[Baileys]

I need to calculate the amount of Zn in a soil sample (mg kg-1), using the following information:

1.000g of soil is digested in 15mL of concentrated nitric acid.

This solution is filtered and the washings collected in a 150mL flask. The solution is made up to the mark.

A 20mL aliquot is taken from this flask, transferred to another 100mL flask, and made up to the mark.

[Borek]

Not enough information.

[Baileys]

The concentration of Zn is 1.6mg L-1 in the diluted solution, going by the calibration graph

[Baileys]

Ok, I'm not entirely sure how to go about this calculation, but I have had a look on the internet and came accross some equations, which I have writte below:

To find the element concnetation in the original sample:

P = (Pf - P0) fd fa

Where:
P is the concentration of the element in the original sample in mg/l; (however, mine has to be mg kg-1)
Pf is the concentration of the element in the test sample in mg/l;
P0 is the concentration of the element in the blank in mg/l;
fd is the dilution factor due to digestion of an aqueous sample;
fa is the dilution factor of the test portion


To find the element concentration in the digested sample:

w = (Pf - P0)faV/m

w is the mass fraction of the element in the solid sample in mg/kg;
V is the volume of the test sample (digest) in litres;
M is the mass of the digested sample in kg.

Would either of these equations work for this solution? I personally think it would be the first equation.

[Borek]

All you need is a dilution factor.

[Baileys]

Can the dilution factor be calculated using the information I gave originally?

[Borek]

Dilution factor - yes. Concentration - no.

[Baileys]

Ok, the concentration values I already have, so it is just calculating fd and fa. This is what I came up with:

If 1000mg soil is diluted into a 0.15 L flask = 0.00015 mg L-1 (fd)

A 20mL aliquot of this taken, and diluted into a seperate 100mL flask = 0.00003 mL  (fa)

[Golden_4_Life]

Bailey: Fortunately you have a straightforward set of dilutions.  Here is the sequence:
1.  1.0 g soil was digested with 15 mL acid = 0.066g/mL = 66 mg/mL total sample

2.  Then you QS'd volume to 150 mL with water, thus
      66 mg/mL x 1/150 = 0.44 mg/mL

3.  then you took a 20 mL subsample and QS final volume to 100 mL with water, thus
0.44 mg/mL x 1/5 = 0.088 milligrams/mL = 88 micrograms/mL is the final concentration value.

4. Run the analysis test (is it Titration or UV Absorbance your using as a Zinc detection device?) on this extracted sample and get the test value.

5. Compare this value to your calibration curve for various levels of Zinc spiked into soil and extracted the identical way.
Y axis = Absorbance value, X axis = zinc concentration levels in the Standard.

6. Drop down the Absorbance value for your sample compared to the zinc level for your   standards, and Bob's your uncle!

[Baileys]

Thank you very much for your response. It has helped me understand the calculation a lot more 

In step 2, you multpiled by 1/150...where did you get 1 from, or why use 1/150?

In step 3, you multiplieed by 1/5...again, where did you get 1/5 from?

[Borek]

1/5 = 20/100

[Train]

I think point #2 in Golden_4_Life's outline has a typo.  It should read 66.7 mg/mL x 15/150 = 6.67 mg/mL because the entire volume of extract was diluted to 150 mL.  At least, that's how I read the original problem.

Actually, I think the 15 mL can be safely ignored for purposes of solving the problem, because it just cancels out.

DF = (150 mL/1.0 g) x (100 mL/20 mL) = fd x fa

However, you need to adjust the units.

Bailey: Fortunately you have a straightforward set of dilutions.  Here is the sequence:
1.  1.0 g soil was digested with 15 mL acid = 0.066g/mL = 66 mg/mL total sample

2.  Then you QS'd volume to 150 mL with water, thus
      66 mg/mL x 1/150 = 0.44 mg/mL

3.  then you took a 20 mL subsample and QS final volume to 100 mL with water, thus
0.44 mg/mL x 1/5 = 0.088 milligrams/mL = 88 micrograms/mL is the final concentration value.

4. Run the analysis test (is it Titration or UV Absorbance your using as a Zinc detection device?) on this extracted sample and get the test value.

5. Compare this value to your calibration curve for various levels of Zinc spiked into soil and extracted the identical way.
Y axis = Absorbance value, X axis = zinc concentration levels in the Standard.

6. Drop down the Absorbance value for your sample compared to the zinc level for your   standards, and Bob's your uncle!