Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: dlow on January 12, 2021, 02:30:12 PM

Hi everyone.
I am wondering if you could clear something up for me.
I am trying to understand why I divide the final ppm of my calibration solution by the density of the original stock.
Just to elaborate. In work we take a weighed aliquot from a bought in stock solution and then dilute it to a specific volume to achieve the desired ppm in solution.
When calculating the final ppm in solution we divide the penultimate ppm in solution by the density of the stock solution.
For example
stock solution concentration 1000ug/ml & density 1.0017g/cm3
Aliquot 5g
Final Volume 250ml
1000 x 5 = 5000 therefore 5000/250= 20ppm in solution and finally 20/1.0017 = 19.9661 final ppm in solution.
I am wondering why we do this conversion from concentration by weight ug/ml to concentration by volume ul/ml. Is it because we are making the calibration solution to volume? Or would we do the same conversion even if the original stock was in concentration by volume anyway i.e. ul/ml.
A point in the right direction would be gratefully appreciated.
Thanks
Dave

Why do you say 1000 x 5 = 5000? Do you mean 1000 ug/ml x 5 ml = 5000 ug? But if, as you say, your aliquot is 5 g, not 5 ml, you don't have 5000 ug in your solution. The volume of your aliquot is 5/1.0017 ml, so the mass of analyte in your solution is 1000 x 5/1.0017 ug. Divide this by 250 to get your ppm.

Thanks for the reply. I think our calculations are the same. I just wanted to know why?
Are you saying we fine tune for the density because we use the weight of the aliquot and not the volume?

Yes

Wonderful.
Thank you