It is a term used in pharmaceuticals and food manufacture mainly. It is an abbreviation for Quantum satis and simply means ‘Add as much of this ingredient as is needed to achieve the desired result, but not more’. So if we take the example below
What you would need to do when deciding what to put in place of the ‘qs’ is read the guidelines for the maximum use rate. With most ingredients you only add up to the maximum allowable amount. When it comes to preservatives, normally you should try and use the minimum recommended amount and then adjust upwards if need be following your tests – this is because all preservatives are potential irritants so you should use the minimum amount to keep your product free of germs. Similarly with a perfume, you only need to use enough to give the desired scent. With water, that can be used up to 100% but for obvious reasons, you put in enough water to the formula to make the whole formula calculate to 100%. Now it may not stop there, you may have to make an educated guess if you are not sure about an ingredient. I got a sample of a fragrance and the supplier suggested the use rate as 1-2% – I decided to put 1.5% of said fragrance which was way too much and in the next batch I knew to reduce significantly, in this case to 0.2%. In sum, what you replace your ‘QS’ with may change after making the physical product and doing your tests.
So there you go, that was pretty straightforward!