Analytical Methods in Environmental Chemistry Questions and Answers

Answer: C

One ppm equals 0.0001%.

Parts per million means 1 part in \(10^{6}\). Per cent means parts per hundred, so multiply the fraction by 100:

\[ \begin{aligned} 1\ \text{ppm} &= \frac{1}{10^{6}} \times 100\% \\ &= 10^{-4}\% = 0.0001\% \end{aligned} \]

Equivalently 1% = 10,000 ppm. The options 0.01%, 0.1% and 0.001% are 100 ppm, 1,000 ppm and 10 ppm respectively.

Answer: C

The technique not used directly for organic determination is AAS, atomic absorption spectroscopy.

AAS measures the absorption of light by free atoms in a flame or furnace, and is used for metals and some metalloids, such as lead, cadmium, mercury and zinc. Organic compounds are destroyed in the atomiser, so AAS cannot identify or measure them directly.

The other three are separation methods for organic compounds. Gas chromatography (GC) separates volatile organics such as pesticides and solvents. High-performance liquid chromatography (HPLC) handles non-volatile and heat-sensitive organics. Thin-layer chromatography (TLC) is a simple method for separating and identifying organic mixtures.

Answer: B

The tool is X-ray diffraction (XRD).

Every crystalline mineral has a characteristic arrangement of atoms, which diffracts X-rays at a set of angles fixed by Bragg's law, \(n\lambda = 2d\sin\theta\). The resulting pattern is a fingerprint, so XRD identifies minerals directly and, with quantitative methods such as Rietveld analysis, measures how much of each is present.

A scanning electron microscope (SEM) images surface shape and texture. Fourier-transform infrared spectroscopy (FTIR) identifies chemical bonds and functional groups, useful but less definitive for minerals. Dynamic light scattering (DLS) measures the size of particles in suspension, not their mineral identity.

Answer: B

The level of cadmium in soil can be determined with an atomic absorption spectrophotometer, which measures the light absorbed by free cadmium atoms at their characteristic wavelength and detects trace metals at mg/kg levels in digested soil. A UV-visible spectrophotometer works on coloured complexes and is far less specific for trace metals. XRD identifies crystalline mineral phases and cannot measure a trace metal concentration, and an IR spectrometer identifies functional groups in molecules, not metal atoms.

Answer: A

The Beer-Lambert law gives absorbance as extinction coefficient times concentration times path length.\[ A = \varepsilon\, c\, l \]\[ c = \frac{A}{\varepsilon\, l} \]\[ c = \frac{0.6}{0.04 \times 3} = 5\ \mathrm{M} \]So the concentration is 5 M. The value 10 M would result if the path length were taken as 1.5 cm, and 15 M and 20 M would need an absorbance of 1.8 or 2.4 with the same constants, so they do not fit the data given.

Answer: C

A concentration of 1 ppm in dilute aqueous solution is 1 mg of solute per litre, since a litre of water weighs about one million milligrams. A 5 ppm solution therefore needs 5 mg of parabenzopyrene in one litre. The molecular weight of 252 is not needed, because the concentration is a mass ratio and not a molar one. A mass of 252 mg would be one millimole, 25.2 mg is a tenth of that, and 252 micrograms is far too small, so 5 mg is the only correct quantity.

Answer: B

Molecular exclusion chromatography, also called gel filtration or size exclusion, separates molecules by size: large molecules cannot enter the pores of the gel beads and elute first, while small molecules enter the pores and elute later. Ion exchange chromatography separates by net charge, adsorption chromatography separates by the strength of adsorption to a solid surface, and thin layer chromatography separates by differential migration on a plate, which is mostly polarity. The pore size of the beads decides the size range that can be resolved.

Answer: D

Electrophoresis uses supported gels as the medium through which charged molecules migrate. Agarose gel is used mainly for nucleic acids and large fragments, polyacrylamide gel is used for proteins and small nucleic acids, and sodium dodecyl sulphate polyacrylamide gel (SDS-PAGE) is used to separate proteins by size. Since each of the three is a support medium, the answer is 'all the above'. Agarose has large pores and suits large molecules, while polyacrylamide has small pores that can be adjusted by the acrylamide concentration for good resolution of proteins.

Answer: A

The Walkley and Black rapid titration method is used for the determination of the organic carbon content of soil, option A. In this method the organic carbon in the soil is oxidised by potassium dichromate in the presence of concentrated sulphuric acid, and the unused dichromate is titrated against ferrous sulphate.

Nitrate (option B), phosphate (option C) and fluoride (option D) are determined by other methods, such as colorimetry or ion-selective electrodes, and not by this titration. The method is a standard one for soil fertility work in India.

Answer: A

A flame photometer can estimate sodium and potassium, option A. The instrument measures the intensity of the light that an element gives out when it is heated in a flame, and the alkali metals sodium and potassium are excited easily at low temperature, which makes the method simple and sensitive for them.

Cadmium and cobalt (option B), chlorine and bromine (option C) and mercury and arsenic (option D) are not excited enough in the flame of this instrument to give a dependable signal, and are measured by atomic absorption, ion chromatography or other methods.