Reef Chemistry Question of the Day # 177 Ethanol Refractometer

[Randy Holmes-Farley]

Reef Chemistry Question of the Day # 177

Refractometers can be designed to measure many different chemicals in water, including salt, sugar, and alcohol.

The advertisement linked below, for example, is for a refractometer that measures 0-80% ethanol (alcohol) in water:

http://www.amazon.com/Alcohol-Refractometer-0-80-Water-Grape/dp/B00CP9OVSQ

Based on just the knowledge that it exactly covers the range 0-80% ethanol in fresh water, why would this refractometer not likely be the best for measuring the salinity of seawater?

A. Ethanol lowers the refractive index of freshwater while salt raises it, so it covers an entirely incorrect range.
B. The range of refractive index it covers is too wide, and so it is not precise enough to measure to a reefkeepers requirements.
C. The range of refractive index it covers is too narrow, and so it does not cover the whole range needed for a reef aquarium.
D. Since it is designed for ethanol and not salt, it cannot measure the refractive index of salt solutions correctly

Good luck!































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[beaslbob]

d

 

[Bruce Burnett]

I say d

 

[Randy Holmes-Farley]

And the answer is...B. The range of refractive index it covers is too wide, and so it is not precise enough to measure to a reefkeepers requirements.

The refractive index of fresh water is 1.3330 and the refractive index of 35 ppt seawater (sg = 1.0264) is 1.3393. Those are really pretty close together.

The whole range we are interested in is shown in a table below from one of my refractometer articles. Importantly, for reef tank salinity, we are really only interested in a range from about 1.3385 (sg = 1.0226) to 1.34012 (sg = 1.0294)

But the refractive index of 80% ethanol in water (which is covered by the the refractometer in the question) is a whopping 1.366. So it covers a far wide range, from 1.3330 to 1.3660. That sounds close, but is far wider than a salinity refractometer.

The entire range we are interested in, from a sg of 1.0226 to 1.0294, is packed into just 4.8% of that ethanol refractometer scale. So there just no way one could distinguish between low salinity and high salinity using a refractometer with such a wide range. Hence I conclude it is unsuitable.

This is the article with the table below:

http://www.reefedition.com/refractometers-salinity-measurement/

 

[jason2459]

Is there any specific reason why the specific gravity is listed at 25C and the refractive index is at 20C?

 

[Dbuzby1]

D

 

[Randy Holmes-Farley]

Is there any specific reason why the specific gravity is listed at 25C and the refractive index is at 20C?

Not really. Those were just the tables I had.