Converting ppm to mg/L?

[sonnus]

I was curious if it would be correct to multiply ppm values in seawater by ~1.026 when converting to mg/L when dealing with seawater? This is assuming that the original ppm was measured as mass/mass (mg/kg) and I'm not sure what the standard is. This is also assuming that the temperature isn't a factor.

I understand that this is a minimal difference but I was just curious if this would be the correct way to convert since we're converting a weight to a volume (mg/kg --> mg/L)? As an example, should 400 ppm calcium be converted to 410 mg/L?

 

[CodyRVA]

I was curious if it would be correct to multiply ppm values in seawater by ~1.026 when converting to mg/L when dealing with seawater? This is assuming that the original ppm was measured as mass/mass (mg/kg) and I'm not sure what the standard is. This is also assuming that the temperature isn't a factor.

I understand that this is a minimal difference but I was just curious if this would be the correct way to convert since we're converting a weight to a volume (mg/kg --> mg/L)? As an example, should 400 ppm calcium be converted to 410 mg/L?

Check this out.

 

[JimWelsh]

I was curious if it would be correct to multiply ppm values in seawater by ~1.026 when converting to mg/L when dealing with seawater? This is assuming that the original ppm was measured as mass/mass (mg/kg) and I'm not sure what the standard is. This is also assuming that the temperature isn't a factor.

I understand that this is a minimal difference but I was just curious if this would be the correct way to convert since we're converting a weight to a volume (mg/kg --> mg/L)? As an example, should 400 ppm calcium be converted to 410 mg/L?

In order to calculate this, you need to make an assumption about the temperature of the seawater, and also its salinity. The correct factor to use is not the Specific Gravity, as you suggest. The correct factor is the density of seawater at the given salinity and temperature. The salinity is usually assumed to be 35, and the temperature is usually considered to be either 20C or 25C, depending upon the context. For S=35.00 and T=20C, the density is 1.024763, and for S=35.00 and T=25C, the density is 1.023343. So, the NSW level of 412.0 "PPM" (mg/kg) translates into 412.0 * 1.024763 = 422.2 "PPM" (mg/L).

Oceanographers typically use mg/kg, because it permits them to ignore temperature (among other reasons). Most people don't notice this distinction. But, it does make a difference, e.g, NSW has 1,284 "PPM" as mg/kg Mg, which is 1316 "PPM" as mg/L (at 20C).

 

[JimWelsh]

Check this out.

I have no idea why that calculator multiplies mg/L by 1.0011423 to convert to PPM, but I'm pretty sure it has no relevance to the question that was asked.

 

[Randy Holmes-Farley Verified Community Expert]

I have no idea why that calculator multiplies mg/L by 1.0011423 to convert to PPM, but I'm pretty sure it has no relevance to the question that was asked.

That number is approximately 1/density of fresh water at 20 deg C, so it is probably converting for fresh water.

 

[sonnus]

All of the test kits I've ever used give results in ppm, are they likely measuring mg/kg? The margin of error of the test kit might make the mg/kg to mg/L irrelevant but I think the conversion would be useful when comparing home tested results to Triton's test results.

 

[Randy Holmes-Farley Verified Community Expert]

All of the test kits I've ever used give results in ppm, are they likely measuring mg/kg? The margin of error of the test kit might make the mg/kg to mg/L irrelevant but I think the conversion would be useful when comparing home tested results to Triton's test results.

I admit that I frequently interchange them when I should not (much to the expressed disappointment of a few readers ), but I'd expect that a kit is reporting in whatever units it says, with the obvious caveat that a kit testing in both fresh and salt can only be truly accurate in both with a mg/L measurement and not ppm.

As to Triton, I am not actually sure what they are using since the data comes out in units like ug/L, but they also use terms such as ppb in the same context, such as their accuracy document. Maybe they are as sloppy as I am, or maybe they really are making the distinction:

http://www.triton-lab.de/fileadmin/user_upload/triton-lab/TRITON_LOD.pdf

That all said, the correction is only about 2.5%, and I'm not sure any measurements we make are that accurate, nor am I convinced that any values we measure need to be that accurate.

 

[sonnus]

I admit that I frequently interchange them when I should not (much to the expressed disappointment of a few readers ), but I'd expect that a kit is reporting in whatever units it says, with the obvious caveat that a kit testing in both fresh and salt can only be truly accurate in both with a mg/L measurement and not ppm.

As to Triton, I am not actually sure what they are using since the data comes out in units like ug/L, but they also use terms such as ppb in the same context, such as their accuracy document. Maybe they are as sloppy as I am, or maybe they really are making the distinction:

http://www.triton-lab.de/fileadmin/user_upload/triton-lab/TRITON_LOD.pdf

That all said, the correction is only about 2.5%, and I'm not sure any measurements we make are that accurate, nor am I convinced that any values we measure need to be that accurate.

I doubt I'll make the correction either but I was just trying to understand it all. It's certainly useful to know about the distinction between the two and to have in the back of your mind when you're comparing numbers from different sources.

 

[JimWelsh]

I assert that "PPM" is ambiguous, and that "1 mg/L" and "1 mg/kg" are BOTH "1 PPM", just one is (w/v) and the other is (w/w), and the "sloppiness" manifests in the lack of specificity about which it is. When your test kit starts out by saying, "take N mL of sample..." instead of "take N mg of sample..." (as most all do), then it is reporting PPM as mg/L. But, when you are comparing your values to commonly published PPM values from serious oceanographers, then the NSW values are usually in mg/kg. That said, most analytical chemists seem to report in units that are (w/v).

While it is true that most hobbyist testing can't generally get closer than about 5% accuracy, I am able to achieve much better than that, with a little care and practice, and volumetric glassware. Not that I can argue that I *need* to be that accurate, except when I am trying to standardize my own test reagents. In that case, a 5% error can easily compound into a 10% error in the final result! I try to keep my error down to below 1% wherever possible. My routine Alk results are typically repeatable to within 0.06 dKH / 0.02 meq/L, and Ca to within less than 3 mg/L (taking the last three months' worth of records as the basis).

 

[sonnus]

Very good info, thank you Jim and Randy!

 

[Randy Holmes-Farley Verified Community Expert]

I assert that "PPM" is ambiguous, and that "1 mg/L" and "1 mg/kg" are BOTH "1 PPM", just one is (w/v) and the other is (w/w), and the "sloppiness" manifests in the lack of specificity about which it is. When your test kit starts out by saying, "take N mL of sample..." instead of "take N mg of sample..." (as most all do), then it is reporting PPM as mg/L. But, when you are comparing your values to commonly published PPM values from serious oceanographers, then the NSW values are usually in mg/kg. That said, most analytical chemists seem to report in units that are (w/v)..

Just as a technical point, I'm not sure how the definition of "ppm" can ever truly allow it to be w/v or v/w and still literally be parts per million. It could be w/w or v/v, but it seems wrong to say it could be w/v.

 

[JimWelsh]

I agree. But, in very wide practice, "PPM" is used to mean w/v. In just about any technical discussion of the definition of the term, both w/v and w/w are usually mentioned, but w/v is most commonly the one mentioned first.

 

[Randy Holmes-Farley Verified Community Expert]

I agree. But, in very wide practice, "PPM" is used to mean w/v. In just about any technical discussion of the definition of the term, both w/v and w/w are usually mentioned, but w/v is most commonly the one mentioned first.

Yes, like me, they are sloppy.