ADVANCED SPECTROSCOPY ASSIGNMENT Answer Guide
AS02 – Mn in steel by UV/VIS
1.
Explain the reasons for the following preparation steps:
a) addition of nitric acid – to dissolve the sample (or at least most of it)
b) boiling to remove oxides of nitrogen – possible interference and toxic
c) addition of ammonium persulfate – dissolve remaining sample components, e.g. C
d) addition of potassium periodate – colour formation; oxidises Mn II to permanganate
e) not adding periodate to the blank – so no purple colour is formed
2.
Explain why the standards were prepared using steels rather than from 1000 mg/L Mn standard. Matrix
matching
3.
Compare the two calculation methods. The Absorbance/mass vs %w/w method is much quicker than
the normal method.
4.
Compare this method to that by AAS for Mn in steel. No need for colour forming step in AAS, therefore
quicker, and probably more accurate.
AS03 – P in copper by UV/VIS
5.
6.
What is the function of the molybdite and vanadate solutions? Colour forming reagents.
Explain why the more common colorimetric method for P known as the Molybdenum Blue method
(because of the colour formed) would be unsuitable for this analysis. The copper matrix forms a blue
colour.
AS04 – Aspirin in tablets by UV
7.
Explain the reason for the samples being treated with NaOH solution and allowed to stand for 30
minutes. To hydrolyse the ester group in the aspirin.
8.
Explain why the samples are then acidified. To form salicylic acid which is what is being actually
measured.
AS05 – Benzoic acid in soft drinks
9.
Explain the difference in results between the acidified and alkaline samples. Benzoic acid in alkaline
solutions forms the salt which absorbs differently to the acid.
10. Explain why this method would not be applicable to soft drinks such as Coca Cola and Fanta. Not really
to do with the colour because this analysis measures in the UV, but the species causing the colour will
also absorb in the UV.
AS08 – Zn in brass by AAS
11.
Do you think that standard addition is necessary in this analysis? Depends on your results, but there is
normally not much difference between the results, so no.
12. Would matrix-matched standards be a better approach? What would the ideal matrix-matched
standard be made from? Yes. Since this is a manufactured product, its matrix is well known. The best
matrix-matched standard would be already certified alloys of the similar composition as in AS02.
13. Is this sample a typical brass in terms of its zinc content? Justify your answer. No. Zinc content is
usually much higher.
AS09 – Analysis of xylene mixture by IR
14.
Explain the reasons for the choice of the analyte peaks. Do not overlap with each other or other peaks
in the spectrum, and are strong peaks.
15. Compare this method with another technique capable of analysing this mixture for each component.
GC would be more accurate but slower.
AS11 – Na & K in cement by flame photometry
16.
17.
18.
Give TWO reasons why mixed standards are superior to separate standards. Matrix matching and less
time involved (includes washing up).
Explain the function of the lithium. Internal standard to compensate for instrument variations.
Explain why calcium is included in the standards. Matrix matching.