0.25 gram of soil sample is digested using 2 mL conc. HNO3 + 0.5 mL conc. H2O2 (i.e. a total of 2.5 ml of conc. acids). Then, distilled water was added to bring the solution in the digestion tube to the final volume of 20 ml. Furthermore, 1 ml of the final solution was taken and diluted in 9 ml distilled water (1 ml+9 ml) and analysed for calcium, aluminum, etc.
For example: Ca = 11.22 mg/L, Al = 42.79 mg/L
How to convert these values to mg/kg?
Thanks in advance.
Try to track your sample in the course of dilution. That is how I do. In your case, you have 0.25g in 20ml solution. In the 1ml you took to dilute, it means you have 0.0125g soil (0.25g/20ml). This is also the amount you have in your diluted 10ml solution, despite you added water to the 1ml solution to raise the volume to 10ml. Then, your ICP values were obtained using the 10ml, which is 0.01 L. In this 0.01L, you have 0.0125g soil, meaning you have 1.25g soil in 1L solution. Now, if you obtain a result of, for example, 11.22mg/L Ca, it means you have 11.22mg/1.25g = 8.976mg Ca/g soil or 8976mg Ca/kg soil. I hope this helps.
Thanks Biyensa and Bayu. Is the initial volume of conc. acid (2.5 ml) used for digestion not considered in the calculation?
I mean, conc of ion (mg/kg) = (mg/L * (20 ml/2.5ml) * d.f)/wt of soil
You can have many solvents added in your solution, but it doesn't affect your soil weight, which still is 0.25g in the 20ml solution. But, the weight changes when you take 1ml from the solution to dilute it. In this 1ml, you only have 0.25/20 = 0.0125g soil. You can now raise the volume to any (in your case 10ml), but the weight now is unaffected in the total volume, it still is 0.0125g in the 10ml solution, but if you take 1ml of this, you only have 0.00125g soil in this 1ml solution. This way you can easily track your soil weight.
Thanks Paul, I see I made a mistake with the units when converting. I have now edited and made correction to it.
Thank you Biyensa. But, how 11.22 mg/L = 11.22 mg/g? I didn't quit understand that. That mass tracking is used only if density is not changing during the dilution process. But when we take 1 ml from the 20 ml solution and dilute by 10 factors, density of the solution changes.
What if I simply use the following equation:
Ca (mg/kg) = (Ca(mg/L) * 20 mL * d.f)/wt of soil (kg)
= (11.22mg/L * 0.02L * 10)/0.00025 kg
= 2.244 mg /0.00025 kg = 8976 mg/kg Ca
Where
0.02 L = final volume of the solution
10 = dilution factor
0.00025kg = 0.25g = soil sample used
I may have complicated it, but that's the easiest way for me to approach, because I don't have formula :)
Yes, the density changes when diluting, but the wt in the total volume doesn't change.
Congrats, you've got a formula!
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