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  #1  
Old 05-22-2010, 08:33 PM
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Default How to convert mg/kg to mg/L

Sorry if this sounds stupid, but we've done a soil prac and found concentration values for 3 elements in the soil in mg/L, but the max values are in mg/kg.

Was wondering how to convert them;

"Maximum concentration in sludge (mg/kg or ppm)
Cadmium - 85
Copper - 3000
Lead - 4300"

Do I have to have the density of the 'sludge' to find the mg/L values?
If this helps...
Density of Cadmium = 8.64 g/cm³
Density of Copper = 8.92 gm/cm3
Density of Lead = 11.34 g/cm³
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  #2  
Old 05-23-2010, 12:03 AM
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Default Re: How to convert mg/kg to mg/L

Quote:
Originally Posted by Unregistered View Post
Sorry if this sounds stupid, but we've done a soil prac and found concentration values for 3 elements in the soil in mg/L, but the max values are in mg/kg.

Was wondering how to convert them;

"Maximum concentration in sludge (mg/kg or ppm)
Cadmium - 85
Copper - 3000
Lead - 4300"

Do I have to have the density of the 'sludge' to find the mg/L values?
If this helps...
Density of Cadmium = 8.64 g/cm³
Density of Copper = 8.92 gm/cm3
Density of Lead = 11.34 g/cm³
Hi, could you please describe if the "sludge" sample is the original soil sample, or did you take a few grams of soil, and digest it to make the sludge?
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  #3  
Old 05-23-2010, 02:50 PM
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Default Re: How to convert mg/kg to mg/L

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Originally Posted by Mrs X View Post
Hi, could you please describe if the "sludge" sample is the original soil sample, or did you take a few grams of soil, and digest it to make the sludge?
The prac we did was Acid digestion of sediments/sludges and soils.
preparing soil samples for AAS...

The sludge part comes from the US department of Agricultures Prevention of Heavy Metal Contamination section where it states the maximum allowed values of certain heavy metals.

The heavy metal concentrations [Quote] "Maximum concentration in sludge (mg/kg or ppm)"
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  #4  
Old 05-23-2010, 08:08 PM
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Default Re: How to convert mg/kg to mg/L

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Originally Posted by Mrs X View Post
Hi, could you please describe if the "sludge" sample is the original soil sample, or did you take a few grams of soil, and digest it to make the sludge?
Oh sorry didn't explain it right, we did an acid digest prac and from this tested it by AAS analysis...the concentrations we found in our soil sample were:
Cadmium - 0.016 mg/L

Copper (II) - 0.054 mg/L

Lead - 0.35 mg/L


What i want to compare them to is the standards (in the opening post) but need to convert those 'max' values which are in mg/kg to mg/L to see if our soil figures are under the max values outlined in the US Agr. document.

From googleing other sites it seems i need the density of the sludge to find the conc. in mg/L??? but have no idea as all the info given is the mg/kg figures for different metals.

Maybe i could convert my figures to mg/kg?? Not sure how to do that.
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  #5  
Old 05-24-2010, 10:39 AM
Mrs X Mrs X is offline
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Default Re: How to convert mg/kg to mg/L

Quote:
Originally Posted by Unregistered View Post
Oh sorry didn't explain it right, we did an acid digest prac and from this tested it by AAS analysis...the concentrations we found in our soil sample were:
Cadmium - 0.016 mg/L

Copper (II) - 0.054 mg/L

Lead - 0.35 mg/L


What i want to compare them to is the standards (in the opening post) but need to convert those 'max' values which are in mg/kg to mg/L to see if our soil figures are under the max values outlined in the US Agr. document.

From googleing other sites it seems i need the density of the sludge to find the conc. in mg/L??? but have no idea as all the info given is the mg/kg figures for different metals.

Maybe i could convert my figures to mg/kg?? Not sure how to do that.
The mg/L value that you have are from acid digesting a known weight of your soil, and diluting that to a known volume. Therefore, work out how many mg are in your sample of soil, and work out what that would mean for the original weight of soil.

If you reply with the weight of soil, and the volume you made the digested solution up to, we can help you with the method.
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  #6  
Old 05-24-2010, 09:36 PM
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Default Re: How to convert mg/kg to mg/L

Thanks for your help, just asked our teacher and he says we don't need to convert the figures, as they are US figures and we are testing in Australia....but it'd be nice to know how they compare to US standards.

I'll write what we did in the prac though, i think we didn't actually measure the vol. of digest.......

Procedure:
1. Weigh sample to three decimal places. (1.029 g)
2. Mix sample to homogeneity using mortar and pestle and sieve.
3. Transfer sample to 150mL beaker and add 10mL 1:1 HNO3. Cover with a watch glass for vapour recovery and reflux at 95C (±5C) for ~10 minutes without boiling.
4. Allow sample to cool and add 5mL conc. HNO3 and reflux for a further 30 minutes. If brown fumes are generated, indicating oxidation of the sample by HNO3, repeat this step over and over until no more fumes are given off, indicating the complete reaction with HNO3.
5. Using a ribbed watch glass or vapour recovery system, allow the solution to evaporate to about 5mL without boiling. Maintain a covering of the bottom of the beaker at all times.
6. Allow to cool.
7. Add 2mL of water and 3mL H2O2. Cover with a watch glass and return to hotplate for warming and to start the peroxide reaction. Care must be taken to ensure that losses do not occur due to excessively vigorous effervescence. Heat until effervescence subsides and cool the vessel.
8. Continue to add 30% H2O2 in 1 mL aliquots with warming until the effervescence is minimal or until the general sample appearance is unchanged.
9. Do not add more than a total of 10mL H2O2.
10. Continue heating whist covered until solution has reduced to about 5mL again. (Without boiling)
11. Add 10mL HCl to the sample digest and cover with a watch glass. Place sample on hotplate and reflux for a further 15 minutes.
12. Filter the digestate through Whatman No. 41 filter paper and collect filtrate in a 100mL volumetric flask. Make up to volume using distilled water.
The sample is now ready for analysis by Atomic Absorption.

Thats the prac....not sure what to do next
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  #7  
Old 05-25-2010, 02:31 AM
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Default Re: How to convert mg/kg to mg/L

Quote:
Originally Posted by Unregistered View Post
Thanks for your help, just asked our teacher and he says we don't need to convert the figures, as they are US figures and we are testing in Australia....but it'd be nice to know how they compare to US standards.

I'll write what we did in the prac though, i think we didn't actually measure the vol. of digest.......

Procedure:
1. Weigh sample to three decimal places. (1.029 g)
2. Mix sample to homogeneity using mortar and pestle and sieve.
3. Transfer sample to 150mL beaker and add 10mL 1:1 HNO3. Cover with a watch glass for vapour recovery and reflux at 95C (±5C) for ~10 minutes without boiling.
4. Allow sample to cool and add 5mL conc. HNO3 and reflux for a further 30 minutes. If brown fumes are generated, indicating oxidation of the sample by HNO3, repeat this step over and over until no more fumes are given off, indicating the complete reaction with HNO3.
5. Using a ribbed watch glass or vapour recovery system, allow the solution to evaporate to about 5mL without boiling. Maintain a covering of the bottom of the beaker at all times.
6. Allow to cool.
7. Add 2mL of water and 3mL H2O2. Cover with a watch glass and return to hotplate for warming and to start the peroxide reaction. Care must be taken to ensure that losses do not occur due to excessively vigorous effervescence. Heat until effervescence subsides and cool the vessel.
8. Continue to add 30% H2O2 in 1 mL aliquots with warming until the effervescence is minimal or until the general sample appearance is unchanged.
9. Do not add more than a total of 10mL H2O2.
10. Continue heating whist covered until solution has reduced to about 5mL again. (Without boiling)
11. Add 10mL HCl to the sample digest and cover with a watch glass. Place sample on hotplate and reflux for a further 15 minutes.
12. Filter the digestate through Whatman No. 41 filter paper and collect filtrate in a 100mL volumetric flask. Make up to volume using distilled water.
The sample is now ready for analysis by Atomic Absorption.

Thats the prac....not sure what to do next
Mrs X is more qualified than I on this extraction stuff, but due to time zones, may not check in for a while.

All the steps are important to the chemistry, but only steps 1 & 12 are important to the conversion. The heavy metals in a 1.029 g soil sample have been dissolved (by various reactions) in 100 mL volume of filtrate. You measured this in the "make up volume" step. You need the total number of milligrams of each metal in the 100 mL (0.1 L) of filtrate, and assign it to the 1.029 g soil sample. (You can do this in one step). Then you may need some prefix manipulation to get the right units. Doing cadmium as an example:
0.016 mg/L x 0.1 L/1.029 g = 0.0016 mg/g = 1.6 mg/kg
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  #8  
Old 05-26-2010, 02:00 AM
Mrs X Mrs X is offline
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Default Re: How to convert mg/kg to mg/L

What a fun experiment! Do you have acid holes in your lab coat now though?
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