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spazthecat
02/09/2008, 02:26 PM
Hi,

I was thinking about calibrating my refractometer with a salt solution rather than distilled water. I work in a hospital and have access to 0.9% sodium chloride IV solution and was wondering if anyone knew what the refractive index of this was? I Googled a bit and couldn't find it.

Thanks,

Andy

Randy Holmes-Farley
02/09/2008, 02:59 PM
It pretty low. Not what I'd use. Not that much better than Di water. Probably something like 8 ppt seawater salinity equivalent.

spazthecat
02/09/2008, 05:19 PM
Yeah, thanks. I didn't think that through before I posted the question. Should have done the math first.....

I ended up making a standard solution with table salt. I put 1/2 tsp in 81.8 ml RO/DI water (I have some syringes to measure the volume).

With that solution my refractometer read 40 ppt. Is it unusual for a refractometer to be off by that much? I'm thinking it may be bad. I noticed the scale moving once or twice as I turned the adjustment screw. What do you think?

Thanks,

Andy

Aquarist007
02/09/2008, 06:04 PM
The recommendation in this forum in the past for refractometer calibration is to use Pinpoint 53.0 mS calibration fluid.

bertoni
02/09/2008, 07:05 PM
If that solution is correct, your refractometer is a lot more inaccurate than most of the numbers I've seen posted. The PinPoint solution is handy.

From the calibration article:

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.

That's the standard for a refractometer, I believe.

Randy Holmes-Farley
02/10/2008, 06:18 AM
I think that using a single half teaspoon may be too likely to cause error. For a DIY solution, I generally recommend a larger recipe:

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

from it:
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.

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ΒΌ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

[[Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]

recife111
06/23/2010, 05:33 AM
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
What salt do you add?

Percula9
06/23/2010, 10:36 AM
Just order pinpoint calibration fluid from Marine Depot.

bertoni
06/23/2010, 12:41 PM
Table salt is fine, especially if it has no additives.

recife111
06/24/2010, 03:42 AM
Isn't Iodized Salt, table salt.

bertoni
06/24/2010, 02:50 PM
Yep, table salt is fine.