate the way of explanation of my question.

Thanks Saif for your kind interest and useful suggestion.

I agree with Tobias although, if we are realistic, the terms ppm and mg/l are used commonly (and wrongly) interchangeably by many people to the extent that in analysis of liquid samples almost every reported ppm result is actually a mg/l value.

Another disadvantage of using ppm is that, in atmospheric analysis, the term is commonly (and in this case legitimately) used to indicate concentrations measured in parts per million by volume. Imagine the confusion if a mass concentration from a GC analysis of a carbon tube extract is reported in ppm. The potential for errors, particularly by inexperienced students, is immense. I have seen it too many times.

Of course in reporting mass concentrations of analytes in solids it could be mathematically correct to use ppm or ppb but I always recommend that students use mg/kg or µg/kg, it is far safer and it makes the mathematics more easily understood when calculating the concentration in the original matrix after analysing an extract or digest solution (another process which, in my experience, students find hard enough without complicating the situation with inappropriate units).

Alan, Thanks for giving very good explanation for mg/lit and ppm. Your study will be useful for researchers.

Many report data in molarity; typically micromoles per litre or millimoles per litre, because that allows easy observance of relative numbers of atoms/molecules. Units of ppm and mg/L persist because of the preferences of non-chemists (e.g. at WHO) and from force of habit e.g. amongst hydrogeologists.

There is no difference: mg/l is ppm due to the fact that water density is 1 gram/cm3.

I guess micromoles/millimoles per liter is the correct unit alternatively mg/L.

Dear Dr. Ebraheem,

The ppm is a mass/mass unit. The mg/L is a mass/volume unit. So, in seawater with about 3.5% salt, 1 litre of seawater weighs about 1.035 kg. In seawater, Ca concentrations of (say) 400 mg/L would have a concentration of 400/1.035 as ppm. A small difference, but an important one. The difference is usually ignored when dealing with freshwaters (rivers, lakes, potable groundwater) because the density correction is typically < the uncertainties in most chemical analysis that might be undertaken. Nevertheless, confusing the units remains incorrect.

Work with brines, with up to 30% salt content, or hydrothermal waters at temperatures up to several hundred degrees Celcius (under pressure) and the difference between mass/mass units and mass/volume units REALLY matters.

If we are analyzing liquid*(water) samples then we should report in mg/lit, for solid samples ppm be written

Dear Dr. McArthur

Thank you very much for your explanation. In my answer I was considering only the normal groundwater samples (fresh to slightly saline water}. But thank you for your correction. Kind Regards

There is another consideration also much overlooked. The volume of aqueous solutions is temperature dependent. One may collect a sample of river water in mid-winter (say 4 degrees C) and analyse it in the lab at 25 degrees C, The result, in mg/L, will not be the true concentration applicable to the field because the volume of the sample will have increased as it warmed by 21 degrees. The effect is small - around 0.4% in the example cited, but use of mg/kg of sample (or, if one really wants to be pedantic, mg/kg of solvent) eliminates this error. Where this correction, or its avoidance, becomes important is when dealing with thermal springs, geothermal energy, and climate change. For the latter, the deep ocean is at a temperature of around 2-4 degrees C. Warm it to 15 or 20 degrees C (as humans are doing their best to do) and that alone could raise sea-level by 5 to 10 metres - enough to cause havoc worldwide, albeit in the far future.

When the solute mass is much smaller than the solution mas, ppm is normally used to measure chemical concentration, usually in a solution of water.

Solute concentration of 1 ppm is equal to solute concentration of 1/1000000 of the solution. ppm is dimensionless quantity, a ratio of 2 quantities of the same unit. For example: mg/kg.

In dilute, aqueous solutions of the type typically found in industrial water systems, "at temperature of 20ºC", the solution density (ρ) of water is 998.2071, nearly is equal to 1, then ppm≈ mg/L.

Milligrams per liter (mg/L) and parts per million (ppm) begin to diverge as the solution density varies from 1. The examples of this error are dense sludge liquids and DNAPL: Dense Non-Aqueous Phase Liquids (density greater than 1) or closed cooling system water with high concentrations of organic compounds and LNAPL: Light Non-Aqueous Phase Liquids (density less than 1).

It is an interesting question and very nicely explained as above by  Alan P Newman and all others. Inspite of the fact as stated above, the concentration of uranium in sea water is reported /written as 3ppb.

Agree with all the panel members and especially the view of Prof. Rathore.

As explained by Professor McArthur, the ppm is a mass/mass unit. The mg/L is a mass/volume unit. It is one of the unique properties of water liquid that we can use either mg/l or ppm units for concentration as normal water density is 1 gram per cubic centimeter.

Ebraheem , Thanks for your interest in this subject. Appreciated your valuable remark.

Jasim, appreciated your post.

water analysis data some time reported in ppm, mg/lit and milli equivalent part per millions . They have a big significance and usefulness/