[matt1234]

Hi there,
I have a question about how to determine ppm or ppb or ppt for solutions. I cant seem to get the appropiate units for each of these. 
For example:
A soln of an enzyme is prepared by dissolving 0.002 g of enzyme in 250 mL of total solution.  Find concentration as ppm (m/v)

I understand how concentration should be mass over volume i just dont understand what i should convert to before performing the division.  I know in this case its mg over liters since the teacher geve us the answer.  What i dont know how to do is find the appropiate prefixes for ppm, ppb, or ppt for each mass over mass, mass over volume, and moles over volume well before performing the division, but i DO know how to convert units.

To sum it up,
Im not sure what to convert to. 

Thank you.

Matt

[steak]

ppm of enzyme component= [mass of enzyme component (g)/total mass of soln (g)] x 10^6

so you need to find the total mass of your solution, and that's:

 [mass of the orignal 250 mL solution + mass of the added enzyme]

Make sure you're using grams, then just follow that ppm equation.  You don't need to worry about volume when finding ppm...you worry about mass.   And the density of H20 is 1g/mL.  So 300 mL of water weighs 300g.  That figure might help you if your solution is water.

Mike

[DrCMS]

You don't need to worry about volume when finding ppm
Mike

Mike read and understand the question before you post that way you might spot in the question ppm (m/v) and not make a dick of yourself next time.

Hi there,
I have a question about how to determine ppm or ppb or ppt for solutions. I cant seem to get the appropiate units for each of these. 
For example:
A soln of an enzyme is prepared by dissolving 0.002 g of enzyme in 250 mL of total solution.  Find concentration as ppm (m/v)

I understand how concentration should be mass over volume i just dont understand what i should convert to before performing the division.  I know in this case its mg over liters since the teacher geve us the answer.  What i dont know how to do is find the appropiate prefixes for ppm, ppb, or ppt for each mass over mass, mass over volume, and moles over volume well before performing the division, but i DO know how to convert units.

To sum it up,
Im not sure what to convert to. 

Thank you.

Matt

Matt just do the maths and then use appropriate units.  For example if the answer comes out as 5000ppb you'd change it to 5ppm if it came out as 0.05ppb you change it to 50ppt etc etc.

0.5g in 1000ml = 500ppm m/v
0.5g in 1000L = 500ppb (not 500000ppm)

[steak]

 

[Dan]

Matt just do the maths and then use appropriate units.

If I understood the original question correctly, which I may not have, "appropriate units" is the problem.

It's all very well asking for concentration in "ppm (m/v)" but what does this mean? g/L? ounces/pint? etc.

As far as I'm aware, in order to make ppm, ppb etc. unambiguous, it should be a unitless quantity - g/g, mg/mg, ounces per ounce or Roman drachms per Roman drachm. The fact that the density of water is 1 g/mL makes the "appropriate unit" in the context of this poorly constructed question g/mL.

The numbers work for water, but switch to a solvent of a different density and you can't start calculating ppm in terms of g/mL because you aren't going to get the right numbers.

The fault, in my opinion, is with the question in requesting ppm with units, which is a fundamental error and a bastardisation of scientific notation. I think Mike was absolutely right. Then again, perhaps I'm just a dick too.

[DrCMS]

Dan have you ever seen something described as for example 5% m/v?

[matt1234]

Thanks for the help guys.  I dont want to start any arguements, i was and still am confused about it. From here on out i will divide similar units then divide by 10^6 and see if i get the appropiate answers. Ill post back when i figure it out if i do.

Edit:
just watched this

http://www.youtube.com/watch?v=SiFy3iOXLdw&feature=related

but its a little to general for my liking. he has a "rule" that says convert all to mg/L for questions with volume and mg/Kg for questions with 2 masses.  thats not the way i like to learn thing but i suppose it will have to do for now.


this also helps.

http://www.youtube.com/watch?v=3aHcAAipr4g&feature=rec-LGOUT-exp_fresh+div-HM

[Dan]

he has a "rule" that says convert all to mg/L for questions with volume

That only works for aqueous solutions though, because 1 L = 1 kg for water.

If you have a solvent with a different density then you should stick with g/g x 10⁶ (which is the same as mg/kg).