DIY vs commercial refractometer calibration

[ajm83]

I made up some DIY solution with the following from RHF's article (using expensive, presumed accurate scales, and 0 TDS water):

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.

I checked this solution in my refrac, it measured at about 30 PPT. I adjusted the refrac to read 35 PPT with this sample, and triple checked with further readings.

Next I checked my TLF Accurasea, this now (after the adjustment above) reads at 39 PPT. The sticker udner the bottle says 35.1 PPT.

So my question is which to believe?!

The background and reason I am checking is that i've always trusted the Accurasea solution, but it didn't match the result I got from my ATI ICP, and water from my LFS read at 1.020 when they told me it was 1.025 (IIRC). Next I got some sea water from the coast and checked that, it read at 1.020...

Is there another way to verify this reading?


edit: another interesting point, when calibrated with the DIY solution, RODI measures at zero, when calibrated with the Accurasea it is significantly off zero.

 

[JimWelsh]

Since you already have access to "expensive, presumed accurate scales", you just need a volumetric flask, and then you could try this approach to measuring the density of various solutions as an alternative method to verify your readings.

 

[Randy Holmes-Farley Verified Community Expert]

I made up some DIY solution with the following from RHF's article (using expensive, presumed accurate scales, and 0 TDS water):

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.

I checked this solution in my refrac, it measured at about 30 PPT. I adjusted the refrac to read 35 PPT with this sample, and triple checked with further readings.

Next I checked my TLF Accurasea, this now (after the adjustment above) reads at 39 PPT. The sticker udner the bottle says 35.1 PPT.

So my question is which to believe?!

The background and reason I am checking is that i've always trusted the Accurasea solution, but it didn't match the result I got from my ATI ICP, and water from my LFS read at 1.020 when they told me it was 1.025 (IIRC). Next I got some sea water from the coast and checked that, it read at 1.020...

Is there another way to verify this reading?


edit: another interesting point, when calibrated with the DIY solution, RODI measures at zero, when calibrated with the Accurasea it is significantly off zero.

The ocean water test is not useful.

I suspect the Accursea is off, but you can try a few things:

1. Buy a different brand standard.
2. Buy a different brand of sodium chloride.
3. Bake the sodium chloride in an oven to be sure it is dry.

 

[ajm83]

The ocean water test is not useful.

I suspect the Accursea is off, but you can try a few things:

1. Buy a different brand standard.
2. Buy a different brand of sodium chloride.
3. Bake the sodium chloride in an oven to be sure it is dry.

A reply from the author himself [emoji4]

Thanks, I'll buy some different sodium chloride in the first instance.
Tbh I rather begrudge paying £15 for 60ml of salt water if I don't have to, especially since the Accurasea seems to have lead me down wrong path in the first place.

The seawater check was just an additional sanity check but I'll not bother again if there's no value in the result.

Thanks

 

[ajm83]

Since you already have access to "expensive, presumed accurate scales", you just need a volumetric flask, and then you could try this approach to measuring the density of various solutions as an alternative method to verify your readings.

Very interesting thread, I'll go through that properly later this evening [emoji106]