Charting Solution Concentrations Using a Refractometer
The Brix scale originated in the food industry and is primarily a unit of measure corresponding to the percent of sugar in a sugar and water solution. The actual Brix value represents the number of grams of cane sugar in a 100 gram cane sugar solution (percent sugar wt./wt.). This direct reading relationship holds true ONLY for sucrose solutions. So, if you are measuring sucrose solutions with your Brix refractometer, you may read percent sucrose directly on the Brix scale. If you are reading non-sucrose solutions on the Brix scale then the readings must be converted into solution concentrations to be useful to the user. Likewise, Refractive index units (nD) represent a physical property of a substance but are not particularly helpful without a table to reference them.
To make both Brix and refractive index units more meaningful, they must somehow be correlated to the concentration of the solution being tested. This is accomplished by creating a chart of solution concentrations relative to the scale. A separate chart must be made for each type of solution being tested. Occasionally, the manufacturer of the solution will provide a reference chart relating the solution’s concentration to Brix or refractive index. If such a chart is not available, it is quite easy to construct one following the instructions below:
Step 1
Mix (preferably by weight) a number of known solutions which bracket the actual concentration to be used. For example, if a 10% solution is most often used, carefully mix 5%, 10%, and 15% solutions.
Step 2
Zero set the instrument.
Take a Brix reading for each prepared sample with the Palm Abbe, and the sample, at or near room temperature (20°C / 68°F).
Record the results and plot them on graph paper. Remember to add a point at 0.0 Brix for water.
Example:
0% = 0 Brix (Water)
5% = 7.5 Brix
10% = 15 Brix
15% = 22.5 Brix
Step 3
Draw a straight line between the plotted points along a path that best represents the center of the data. Do not be concerned if some of the points fall slightly off the line. The concentration for future measurements of the same fluid can be determined by matching the Brix value against the corresponding concentration on your graph. A separate graph must be constructed for each type of fluid used.
You can use this chart to convert Brix readings into product concentration readings or, since the releationship in this example happens to be linear, you can use a simple slope-intercept equation, such as: Product % w/w = 0.6667 * Brix Reading