Graham’s law is the rates at whish gases diffuse are inversely proportional to the square roots of their densities. This principle is made use of in the diffusion method of separating isotopes. The law was formulated in 1829 by British chemist Thomas Graham (1805-1869).
Gravimetry is the quantitative measurement of an analyte by weighing a pure, solid form of the analyte. Since gravimetric analysis is an absolute measurement, it is the principal method for analysing and preparing primary standards.
A typical experimental procedure to determine an unknown concentration of an analyte in a solution is as follows:
- quantitatively precipitate the analyte from the solution
- collect the precipitate by filtering and wash it to remove impurities
- dry the solid in an oven to remove the solvent
- weigh the solid on an analytical balance
- calculate the analyte concentration in the original solution based on the weight of the precipitate.
Kjeldahl flask is a round bottom flask with a long wide neck that is used in the determination of nitrogen by Kjeldahl’s method. The method was developed by the Danish chemist Johan Kjeldahl (1849-1900).
Ostwald viscometer, also known as U-tube viscometer or capillary viscometer is a device used to measure the viscosity of the liquid with a known density. The method of determining viscosity with this instrument consists of measuring the time for a known volume of the liquid (the volume contained between the marks A and B) to flow through the capillary under the influence of gravity. Ostwald viscometers named after the German chemist Wilhelm Ostwald (1853-1932).
The instrument must first be calibrated with materials of known viscosity such as pure (deionized) water. Knowing the value of viscosity of one liquid, one can calculate the viscosity of other liquid.
where η1 and η2 are viscosity coefficients of the liquid and water, and ρ1 and ρ2 are the densities of liquid and water, respectively.
Potentiometric titration is a volumetric method in which the potential between two electrodes is measured (referent and indicator electrode) as a function of the added reagent volume. Types of potentiometric titrations for the determination of analytes in photoprocessing solutions include acid-base, redox, precipitation, and complexometric.
Potentiometric titrations are preferred to manual titrations, since they are more accurate and precise. They are also more easily adapted to automation, where automated titration systems can process larger volumes of samples with minimal analyst involvement.
A titration curve has a characteristic sigmoid curve. The part of the curve that has the maximum change marks the equivalence point of the titration. The first derivative, ΔE/ΔV, is the slope of the curve, and the endpoint occurs at the volume, V', where ΔE/ΔV has the maximum value.
Soxhlet extractor is a laboratory apparatus designed to extract substances with a low solubility in the extracting solvent. The method described by the German chemist Franz von Soxhlet (1848-1926) in 1879 is the most commonly used example of a semi-continuous method applied to extraction of lipids from foods. In the Soxhlet extractor, the sample soaks in hot solvent that is periodically siphoned off, distilled and returned to the sample. During each cycle, a portion of the non-volatile compound dissolves in the solvent. After many cycles the desired compound is concentrated in the distillation flask. The solvent in the flask is then evaporated and the mass of the remaining lipid is measured.
Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.
The accuracy of an analytical measurement is how close a result comes to the true value. Determining the accuracy of a measurement usually requires calibration of the analytical method with a known standard. This is often done with standards of several concentrations to make a calibration or working curve.
A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight.
A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is usually standardised against a primary standard.
Tear gases is the common name for substances which, in low concentrations, cause pain in the eyes, flow of tears and difficulty in keeping the eyes open. Tear gases are used mainly in military exercises and in riot control, etc., but have also been used as a method of warfare. Irritating gases have been used in war since ancient times but it was not until after the Second World War that a more systematic search for effective substances was started. Among a long series of substances, three have become of greater importance than the others. These substances are chloroacetophenone (codename CN), orto-chlorobenzylidene-malononitrile (CS) and dibenz(b,f)-1,4-oxazepine (CR).
Generalic, Eni. "Kjeldhal’s method." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. 21 Jan. 2025. <https://glossary.periodni.com>.
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