I stayed last week on the Scottish Isle of Arran and spent one morning enjoying a very practical application (‘Utilization’) of chemistry by going round the Isle of Arran distillery. As well as seeing fermentation of malted barley and large scale distillation using copper stills I found a good practical application of the use of parts per million.

The copper still (left) and the distillate (right) from the Isle of Arran distillery

Measuring concentration in parts per million (ppm) is now part of the Guidance notes of sub-topic 1.3: Reacting masses and volumes on the new programme. One part per million means one part per 1.00 x 10⁶ parts i.e. one in a million or 0.0001%. It is dimensionless but is actually a confusing unit as it may refer to the mass fraction, the mole fraction or the volume fraction so really more clarity is needed when using it. However, most chemists use it to express aqueous concentrations, and take it to mean how many milligrams of solute are dissolved in one litre (dm³) of aqueous solution. This means that in fact it is not dimensionless as the unit is mg dm^-3 but since 1 dm³ of water has a mass of approximately 1 kg then it is effectively mg kg^-1 which is dimensionless.

On Arran most of the single malt whiskey they produce is peat free. However many aficionados of malt whiskey prefer the peaty taste of the ‘malts’ that are produced on the nearby Isle of Islay where the barley is smoked by burning peat. These single malt whiskeys contain a small amount of phenol which gives the whiskey its distinctive peaty taste. The Arran distillery makes one ‘peaty’ malt whiskey and it goes by the name of Machrie Moor. I am now the proud owner of one of a limited edition of 6000 cask strength bottles of 14 year old Machrie Moor which has matured for 14 years in an oak cask. It states clearly on the bottle (right) that the peating level is 20 ppm, i.e. it contains 20 mg of phenol per litre of whiskey.

Incidentally the Scots also use one other unit (that is not on our chemistry syllabus) when talking about whiskey. It is measured in ‘drams’ as in “would you like a wee dram?” A dram has been described in the book ‘Raw Spirit’ by Ian Banks as “a measure of whiskey that is pleasing to guest and to host” (i.e. to the person giving it and to the person receiving it) - now that is a truly scientific definition!

Cheers!