Spectrophotometry Fundamentals Questions and Answers

Answer: B

A colorimeter uses a coloured filter passing a band tens of nanometres wide, which is cheap and rugged but far less selective than a monochromator. Prisms and gratings belong to spectrophotometers proper, and an interferometer belongs to a Fourier transform instrument. The wide band means the Beer-Lambert law is obeyed less well, so a colorimeter suits routine determinations with broad absorption bands rather than precise or crowded spectra.

Answer: C

A broad band and a single absorber mean the wide filter passband does little harm, since there is nothing else to confuse it and the absorptivity varies slowly across the band. High precision, mixtures and the far ultraviolet all demand a proper monochromator. Matching the selectivity of an instrument to the selectivity the problem requires is what stops money being spent on capability that will never be used.

Answer: D

A solution appears the complement of what it absorbs, so absorbing blue leaves yellow and orange to reach the eye. It cannot appear the colour it removes. That the analytical wavelength is the absorbed one rather than the apparent one catches people out constantly: a yellow solution is measured in the blue. Reading the colour wheel backwards is the quickest way to choose a filter for an unfamiliar coloured solution.

Answer: A

A red solution transmits red and absorbs its complement, so it is measured in the green around 500 to 550 nanometres. Measuring in the red would find almost no absorption at all and give a hopelessly insensitive determination. Ultraviolet and the sodium line are unrelated to the colour. Choosing the wavelength the sample absorbs rather than the one it appears is the single most common practical decision in visible photometry.

Answer: B

Absorbances add, so the total at one wavelength is one equation in two unknown concentrations, and a second wavelength supplies the second equation. The species generally absorb at both wavelengths, which is precisely why the simultaneous equations are needed. The law holds perfectly well for mixtures and is what makes the addition valid. Counting unknowns and counting independent measurements is a habit that settles how much data any analysis needs.

Answer: C

Two unknown concentrations need two independent equations, so two absorbance measurements at two wavelengths suffice, provided the two species' absorptivity ratios differ between them. If the ratios were identical the equations would not be independent and no number of measurements would help. That independence rather than count is the real requirement is worth remembering, and it decides which pair of wavelengths to choose.

Answer: D

Choosing wavelengths where one species dominates at each makes the equations well conditioned, so a small measurement error produces a small error in the result. Steep sides make the readings wavelength-sensitive, the region is irrelevant, and equal absorbances would make the equations degenerate. The quality of a simultaneous determination depends on how different the two equations are, and picking each wavelength near a different maximum is how that difference is maximised.

Answer: A

The first two are correct and the third is not: a detector measures intensity, from which transmittance and then absorbance are computed. The distinction matters because every source of error acts on the intensity measurement, and its effect on the computed absorbance depends on where on the logarithmic scale the reading falls - which is precisely why the low-error band exists at all.

Answer: B

Both are true and the second is the reason: lengthening the path raises the absorbance in proportion, bringing a reading that would otherwise sit near zero into the band where relative error is least. Dilution works the other way for concentrated samples. Two of the Beer-Lambert law's three terms are under the operator's control, and using them to place the reading in the best part of the scale is the practical skill the law supports.

Answer: C

The reason is true but the assertion has the direction wrong: extra light at the detector makes the sample appear to transmit more, so the computed absorbance is too low rather than too high. That is why the calibration bends towards the concentration axis. A mechanism correctly identified but applied in the wrong direction is the most easily missed kind of error, and tracing it through the logarithm settles it.