Difference between Seawater and Brine refractometers?

Question:

>Good Morning,
>
>I was just reading your website about the Sea water calibrated
>refractometers. I work within the Marine Aquaculture industry and I am
>intrigued as to your findings.
>Could you explain exactly why, scientifically if you could how magnesium,
>calcium and other salts in the mix affect the refractive index?
>
>As I am aware salinity is a measurement of the dry weight of salt and in
>a 35ppt mix of pure sodium (brine shall we say) the measurement would be
>the same as a 35ppt mix of sea water, with calcium and magnesium and
>trace others totalling up the dry mass weight. I appreciate that the
>molecular weights will differ slightly but in an overall mix very very
>minor as the solution can only hold a certain value of sodium/calcium/mag
>before one is precipitated out. Could you put me right on this as I am
>confused as to how there a 1.8ppt difference. Unless it is the molecular
>weights, or is it a physical different that causes the refractive index
>to change.
>
>Kind Regard
>
>Craig

Answer:

Craig

Refractive index is specific to the particular salt and also to the salt
concentration as you know.

I am a material scientist by degree but have not investigated the reason
behind why the different salts cause an optical change but accept that it
does.

An analogy to this is that different liquids boil at different
temperatures which I accept but have not investigated whether this is due
to bond strength or some other physical reason.

If you make up the two different 35ppt salt solutions that you suggest
then they will both have the same salinity however the other parameters
will change such as density, possibly freezing point and in this case
refractive index.

A refractometer is calibrated to read a refractive index and then
reference it against a scale that is printed on the lens, the scale is
specific to the material being measured, be it brine, antifreeze or any
other liquid and so you can not interchange the refractometers.

The amount of magnesium and calcium is relatively small compared with the
NaCl but not insignificant and is enough to make a difference to the
refractive index. Even changes in the magnesium level can have an effect
on refractive index:-

3 solutions all at 35ppt measured with the same refractometer

Standard - Magnesium - 1280 - Salinity reading 35ppt
Low 800ppm Magnesium - 800 - Salinity reading - 34.2ppt (2.2% error)
High 1500 Magnesium - 1500 - Salinity reading - 35.4ppt (1.1% error)

This illustrates the effect of small changes to just one element.

Hopefully this gives you a clearer understanding of why we spent the time
developing a refractometer specific for the solution being measured rather
than one off the shelf

Regards
Stuart