Reef Chemistry Question of the Day #218 Vodka and Refractometers

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #218

Suppose that you monitor the salinity of your 120 gallon aquarium using a true seawater refractometer, and it reads 35.0 ppt, or about specific gravity = 1.0264.

You then add 15 mL of vodka (essentially 40% ethanol in fresh water) to the aquarium and let it mix in. After mixing, you remeasure the water with your refractometer. You recognize that the real effect is likely very small and probably impossible to detect with a hobby device, but what effect takes place?

A. The salinity displayed by a perfect refractometer will decline because the vodka has no salts and hence has lower salinity than the aquarium. In other words, you are diluting the salts and see an apparent drop in salinity.

B. The salinity displayed by a perfect refractometer will rise because vodka has a higher refractive index than seawater. In other words, the refractometer sees the vodka as added salt and you see an apparent rise in salinity.

C. The salinity displayed by a perfect refractometer will decline because vodka has a lower refractive index than seawater. In other words, the refractometer sees the vodka as "negative" salts and you see an apparent drop in salinity.

D. The salinity displayed by a perfect refractometer will rise because 35 ppt ethanol in water has a higher refractive index than 35 ppt seawater. In other words, the refractometer sees the ethanol as added salt salts because ethanol has a larger effect on refractive index than does a similar amount of salt.

Good luck!





























.

 

[IslandLifeReef]

B.

 

[Alfrareef]

I must go with D. Vodka has a larger effect on everything...

 

[SDReefer]

I'm going to go with B since "refractive index changes in step with how much of each component is present."

 

[SashimiTurtle]

C, if you can tell any difference at all.

Pure ethanol alcohol has a specific gravity of 0.787 ish... depends on temp
Pure water has a specific gravity of 1.000
The SG of 80 proof (40%) vodka at about 60F is 0.9528
Adding anything less than 1.026SG, will lower your total SG.

 

[JimWelsh]

C, if you can tell any difference at all.

Pure ethanol alcohol has a specific gravity of 0.787 ish... depends on temp
Pure water has a specific gravity of 1.000
The SG of 80 proof (40%) vodka at about 60F is 0.9528
Adding anything less than 1.026SG, will lower your total SG.

But, why are you focusing on specific gravity? This is clearly a question about refractive index. We are talking about a "refractometer", after all....

 

[Martin Kuhn]

B

 

[CNDReef]

C

 

[jason2459]

I tried testing this and came to the conclusion vodka is impossible to test so this is a trick question.

In order to do this properly and get only a drop or so on my refractometer I drank the bottle so there was a drop left but I couldn't see a darn thing after that.

 

[dragon99]

B, but I'm not sure a even a perfect refractometer has enough significant digits to measure it.

 

[Doubbler]

I'd say 15 ml of vodka in a 120 gal tank isn't going to be even close to enough to alter salinity.

 

[BluewaterLa]

Hmm. So whats the answer to this @Randy Holmes-Farley

 

[Randy Holmes-Farley]

And the answer is...

B. The salinity displayed by a perfect refractometer will rise because vodka has a higher refractive index than seawater. In other words, the refractometer sees the vodka as added salt and you see an apparent rise in salinity.

The refractive index of pure fresh water is about 1.3330 and the refractive index of 35 ppt seawater is approximately 1.3394.

The refractive index of pure ethanol (1.3614) is higher than water, and so ethanol/water mixtures such as vodka have higher refractive index than pure water. Consequently, the ethanol in vodka makes it "look like" it has salt in it (to a salinity refractometer).

In fact, the refractive index of vodka with 40% ethanol by volume is about 1.3562. That value is substantially higher than the 1.3394 of 35 ppt seawater, and hence adding vodka to seawater raises the refractive index and so raises the "observed" salinity.

The effect is, of course, rather small. Adding 15 mL of vodka to 120 gallons (454 L) will boost the final refractive index from 1.3394000 to about

(15 mL x 1.3562 + 454,000 x 1.3394)/(15 + 454,000) = 1.3394006

So you would not see it with any standard refractometer, but it is real.

Why is the answer not D?

D. The salinity displayed by a perfect refractometer will rise because 35 ppt ethanol in water has a higher refractive index than 35 ppt seawater. In other words, the refractometer sees the ethanol as added salt salts because ethanol has a larger effect on refractive index than does a similar amount of salt.

The refractive index of 35 ppt (3.5 weight percent) ethanol in water is about 1.3351. That value is lower than 35 ppt seawater (1.3394). The ethanol is raising the refractive index, but not by as much as the salts of seawater at a similar percentage concentration will do. But the concentration in vodka is far higher, and at that concentration, it causes a much bigger effect than the salts in 35 ppt seawater.

 

[Alfrareef]

I knew it. Vodka causes a much bigger effect than seawater.
Thank you sir