CHAPTER 1: CO 2 CAPTURE
4. Experimental Equipment and Techniques
4.3 Introduction to the refractive index
- Measuring the power input required to keep the oscillator vibrating at a constant amplitude. The higher the viscosity, the more power is needed to maintain the amplitude of oscillation. The vibrating probe accelerates the fluid and power input is proportional to the product of viscosity and density.
- Measuring the decay time of the oscillation when the vibration of the resonator is switched off.
The higher the viscosity, the faster the signal decays.
- Measuring the frequency of the resonator as a function of the phase angle between excitation and response waveforms. The higher the viscosity, the larger the frequency change for a given phase change.
The main advantage of vibrating viscometers is the small volume of sample required for their use that leads to ease of operation over extreme conditions of temperatures and pressures. Furthermore, these instruments are of simple mechanical construction than those of oscillatory bodies. Easy cleanout and prospect of construction with easily available materials is another important feature of vibrational viscometers [223].
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! Types of refractometer
Measurement of the refractive index has been used for many years to identify and characterize liquids and solids .as refractive index measurements are quick and liable, they have become the state of the art in various industries around the world. Due to the different requirements for refractometers, these instruments have generated with different designs, operation, and precision.!
There are different types of refractometer including abbe refractometer, the digital refractometer, automatic refractometer and hand refractometer [224].
4.3.1.1! Abbe refractometer
The first laboratory bench-top device for the high precision determination of the refractive index of liquids was the Abbe refractometer. The measuring principle of this refractometer is based on the principle of total reflection. This way the purity of the substances and the concentration of the solution can be determined. For the measurement, two prisms that are made of glasses are used between which the sample should be tested is placed. These prisms are called illuminating and refracting prisms. The light from a radiation source is allowed to enter the apparatus and hits the sample from the illuminating prism, gets reflected at a critical angle at the bottom surface of reflecting prism. The refractive index of the refracting prism is high about 1.75 and the samples used in this refractometer should have a refractive index less than that of refracting prism. The incident light rays propagate through the double prism and sample only if their angles of incidence at the interface are less than the critical angle of total reflection. A microscope and a mirror are used to measure the position of the border between light and dark areas. A compensator knob is used to make the edge of light and dark area clear/sharp. Also, with the help of a rotary knob, the light and dark portion at the intersection of the microscope’s crosshairs is adjusted. The resultant refractive indexes can then be read from a Vernier scale piece. Furthermore, a pipe with an inlet and outlet as well as a thermometer is employed in this instrument to circulate water to control instrument and fluid temperatures [224].
4.3.1.2! Handheld refractometers
The handheld refractometers are cost-effective and simple to use with rapid concentration measurement of liquids and semi-solid samples. These refractometers are popular in a multitude of applications including determination of salt content in marine aquaria, water content in honey, serum protein content and Brix, and potential alcohol content. These cylinder-shaped refractometers do not require any battery for operation. Handheld refractometers have an illuminator flap instead of an illuminating prism which creates a diffused light and keeps the sample in place. The light propagates through the sample and enters the measuring prism. To measure the refractive index of the sample, only a small drop of liquid is sufficient. The corresponding refractive indexes can be then read from the eyepiece scale while the device is placed refractometer in the direction of some light source (such as lamp or sun). The measured refractive index can be converted into other units such as Brix degrees, alcohol or glycol %, etc.
For temperature compensation of liquids, handheld refractometers offer integrated Automatic Temperature Correction (ATC) [224].
4.3.1.3! Digital refractometer
Digital refractometers operate in the same way as handheld refractometers, except they are provided with an automatic determination and readout of the boundary line. They offer reduced inter-operator variability, and higher precision than manual handheld refractometers, and are typically available with a selection of common scales [224].
4.3.1.4! Automatic refractometer
Automatic refractometers eliminate differences in measured results between operators and provide the highest level of accuracy. In contrast to manual refractometric measurements under uncontrolled environmental conditions, it is possible, for example, to measure the refractive index of the sample at different temperatures or light wavelengths. Automatic refractometers are found mainly in laboratory applications, where precise measurement under highly controlled conditions is required. Modern instruments are equipped with light-emitting diodes (LEDs) as light sources,
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which have largely replaced older tungsten halogen or sodium vapor lamps because of their exceptionally long lifetime of 100 000 hours. An interference filter guarantees the maintenance of the correct wavelength. To enable comparability refractometric measurements, these are usually carried out at the standard wavelength of 589.3 nm (sodium D-line). The measuring principle is the same as other refractometers, where monochromatic light is incident at different angles of incidence on the prism and the sample. The intensity of the reflected light is determined by a charge-coupled device (CCD) scanner and the resulting position of the light / dark boundary is determined automatically.!Dark, cloudy, and even opaque samples such as mustard, ketchup, or mayonnaise can easily be measured with the help of a fully automated, modern refractometer. For these applications, 2-3 drops or a few grams of sample needed to be applied using a pipette or a spatula onto the measuring prism. To avoid stray light and evaporation, a sample cover is placed over the sample and prism. After measurement, the sample is removed by wiping with a soft paper cloth. The arduous purification of two prism halves (as is necessary for conventional refractometers) is thus avoided. Automatic refractometers are used in diverse industries in the processing of food and drinks to determine sugar content and for quality control. Refractive index is often represented on the Brix scale (° Brix) for these applications. This value, which is displayed directly on the device, technically refers to the content of the sucrose-dry substance in a pure water solution. However, it is also used as the characteristic value in the quality control of juices and other food products, and sometimes even in the analysis of other products, such as petroleum [224].
CHAPTER FIVE: EXPERIMENTAL SECTION