Calculating Phosphate Concentration in Water: A Scientist's Perspective

[caliban07]

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?

 

[caliban07]

Way*

 

[Borek]

Define "phosphate". In typical application it will mean just the PO₄^3-, but you most likely dissolved some salt, of which PO₄^3- is only a fraction.

50 ppm suggest you did calculations right, just not taking above into consideration.

 

[caliban07]

Define "phosphate". In typical application it will mean just the PO₄^3-, but you most likely dissolved some salt, of which PO₄^3- is only a fraction.

50 ppm suggest you did calculations right, just not taking above into consideration.

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.

 

[Borek]

KH₂PO₄ has a molar mass of 136 g/mol (assuming anhydrous salt), PO₄^3- is 95 g/mol, so in the end you will be closer to 95/136*50 = 35 ppm of phosphate.

 

[epenguin]

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!

 

[caliban07]

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!

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.

 

[Borek]

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.