precent w/w question

Was asked this question the other day, does one need density to do %w/w

example, simple formula

siligel 0.3%
glycerin 9%

now if we're making 100g

do you add 9 g of water which comes out to 8.5mls ( if you look at the amount you added to get 9g)

or do you add 7.143mls which is taken for Glycerol's density of 1.26g/ml which calculates to that 7.143ml = 9g glycerol

Any takers?



  • PerryPerry Administrator, Professional Chemist
    When using weight measurements everything is in terms of Grams.  Volume does not matter.

    If you want 90% water in a 100g batch, you add 90g of water
    If you want 9% glycerin in the same batch, add 9g of glycerin

    Density & volume do not matter.
  • Yep. thanks
  • @Perry
    what is your method when going through your protocol? Say when using %w/w and you have to heat and weigh your liquids and solids? do you weigh all your liquids first on a scale then place on a hotplate then add your solids?

    If so, how about if the protocol calls for a strict sequence where you can't add all liquids at once? Do you then add all ingredients in your vessel then heat and stir?

  • PerryPerry Administrator, Professional Chemist
    Typically, you weigh your ingredients in weigh boats (something like these

    You can pre-weigh all your ingredients, but you can also weigh as you go.  You should follow the proper sequence and not add any ingredient at the same time. I always tried to imagine what a compounder who was making a 3000 gallon batch would be able to do. Ultimately, your manufacturing procedure needs to be scalable.
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