Plant Physiology

Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu

Determining the Sucrose Concentration of Sugar Maple Sap by the Refractive Index Method

The concentration of sucrose in a sample can be determined by measuring the refractive index of the sample. This measurement is based on the principle that the index of refraction of a solution containing sugar is proportional to its concentration. We will use an Atago Hand Sugar Refractometer to measure sugar concentration. (Note: other types of refractometers will work equally well, but in order to obtain a concentration value, the user needs to refer to standard tables or run a standard curve with sucrose solutions of known concentration).

Open the prism cover of the refractometer. Rinse with a few drops of water from a squirt bottle and then gently blot dry. Be careful so that you don't scratch the lens/prism!
Place a few drops of sample on the face of the prism and then close the cover.
Turn the instrument until the window hole is oriented to a light source.
Adjust the eyepiece to give a sharp focus on the scale and the border line between the dark and light field is read directly as percentage sugar.
Measure the temperature of the sap sample.
If the sap temperature is not 20 C, correct the reading (by referring to the table provided with the instrument).