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Determination of water soluble solids

Water soluble solids (WSS) are estimated by refractometry, taking into account that a straight interdependence exists between brix and refractive indices in certains solutions. A hand refractometer (Fisher Scientific, catalogue No. 13 - 946 - 60A) will be used for WSS determination in leaves and sugar cane juice. The measuring range of the instrument is 0 - 32% (accuracy, 0.2%)

Calibration procedure

1/ Keep distilled water and instrument at room temperature (20oC) for 1 hours.

2/ Open the prism cover and put one or two drops of distilled water on the prism. Close the

    prism gently.

3/ Direct the prism towards a light source and look at the scale through the eyepiece.

4/ Rotate the view control to a clean image of the scale. Real the scale. It the scale is correct,

    the boundary line meets with “0" position. No calibration is needed. IF the boundary line is

    out of “0" position, calibration the boundary line to indicate “0" position by turning the

    calibration screw with a screwdriver.

5/ Calibration is completed wipe off the distilled water from the prism with a soft clean cloth

    or tissue paper.

Instructions.

1/ Open the prism cover and put one or two drops of the sample in the prism.

2/ Close the prism cover gently

3/ The sample volume must cover the prism surface entirely

4/ Direct the prism towards a light source and look at the scale through the eyepiece

5/ Rotate the view control to focus a clean image of the scale. Real the scale where the 

    boundary line interrupts it.

6/ After every measurement, wipe the sample from the prism with a soft cloth of tissue paper 

    and water. Keep the prism clean and dry to get the correct reading.

7/ Calculate WSS content (Brix %) from the determined refractive indice (nD), see table 1

 Table 1 : Brix (%) vs refractive index (nD)

 Brix (%) Refractive indices (nD) Brix (%) Refractive indices (nD)

0                1.33299                17                1.35890

1                1.33423                18                1.36053

2                1.33588                19                1.36218

3                1.33733                20                1.36348

4                1.33880                21                1.36551

5                1.34027                22                1.36719

6                1.34176                23                1.36888

7                1.34027                24                1.37059

8                1.34477                25                1.3723

9                1.34629                26                1.3740

10              1.34783                27                1.3758

11            1.34927                  28                1.3775

12            1.35093                29                1.3793

13            1.35250                30                1.3811

14            1.35408                31                1.3829

15            1.35567                32                1.3847

The hand refractometer is designed to be used at  20oC, and when used above 20oC, the reading value (brix %) have to be correcting referring to table 2 by adding the following value

Table 2 : Temperature compensation table

Reading (brix %)                                           

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