Refractometer question

[Cohibaman]

I made up a liter of DIY refrac calibration solution to verify my refractometer (36.5 grams of Morton kosher salt dissolved in 963.5 grams of RO). The reading was spot on at 0 with RO and the DIY solution read 35 like it should. So far, great!

But help me understand why when I dissolve 35 grams of instant ocean salt mix in 965 grams of RO, the refractometer reads 30 rather than 35 as I expected.

Mike

 

[Mr. Limpid]

Because all salt mixes and batches of same manufacture differ. Try checking the PH of a fresh batch of salt you be surprised in the results. Also each manufacture of refac. have different ways to set to 0. Mine I have to have the refractor at 65deg. and use RO water (that is at 0 TDS).

 

[Randy Holmes-Farley]

But help me understand why when I dissolve 35 grams of instant ocean salt mix in 965 grams of RO, the refractometer reads 30 rather than 35 as I expected.

There is a lot of moisture in salt mixes. So part of the weight of it is water. Calcium and magnesium salts are likely all hydrates, and the magnesium ones are half water.

 

[Cohibaman]

I agree with you that all salts vary, even batch-to-batch. But since salinity is a weight per weight measurement of salts dissolved in pure water, shouldn't 35 grams of instant ocean dissolved in 965 grams of RO water make a 35 ppt salinity solution? I fully understand that the refractive index of sodium chloride is different than seawater mix, hence the 36.5 gram sodium chloride cal solution makes the refractometer read 35. But it seems to me that since I KNOW the 35 ppt instant ocean mix I made IS truly 35, why wouldn't I use that to calibrate my refractometer, thereby standardizing the instrument to the actual mix I'll be using in my tank?

 

[Randy Holmes-Farley]

We probably posted at the same time, but check my answer which is the reason.

But it seems to me that since I KNOW the 35 ppt instant ocean mix I made IS truly 35,

It isn't 35 ppt solids.

 

[Cohibaman]

Thanks Randy! Yep, we must have posted at the same time.

I assume that the hygroscopic nature of the instant ocean (or any other salt mix) vs kosher salt is why you chose sodium chloride for the DIY cal solution?

The reason I'm so anal about this is I'm trying to do hyposalinity treatment. So basically since the 0 and 36.5 on my refrac were spot-on at 0 and 35, just trust its 12 ppt reading for hypo?

 

[disc1]

If I understand your question right, you are asking why we use a 36.5ppt NaCl solution to calibrate a refractometer for 35ppt seawater.

The answer is that refractometers don't measure salinity nor do they measure specific gravity. Refractometers measure refractive index. So if you want to calibrate your refractometer you need to match the refractive index of seawater, not its salinity. Since seawater has other elements besides NaCl in it and those elements affect refractive index differently than NaCl you shouldn't expect an NaCl solution and a seawater solution with the same salinity to have the same refractive index. So you must make a NaCl solution that does match refractive index and that happens at 36.5ppt salt.

 

[theoregonian]

I think they add anti caking agents to the salts... Would that throw the number?

 

[bertoni]

If the anti-caking agents made up a significant amount of the salt product, they would influence the results, but I don't think that they are an issue, in practice.

 

[disc1]

But help me understand why when I dissolve 35 grams of instant ocean salt mix in 965 grams of RO, the refractometer reads 30 rather than 35 as I expected.

Mike

Aha, I didn't read your question right. The answer to this question is that a significant amount of the weight of that salt mix is water. The water doesn't count, but you never know how much is there so you can't really account for it.

 

[Randy Holmes-Farley]

I assume that the hygroscopic nature of the instant ocean (or any other salt mix) vs kosher salt is why you chose sodium chloride for the DIY cal solution?

Yes, that's correct.

 

[Cohibaman]

I knew there was a reason, just didn't know what it was. Thanks Randy et al.

 

[Randy Holmes-Farley]

:thumbsup:

Happy Reefing.

 

[Cohibaman]

BTW, for those looking for a nicer refrac than the $25 to $50 ones I've seen on eBay, LFS, or on-line pet supplies, take a look at the Vee Gee salinity refractometer. It's quite a bit more substantial (heavy), sports glass prism & optics, and has auto temp comp. The calibration on mine was just about perfect from the factory, but it has a slick ring you simply rotate for calibration rather than the typical small screw. It's a little pricey, but I think it's worth the extra $. :beer:

 

[Wills612]

Will the RHF refractometer DIY calibration fluid cary over to the Vee Gee STX-3?

 

[Randy Holmes-Farley]

It will proved a 35 ppt seawater standard (seawater sg = 1.0264)for any refractometer.