Digital refractometer

FIGURE 15.12 A digital refractometer with a diode array detector. 

Just like refractive index, the °Brix scale is quite dependent on the temperature. Manual Abbe refractometers do not compensate for this temperature effect. Special correlation tables are used to adjust the readings to a standard temperature, 20°C. Digital refractometers, on the other hand, can operate over a fairly wide range of sample temperatures (+15 to +40°C) and automatically apply these temperature corrections. See Workplace Scene 15.2. 

Note If a digital refractometer is used, follow the manufacturer s operating instructions. 

The basic method is to use a manual Abbe refractometer to determine refractive index. Various automated or electronic instruments exist which automatically perform some of the steps of the manual procedure. The first requirement is that the sample be a solution. In some instruments, the solution is placed between two prisms, and the image of the critical ray boundary is adjusted to meet a reference mark for this adjustment, the refractive index and equivalent °Brix can be read from a scale. The sample temperature must be known, or the instrument must have temperature compensation. Some automatic digital refractometers use the same methodology of sample presentation, but automate the matching of the critical boundary to the reference marker. 

Today, there are modern digital refractometers available that determine the refractive index of a liquid electronically (Figure 24.6). Once the instrument has been calibrated, it is only necessary to place a drop of your liquid between the prisms (see... 

The Rudolph J-series, a modern digital refractometer. To make a measurement, place the sample on the lower prism (see the inset) and close the lid. 

Brix Value. Brix is one of the most common quality factors of fruits. Beside water, soluble sugars, determined as Brix, is the major component in most fruits— say 10-20% by fresh weight. Therefore, in a spectmm of typical fruit, there is an adequate amount of information related to the sugar absorption. The reference measurement of Brix is also simple. The value can be precisely determined by duplicate analyses of fruit juice with a temperature-compensated digital refractometer. The critical factor is to measure Brix value as fast as possible after juice extraction. The evaporation of water from the juice will increase the Brix reading. The use of gauze to squeeze the juice from the flesh may seem to be a convenience and is a common way to obtain... 

Let us dwell on Figure 6.4 for a moment. The standards and sample solutions are introduced to the instrument in a variety of ways. In the case of a pH meter and other electroanalytical instruments, the tips of one or two probes are immersed in the solution. In the case of an automatic digital Abbe refractometer (Chapter 15), a small quantity of the solution is placed on a prism at the bottom of a sample well inside the instrument. In an ordinary spectrophotometer (Chapters 7 and 8), the solution is held in a round (like a test tube) or square container called a cuvette, which fits in a holder inside the instrument. In an atomic absorption spectrophotometer (Chapter 9), or in instruments utilizing an autosampler, the solution is sucked or aspirated into the instrument from an external container. In a chromatograph (Chapters 12 and 13), the solution is injected into the instrument with the use of a small-volume syringe. Once inside, or otherwise in contact with the instrument, the instrument is designed to act on the solution. We now address the processes that occur inside the instrument in order to produce the electrical signal that is seen at the readout. 

The composition of permeates was deduced refractomet-rically by digital differential refractometer (Atago DD-5, Atago Co. Ltd., Japan). 

A sample about 6.3 x 12.7 mm with a flat polished surface is placed on the prism of a refracto-meter. Generally, the refractometer will provide a digital representation of the refractive index. 

The sulfonate concentration in the microemulsion was determined from the equilibrated microemulsion phase volume and the known weight of sulfonate in the system the assumption that the microemulsion phase contained all of the sulfonate was justified for all microemulsions. The volume fractions of oil and brine in the microemulsion were determined from the excess volumes of oil and brine, respectively. The microemulsion density and index of refraction needed to calculate the specific refraction (Eq. (1)) were measured on a Mettler-Paar DMA 40 digital density meter with accuracy of 0.0001 g/cm and a Zeiss Abbe refractometer ( 0.0001), respectively the temperature was controlled with an Exacal 100 and Endocal 150 constant temperature circulator-baths connected in series. Interfacial tensions between the microemulsion and equilibrated excess phases were measured on a University of Texas Spinning Drop Tensiometer or a Spinning Drop Tensiometer from S S Instrument Mfg. measurements were carried out until equilibrium values were obtained as indicated by constant readings over a period of at least 1 hour. 

Dispersion of the injected samples was monitored using a differential refractometer (Waters model 2410) at the outlet of the dispersion tube. Detector voltages, V t), were measured at accurately 5 s intervals with a digital voltmeter (Agilent 34401 A) with an IEEE interface. Binary difflr-sion coefficients were evaluated by fitting the dispersion equation ... 

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