If you compare figures on the concentration of brine, you’ll quickly come across different units: percent, grams per litre and grams per kilogram. They all describe how much salt a solution contains, but each uses a different reference quantity. That’s why the numbers can’t always be compared directly.
We explain what brine is, how it forms and its role in salt production in the article “What is brine? Definition, formation and salt production”.
Here, we go a step further: how do you read figures on salt content correctly and compare them with one another?
What does salt concentration mean?
Put simply, salt concentration tells you how much salt a solution contains. For a figure to be unambiguous, though, it also needs to be clear what that amount relates to.
IUPAC distinguishes, among other things, between mass fraction and mass concentration. Mass fraction relates the mass of a component to the total mass of the mixture, while mass concentration relates it to the volume of the mixture.
That’s where the difference between figures such as 10 percent, 100 g/kg and 100 g/l comes from.
What does a percentage mean?
With a percentage, the first thing to establish is the reference quantity. For salt solutions, the mass fraction is often used.
A sodium chloride solution with a mass fraction of 10 percent contains 10 grams of salt in a total of 100 grams of finished solution. Scaled up to one kilogram, that’s 100 grams of salt per 1,000 grams of solution, or 100 g/kg.
What counts here is the mass of the finished solution. If you add 10 grams of salt to 100 grams of water, you end up with 110 grams of solution, so the salt content is: 10 g / 110 g ≈ 9.1 %
So the two figures don’t describe the same composition.
A percentage needs a clear reference quantity
A percentage isn’t always unambiguous.
With a mass fraction of 10 percent, 100 grams of finished solution consist of 10 grams of salt and 90 grams of water.
In laboratory and practical settings, you’ll sometimes also see the notation “10 % m/v”. This means 10 grams of a substance in 100 millilitres of solution – a mass-volume figure, which describes something different from a mass fraction.
To avoid ambiguity, it therefore makes sense to state the reference quantity explicitly. Mass-volume figures, for example, can be given directly as g/100 ml or g/l.
What does g/kg mean?
Grams per kilogram compares two masses.
100 grams of sodium chloride per kilogram of finished solution corresponds directly to a mass fraction of 10 percent: 100 g / 1,000 g = 0.10 = 10 %
Here, too, the exact wording matters. “100 g of salt per kilogram of solution” isn’t the same as “100 g of salt per kilogram of water”.
If you add 100 grams of salt to a kilogram of water, the finished mixture weighs 1,100 grams, and the mass fraction of salt is around 9.1 percent.
What does g/l mean?
Grams per litre gives a mass in relation to the volume of the finished solution.
A concentration of 100 g/l sodium chloride means that one litre of finished solution contains 100 grams of sodium chloride.
It doesn’t mean that 100 grams of salt were added to one litre of water.
IUPAC calls the mass of a component divided by the volume of the mixture the mass concentration.
Why isn’t 10 percent automatically 100 g/l?
This is where density comes in.
A litre of salt solution doesn’t necessarily weigh exactly one kilogram. As the salt content rises, so does the density of an aqueous sodium chloride solution.
For a 10 percent NaCl solution, the density at 20 °C is around 1.0707 g/ml, so one litre weighs about 1,070.7 grams.
10 percent of that is sodium chloride: 1,070.7 g × 0.10 ≈ 107.1 g
A 10 percent NaCl solution at 20 °C therefore contains around 107 grams of sodium chloride per litre, not 100 grams.
Without knowing the density, you can’t convert a mass fraction exactly into g/l.
The table below shows the difference for three concentrations:
| Mass fraction | g/kg finished solution | Density at 20 °C | Salt per litre |
|---|---|---|---|
| 5 % | 50 g/kg | 1.0340 g/ml | approx. 51.7 g/l |
| 10 % | 100 g/kg | 1.0707 g/ml | approx. 107.1 g/l |
| 20 % | 200 g/kg | 1.1478 g/ml | approx. 229.6 g/l |
Density values for aqueous sodium chloride solutions at 20 °C. Source: CRC Handbook of Chemistry and Physics.
At low concentrations, the difference is fairly small because the density is still close to that of water. The higher the salt content, the bigger it gets.
Why does temperature matter?
With a mass fraction, a change in temperature doesn’t affect the ratio of salt mass to total mass, as long as no water or salt is added or removed.
With a figure in g/l, however, volume comes into play, and liquids change volume with temperature. That’s why precise density and volume figures always state the temperature at which they were measured.
The values in the table, for example, apply at 20 °C.
This is another reason why you can’t convert a percentage exactly into g/l without more information.
What does this mean for brine?
In the German spa and medicinal water sector, there are specific requirements for the conventional designation “Sole” (brine). The German Spa Association (Deutscher Heilbäderverband) specifies at least 5.5 grams of sodium ions and 8.5 grams of chloride ions per litre.
In a sodium chloride solution, these ion levels correspond to roughly 14 grams of sodium chloride per litre.
This example shows why the unit matters: the threshold is given in grams per litre, not as a percentage by mass.
With weak salt solutions, the figures in g/l and percent by mass can be fairly close. But they’re still different quantities, and an exact conversion requires the density of the solution.
What happens to the concentration when water evaporates?
When water evaporates from a salt solution, the dissolved sodium chloride initially stays behind. The same amount of salt is then spread through less solution, so the concentration rises.
This can’t go on indefinitely. At any given temperature, water can only hold a limited amount of sodium chloride in solution.
At 20 °C, a saturated aqueous sodium chloride solution contains around 26 percent sodium chloride by mass. If more water is removed from a solution that’s already saturated, salt can crystallise.
In simplified form, the relationship looks like this:
Water evaporates → salt concentration rises → saturation is reached → salt can crystallise
This links the concentration figure directly to a process that also played a major part in historical salt production.
Percent, g/kg and g/l side by side
| Figure | Reference quantity | Example |
|---|---|---|
| 10 % mass fraction | Salt mass in relation to total mass | 10 g salt in 100 g solution |
| 100 g/kg solution | Salt mass in relation to total mass | Equivalent to 10 % mass fraction |
| 100 g/l | Salt mass in relation to solution volume | 100 g salt in 1 l finished solution |
The first two can be converted directly into each other. To convert between mass fraction and g/l, you also need the density.
Frequently asked questions about salt concentration
Is 10 percent the same as 100 g/kg?
Yes, if both figures refer to the mass of the finished solution. 100 grams of salt in a total of 1,000 grams of solution corresponds to a mass fraction of 10 percent.
Is 10 percent the same as 100 g/l?
No. A mass fraction compares mass with mass, while g/l relates a mass to a volume. Because of its density, a 10 percent NaCl solution at 20 °C, for example, contains around 107 grams of salt per litre.
Why isn’t the number on its own enough?
Because a number only becomes unambiguous together with its unit and reference quantity. 10 percent, 10 g/kg and 10 g/l are three different concentrations.
When is a sodium chloride solution saturated?
At 20 °C, a saturated aqueous sodium chloride solution contains around 26 percent NaCl by mass. How much of a substance can dissolve depends on factors such as temperature.
Conclusion
When it comes to salt concentration, the unit matters just as much as the number. A mass fraction and a figure in g/kg both refer to masses and can be converted directly into each other. A figure in g/l, by contrast, relates the amount of salt to a volume, so an exact conversion between the two requires the density of the solution.
With brine in particular, this distinction helps you read concentration figures correctly. It also explains why a salt solution becomes more concentrated as it evaporates, and why salt eventually crystallises once the concentration is high enough.
This article is for general information purposes only.
