We are frequently told about the importance of reading labels on food products to find out exactly what is hidden in them. This is supposed to ensure that we don’t exceed sensible limits for sugar, salt, fat, alcohol or whatever.
So, I started by reading the label on a bottle of fizzy mineral water. The analysis results on the label tell me that it contains 1 708 mg.L-1 of sodium ions (post 16.39). However, most nutritional guidelines don’t tell us how many sodium ions we should consume – they tell us a safe upper limit to our daily salt consumption. So, the company that sells this water helpfully tell me that this is “équivalent à 0,53 g chlorure de sodium (sel) par litre” [equivalent to 0.53 g of sodium chloride (salt) per litre].
The label doesn’t say what it means by “equivalent to” but I guess it means the amount of salt that you would need to eat to obtain 1 708 mg = 1.708 g of sodium ions. That’s very strange because salt is a mixture of equal numbers of sodium ions and chloride ions (post 16.39). But the label tells me that a litre of water containing 1.708 g of sodium ions has less sodium ions than 0.53 g of salt.
If you’re not sure why this is strange – read this paragraph. Let’s represent the mass of sodium ions by mNa and the mass of chloride ions by mCl. Then the label tells us that
mNa + mCl = 0.53 g and mNa = 1.708 g.
This can only be true if the mass of chloride ions in the water is negative. Not surprisingly, the analysis results tell us that this isn’t true.
So what mass of salt do you need to eat to obtain 1.708 g of sodium? The relative atomic masses (post 16.27) of sodium and chlorine are 23 and 35.5. So, the mass of salt that contains 1.708 g of sodium is
1.708 x (23 + 35.5)/35.5 = 2.8 g.
Remember that, salt consists of equal numbers of sodium and chloride ions (post 16.39).
According to the British National Health Service, adults shouldn’t consume “more than 6 g salt a day (2.4 g sodium)”. You can check that these numbers are consistent by doing a calculation like the one above. So, it appears that a litre of my fizzy water would provide a bit less than half my maximum daily salt intake. Fortunately, I don’t drink that much fizzy water in a day.
We can conclude that it’s not enough just to read labels – we need to think critically about what they say. As I’ve written before, many politicians, journalists and “educators” believe that science is a bunch of facts that we are supposed believe because we have been told to believe them by experts. The label on my fizzy water is a warning that we should think for ourselves.