1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Concentration in solution

  1. Apr 24, 2015 #1
    If I add 1 gram of phosphate to 20 litres of water. What would the concentration of phosphate in solution in ppm?

    I have somehow come up with 50ppm does this sound right?

    To achieve 1ppm phosphate in 20l of water I would require 0.02g?

    Or am a weigh off?
  2. jcsd
  3. Apr 24, 2015 #2
  4. Apr 24, 2015 #3


    User Avatar

    Staff: Mentor

    Define "phosphate". In typical application it will mean just the PO43-, but you most likely dissolved some salt, of which PO43- is only a fraction.

    50 ppm suggest you did calculations right, just not taking above into consideration.
  5. Apr 24, 2015 #4
    Thanks. It's actually mono potassium phosphate used as a plant fertiliser or on this case as a nutrient for autotrophic bacteria during nitrification.

    I was more interested in the math. I gather that what you are saying is it would not be pure phosphate and that the same would apply with the mono potassium phosphate? It doesn't really have to be that precise and the calculation was purely hypothetical based on the molecular mass of phosphate.
  6. Apr 24, 2015 #5


    User Avatar

    Staff: Mentor

    KH2PO4 has a molar mass of 136 g/mol (assuming anhydrous salt), PO43- is 95 g/mol, so in the end you will be closer to 95/136*50 = 35 ppm of phosphate.
  7. Apr 25, 2015 #6


    User Avatar
    Homework Helper
    Gold Member

    Sorry I think the original answer 50 ppm is right. Ppm is a simple 'practical' measure. Parts per million of that particular product, whatever it is. I.e. Grams per million grams of water - in practice per million litres.

    Now sure one ppm of one of the several forms of phosphate wouldn't have the same moles of phosphate (or of potassium) as another form, but if the difference between 35 and 50 was significant for the application, then each product would have to come with a different recommendation of how many ppm to use.

    Ppm is less scientific and flexible but reccomended molarity would not be helpful to the majority of users of plant fertiliser!
  8. Apr 25, 2015 #7
    Yes thank you. I wanted to split the phosphate away from the potassium to make the maths easier. I thought if I could get the phosphate part right I could work them as a whole if needed.

    It doesn't have to be overly precise it since fertilisers come dry it is useful to know exactly what you are putting in without having to use inaccurate online calculators.
  9. Apr 25, 2015 #8


    User Avatar

    Staff: Mentor

    I am far from saying 50 ppm is "wrong". It is just ambiguous and can be misleading once you try to get some "real" numbers. I have no problems with you saying your solution is 50 ppm as long as we agree it is just an approximation.

    As it is often the case, same thing has different meanings for different groups of users. It is just a matter of being sure you use it correctly in the context and you are correctly understood by the others.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Concentration in solution
  1. Solution Concentration (Replies: 1)