Homemade refractometer standard

[Randy Holmes-Farley Verified Community Expert]

Thanks. So your saying it should be reading about 35ppt on a hydrometer?

Close, but not exact. I need to look up the actual value.

 

[Cory]

Taking the data for refractive index and density of sodium chloride solutions at 20C and various concentrations found here http://chemistry.mdma.ch/hiveboard/rhodium/pdf/chemical-data/prop_aq.pdf and then deriving cubic polynomial equations to describe both, the refractive index of a 3.65% NaCl solution at 20C = 1.33938 and the density = 1.024285. Using a value of 0.998203 for the density of pure water at 20C, this equates to a SG of 1.02613. Seawater at 20C with this SG has a salinity of about 34.4 PPT (rounded to the nearest 0.1 PPT).

EDIT: Responding to Cory, not joekool.

Are you saying that it should 1.0263?

 

[JimWelsh]

By my calculations, yes. I'm interested in seeing what Randy comes up with, though. EDIT: I said 1.02613, not 1.0263.

 

[Cory]

Close, but not exact. I need to look up the actual value.

Well if the standard should read close to 1.026 this is way off the deep six hydrometer is inaccurate. Refractometer reads the solution at 1.026.

 

[Cory]

Here is what the refractometer reads:

 

[Randy Holmes-Farley Verified Community Expert]

The Deep Six appears to be off.

My 3.65 weight percent NaCl solution which is the 35.0 ppt refractometer standard should read 34.3 ppt on a hydrometer, or a sg of about 1.0261 at 20 deg C, which is the same as what Jim calculated. My calculations are based on Table 5 from this article of mine:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

FWIW, when I tested a Deep Six, I also found it lacking accuracy:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated! ? Advanced Aquarist | Aquarist Magazine and Blog

from it:

So how do these hydrometers measure up? In my tank the water was measured to be S=35 ± 0.5 by conductivity. Using the Deep Six swing arm hydrometer I got readings of S=32.5 ± 0.5 at 81 °F and S=32 ± 0.5 at 68 °F. Using the SeaTest I got S=34.5 ± 0.5 at 81 °F and S=34 ± 0.5 at 68 °F. For the standard type Tropic Marin hydrometer, I got a 77 °F/ 77 °F specific gravity of about 1.0265 ± 0.0003 (Figure 4), which compares well to the expected value of 1.0264.

 

[Cory]

Thanks Randy. I'm not sure what the temp was for this sample but probably close to 20. Is that brand of hydrometer temperature corrected?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks Randy. I'm not sure what the temp was for this sample but probably close to 20. Is that brand of hydrometer temperature corrected?

The Deep Six? Yes, it is supposed to be.