Salinity/Refractive Index Question

[Pazernaker]

So this is a weird one and one I know I'm going to feel REALLY dumb for asking. I've been using my hand held store bought meter that I calibrated with a fluid still in date and have been getting a refractive index of about 1.025. No problems, everything is good.

I discover we have an expensive lab bench Anton-Paar Abbemat 500 found here. I think to myself, "Perfect, everyone should DEFINITELY know their salinity out to 6 decimal places!" Well, now I have more questions...

Readout for salinity came as follows:
Salanity, NaCl 3.56g/100g
RI SetTemp: 20.00 C
RI Temp: 20.00 C
RI Wavelength: 589 nm
RI 1.339163 nD

I re-ran it at 25C (because ambient) and came up with an RI of 1.338601. Sample size was about 3 ml's taken from my tank into a sterilized glass container 2 hours before testing. Light shaking was done to guarantee homogeneity. What am I missing here, and why am I not getting anywhere close to the refractive index as my calibrated hand unit? Is a salinity of 3.56g/100g in line with my measured 1.025 at home? If it matters, I use instant ocean reef salt and did a 20% water change a week and a half ago with RODI water from my LFS.

Thanks,
Ryan

 

[The Farmer]

Did you run a test on your calibration fluid?

 

[KStatefan]

So this is a weird one and one I know I'm going to feel REALLY dumb for asking. I've been using my hand held store bought meter that I calibrated with a fluid still in date and have been getting a refractive index of about 1.025. No problems, everything is good.

Are you sure your hand held meter is giving you the refractive index? I would assume it is giving you specific gravity. Seawater at 1.0264 SG has a refractive index of 1.3394

 

[Randy Holmes-Farley Verified Community Expert]

http://reefkeeping.com/issues/2004-06/rhf/index.htm

I show a table of RI vs seawater salinity in table 1 above, and a table of NaCl solutions in table 2.

How did you make the 3.56g/100 g solution?

 

[Pazernaker]

Are you sure your hand held meter is giving you the refractive index? I would assume it is giving you specific gravity. Seawater at 1.0264 SG has a refractive index of 1.3394

Jesus Christ, you're right... I knew there was something way off here.

 

[Pazernaker]

http://reefkeeping.com/issues/2004-06/rhf/index.htm

I show a table of RI vs seawater salinity in table 1 above, and a table of NaCl solutions in table 2.

How did you make the 3.56g/100 g solution?

I didn't make the solution of 3.56/100, that's one of the units of measure the device spit out for me when I tested my tank water.

 

[Pazernaker]

Did you run a test on your calibration fluid?

Have not, but I will bring that into the office next time I'm on site. I just bought the fluid a week ago, though, so I'd be shocked if it was out of spec.

 

[Randy Holmes-Farley Verified Community Expert]

I didn't make the solution of 3.56/100, that's one of the units of measure the device spit out for me when I tested my tank water.

Do you know whether it means 3.56 g per 100 g water, or 3.56 g per 100 g total?

 

[Pazernaker]

Do you know whether it means 3.56 g per 100 g water, or 3.56 g per 100 g total?

I don't, I ran it on the salinity mode, so I assume it means 100g of test solution yields 3.56 grams of NaCl

 

[Randy Holmes-Farley Verified Community Expert]

I don't, I ran it on the salinity mode, so I assume it means 100g of test solution yields 3.56 grams of NaCl

If true, that gives it a seawater salinity equivalent of about 33.66 ppt, or a specific gravity of about 1.0253.

 

[Pazernaker]

If true, that gives it a seawater salinity equivalent of about 33.66 ppt, or a specific gravity of about 1.0253.

OK, that's bang on what my refractometer at home says. How did you convert grams of salt/100 to specific gravity?

 

[Randy Holmes-Farley Verified Community Expert]

OK, that's bang on what my refractometer at home says. How did you convert grams of salt/100 to specific gravity?

The table I mentioned in my article linked above.

 

[Pazernaker]

The table I mentioned in my article linked above.

I really need to read things in their entirety...

 

[Randy Holmes-Farley Verified Community Expert]

lol

I should also clarify that the 1.0253 is the sg of seawater matching that salt solution refractive index. It is not necessarily the sg of the salt solution itself, which is a little different.