Leica 13104950, 13104810 6English, 0REFRACTOMETER USE, Techniques and Refraction Principles

8.0REFRACTOMETER USE, TECHNIQUES AND REFRACTION PRINCIPLES

Refractive Index measurements taken on the LEICA ABBE MARK

IIand LEICA ABBE MARK II Plus Refractometers are affected by temperature. Different values can be obtained when the same sample is measured at different temperatures. For example: as the temperature of a sugar solution sample rises, its density and refractive index decrease. This causes an apparent decrease in concentration when using the non-temperature compensated % Solids mode. The % Solids-TC (Temperature Compensated) mode automatically corrects the reading to the correct value at 20OC.

For every 1OC change in temperature, an aqueous solution con- taining sugar will change readings by approximately 0.08%. This is the average value for samples of 0 to 85% at 15OC to 45OC. As temperature rises the % Solids reading decreases. As tempera- ture decreases, the % Solids reading increases.

The LEICA ABBE MARK II Refractometer temperature will rise approximately 1OC every 10 minutes for 1 hour until it stabilizes. An instrument that has been turned on for more than 1 hour in a 20OC room could read 26OC. The rise in temperature is principally due to heat generated by the power transformer inside the instrument housing.

The sample placed on the measuring prism surface of a LEICA ABBE MARK II Refractometer that has been turned on for more than an hour in a 20OC environment will assume the temperature of the prism (i.e., approximately 26OC). Although the sample is now warmer and would indicate a lower concentration when measured in the uncompensated Brix mode, the BX-TC mode automatically compensates for the temperature difference and displays the concentration of the solution as if its temperature were 20OC. Without compensation for the 6O rise, the readout of a 70% solution would be 0.5% less (6OC x 0.08%) or 69.5%.

The measuring prism surface temperature can be lowered by running cooling fluid through the water jackets in the prism housing. However, the heat generated inside the instrument housing still affects the prism temperature. For this reason it is necessary to run fluid slightly below the desired temperature. A thermometer placed in the fluid line may measure several degrees below the actual displayed temperature of the measur- ing prism surface displayed when the temperature button is pressed. The opposite is true if the prism temperature has to be elevated above the ambient temperature.

The fluid circulation system allows the prism to be adjusted for temperatures up to 80OC. This may be necessary when using samples that solidify at lower temperatures.

Measuring viscous samples, such as high Brix sugar syrups, may be more difficult than measuring products with lower total dis- solved solids.

Stir the sample on the measuring prism surface with a rubber spatula or an angled piece of clear plastic tubing. Completely cover the surface making sure all the air pockets between the sample and the measuring prism surface are removed.

For removing carbonation from carbonated soft drink samples, open and close the prism cover two or three times after loading. Doing this provides the best possible reading line and the best way to judge where the shadow line intersects the crosshair reticle.

NOTE: Remember the accuracy of the instrument is only as good as the clarity of the shadow line.

8.1 High Index/High Dispersion Samples

High refractive index samples with high dispersion properties may require additional use of Catalog #13K30650 589nm bandpass filter accessory. This accessory will insure that the shadowline will be sharp for these samples.

9.0 CALIBRATION WITH TEST GLASS

Refer to Chart 1, Refractive Index of Triple Distilled Water at Various Temperatures to compare the refractive index reading with the calibrated temperature reading.

To calibrate the LEICA ABBE MARK II Refractometer use:

•a refractive index test glass (Catalog No. 13104510)

•a bottle of 1- Bromonaphthaline (Catalog No. 13104520)

•a 1/16" (No. 4) Allen wrench (Catalog No. 13X30600)

•lens tissue

The refractive index test glass is imprinted with the refractive index scale. The bottom and front surfaces of the glass are pol- ished. It is important that these surfaces are very clean when beginning calibration procedures.

To calibrate:

1.Turn the “Mode” selector switch to the refractive index position.

2.Apply a single drop of 1-Bromonaphthaline to the surface of the measuring prism surface.

3.Place the refractive index test glass on the measuring prism surface with the imprinted side facing up. Position the polished end of the test glass toward the illuminator - about 1/8" (2mm) from the edge (Figure 4).

4.Align the illuminator so that the light is directed onto the pol- ished face of the test glass. Cover the illuminator lens with lens tissue to diffuse the light.

5.Adjust the illuminator arm, dispersion correction wheel, eye- piece and adjustment control so that the shadowline is centered, in focus, free of color. This is the same procedure used with a

Back Plate Illuminating Prism

Test Glass

Fastening

Screws

Measuring

Prism

Test Glass Position

1/8” [2mm] from edge)

Base Plate

Figure 4

Chart 1: Refractive Index of Triple Distilled Water at Various Temperatures

Temp OC	Refractive Index
18	1.3332
19	1.3331
20	1.3330
21	1.3329
22	1.3328
23	1.3327
24	1.3326
25	1.3325
26	1.3324
27	1.3323
28	1.3322
29	1.3321
30	1.3319

Source: Molar Refraction of Organic Compounds

6English