Methods Manual for Salt Lake Studies/Salinity/measuring refractive index

Authors: PSJ Coleman,

Refractive Index

Refractometers measure the deflection of light through a drop of water. This is affected by the amount of salts dissolved in the water. It is unknown what impact the varying salts in solution have on the refractivity of the brine, but it would seem logical that different ionic solutions may refract light differently, suggesting that, for example, specific gravity results obtained from a refractometer may not match those obtained from the same brine with a hydrometer.

Handheld refractometers cost in the region of $400 each. The instruments do not take kindly to being entirely immersed in salt water, but are reasonably easy to repair unless the prism is scratched or damaged, in which case the entire unit is usually replaced. Refractometers may be purchased that display a range of measures (specific gravity, salinity parts per thousand, brix and salinity percent). Use one that displays SG and/or salinity parts per thousand. The reading is affected by temperature. The instrument should come with a manual that explains how to correct the readings

As with other indirect methods of measuring salinity, such as electrical conductivity, the converted reading should never be reported without also reporting the method was used to obtain the value.