Question

I want to measure the amount of Lead Pb in a soil sample ?

I have done all the digestion calculation and procedures and I used only 2 grams of soil .

The instrument data gave me the following concentrations :

Sample of soil 1= 2.06 ug/L

Sample of soil 2= -1.18 ug/L

So how much Lead in one gram of soil?

How I know if it's high and above the limit of allowed conimination !?

Answer 1

Ans. Given,

            Amount of soil samples taken for digestion = 2.0 gram

            Final volume of digested sample made upto = 25.0 mL

            [Pb] in soil 1 = 2.06 ug / L

            [Pb] in soil 2 = - 1.18 ug / L        [Note: concentration can’t be zero.]

#1. 2.0 gm soil ----> digestion ----> final volume of aliquot is 25.0 mL

Thus, all of the Pb present in 2.0 g soil sample is now present in 25.0 mL of the final volume aliquot.

#2. Analyzing a part of 25.0 mL aliquot gives [Pb] = 2.06 ug/L for soil 1.

It means, 1 Liter of the solution contains 2.06 ug Pb.

Now,

            Since, 1000 mL of sample has 2.06 ug Pb

            Or,        1 mL     -           -       (2.06 / 1000) ug Pb

            Or,        25 mL   -           -       (2.06/ 1000) x 25 ug Pb    

                                                = 0.0515 ug

Thus, total Pb content in 2.0 g sample of soil 1 = 0.0515 ug

Amount of Pb in 1 gram soil = Amount of Pb in 2.0 g soil / 2

                                    = 0.0515 ug/ 2= 0.02575 ug

Therefore, 1.0 gram of soil 1 contains 0.02575 ug of Pb.

Note: The final result depends on “the volume of digested sample made upto prior to analysis”

#3. Coming soil 2.

Concentration can’t be negative.

The -ve value can be a typo error.

Or, it may be due to errors during sample processing, digestion of soil sample, contamination of the sample blank, solution handling and volume make up, etc.

Please get the correct value and calculate as done in #2.

#4. It is a bit tricky.

In general, most of the geographical regions publish the mineral composition and heavy metal contamination reports prepared by sophisticated and authorized laboratory. Refer to one related to your geographical location.

The heavy metal content (like other minerals, etc.) of soil may vary if contaminated by household or industrial wastes.

So, it is difficult to say which report to rely upon because you don’t know when and how the soil has been contaminated in past. The best way to assure about result is to run samples in triplicates or more to get a concordant result.