10 February 2009

Molarity Article by Steffi Holt

Glucose diagram: Fig 1
Sucrose diagram: Fig. 2

Periodic Table:

Molarity is a unit used to measure the concentration of a solution. It is based on the molecular weight of 1 molecule of the solute (calculated from the periodic table) in a certain volume of solvent & relates to how many molecules of solute are contained within that volume of solvent.

Solute = thing being dissolved
Solvent = liquid doing the dissolving
Solutoin = Solute + Solvent mixed together

The most relevant solution for bartending is sugar syrup, so we shall work with that as an example of how to calculate the molar strength of a solution.
The solute in sugar syrup is normal sugar, or said in a chemical way, sucrose. The molecular weight is calculated by adding up the weights of all the atoms in one molecule, so to do this you need to know the structure of the solute & the weights of the atoms. This can be found on the periodic table of elements – each atom has its weight displayed below its letter. There also has to be only 1 solute being dissolved in the pure solvent (usually water).

For the structure of Sucrose see fig 1

The chemical structure of sucrose is C12H22O11 meaning its molecular weight is 12 x molecular weight of Carbon(12) added to 22 x the molecular weight of Hydrogen(1) added to 11 x the molecular weight of Oxygen(16). This totals 342, which is the molecular weight of sucrose.
Handily enough, the scientists who came up with Molarity decided that 1 mole of any substance should be the molecular weight in grams. Meaning you have 1 Mole of sucrose when you have 342 grams of it.

To make a 1 molar solution of sucrose, you would measure out 342g of sucrose and add enough pure water to make the total volume of liquid 1 litre. It is important that you don’t just add 1 litre of water as the total volume would be too much – it needs to total 1 litre with the solute in it. You also need to make sure the water is the same temperature every time as liquids change volume with temperature – I think for making sugar syrup hot, but not boiling water is the most efficient.

This all doesn’t sound too relevant until you look at bars that make their own sugar syrup. Can they guarantee that it is exactly the same concentration every day? A lot of drinks depend on their sweet-sour balance being just right, so everything should be done to ensure the sugar syrup is always at the same strength as even a small difference could change a drink.

Some bars that make their own syrup use either a 1:1 ratio or a 2:1 ratio, but they measure the amount of sugar & water in the mixing bottle by eye which is of course different depending who does it.
Some other bars get the chefs to heat water with some sugar in it – even if you use the same amount of water & sugar each time, the water will evaporate a different amount every day depending on the surrounding temperature/humidity/size of pan etc.

We get around this by tasting every drink we make to check the balance, & as everyone’s palate is different there can be no quick fix to ensuring a perfectly balanced drink every time, but I do believe that the more constant our sugar syrup is, the less things we have to keep an eye on that could alter the flavour of a drink.

A factor that makes Molarity difficult to use in real-life situations is the water. If anything is already dissolved in the water then the Molarity of the solution changes & not many of us have access to distilled water in our bars!
If we were doing scientific experiments with the solution, this would be unacceptable, but as we are using drinking water which should have the same ‘other solutes’ in it pretty constantly, then this shouldn’t affect our syrups on a day to day basis. Obviously filtered water is preferable to stuff just out of the tab as well.

We also can’t really calculate a more complicated solution. There has to be only 1 thing to be dissolved each time – as soon as you have more than 1 solute then you have to make up pure solutions of each one & then mix them together. You can’t calculate a 1M solution of say powdered coffee as there are many molecules making up that coffee & we don’t know what most (or any!) of them are to calculate the molecular weight.

This does become helpful when you look at other sweeteners however. Glucose has a molecular weight of 180 as it is much smaller, so you need only 180g in a 1L solution to have a 1M solution. A 1M solution of glucose and a 1M solution of sucrose have exactly the same amount of ‘sweet’ molecules in it, so they taste as sweet as each other, but you need to dissolve much less glucose to get the same concentration. You can buy glucose powder to use instead of using ‘table sugar’ which is sucrose.

So maybe next time you make up some sugar syrup, try making a 1M and 2M solution of it to see how it compares to your regular solution & maybe calculate the Molarity of the solution you usually make.

Grams of sucrose used = Moles of sucrose
342 (no of grams in 1 mole)

Moles of sucrose = Molarity of solution.
Litres of solution when made up

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