Determinations Uvitex

This fluorometric method determines Uvitex OB ultraviolet absorber in polystyrene in amounts down to 0.02%. 

The ultraviolet spectra in Figures 5.3 (b) to (d) show that, due to the presence of the Wingstay T maximum at 275 nm, the correction procedure would not be applicable to the determination of Uvitex OB in solutions which also contain a similar level of Wingstay T. However, it is quite feasible to determine Uvitex OB accurately in the presence of up to twice its concentration of butyl stearate or mineral oil, as the background due to these substances occurring between 275 and 418 nm is low and is sufficiently linear to permit application of the correction procedure. Thus, in some, but not all cases, this interference can be overcome by the use of a correction procedure. 

In a typical method, 0.2 g of the PS sample was shaken with chloroform to achieve a solution. The sample was excited by UV radiation of wavelength 370 nm from a mercury vapour lamp and the fluorescence spectrum of the sample recorded over the range 400-440 nm. The reading from the fluorimeter was noted and the Uvitex OB concentration in the PS determined by reference to a prepared calibration graph. 

METHOD 32 - DETERMINATION OF UVITEX OB ULTRAVIOLET ABSORBER IN POLYSTYRENE. FLUOROMETRIC PROCEDURE. 

This method is capable of determining down to 0.02% Uvitex OB in polystyrene with an accuracy of + 5%. 

The determination of Uvitex OB illustrates an example of the application of ultraviolet spectroscopy to the determination of additives in foodstuff simulent extraction liquids. 

Table 5.1 gives the formulation of a polyethylene and a polystyrene used in some Uvitex OB extractability studies. In addition to the ultraviolet stabiliser, liquids obtained in extractability tests carried out on these plastics would contain various other substances, some of which are ultraviolet absorbers and which may be present in the extractant at higher concentrations than Uvitex OB. In this case the polymer also contained Santonox R, Wingstay T, phenolic antioxidant and styrene monomer. The presence of such ultraviolet absorbing substances in the extractant will interfere in the determination of Uvitex OB at 378 nm. In applying spectroscopic methods of analysis to extractants, consideration must always be given to the possibility of interference by any polymer additives present other than that which one is required to determine. 

Figures 5.3(b) to (d) are ultraviolet spectra in the 250 to 418 nm region of synthetic solutions in the 5% sodium carbonate extractant of Uvitex OB (53 ppm), and up to three times this concentration of the other three polymer additives. Each of these additives would seriously interfere in the determination of Uvitex OB by evaluation of its absorption maximum at 378 nm. In the correction procedures, measurements are made at not only at this maximum but also at the two Uvitex OB minima at 275 and 418 nm (Figure 5.3(a)) Using suitable calibration and calculation procedures it is then possible to calculate the corrected optical density at 378 nm due to Uvitex OB alone, provided that in the region 275-418 nm, background absorption due to any other substances present is low, fairly linear, and not too steep.

Figure 5.2 Calibration Curve. Ultraviolet spectroscopic determination of Uvitex OB at 378 nm in aqueous extraaants (British Plastics Federation). Reproduced from Author s own files)...

Table 5.1 Interference by other Polymer Additives in the Ultraviolet Spectroscopic Determination of Uvitex OB in Foodstuff Extractants ...

Uvitex OB 5 0.052 The presence of Santonex R at this level would not cause more than 10% error in the determination of Uvitex OB at 378 nm... 

Uvitex OB 5 0.05 Optical density at 378 nm due to Uvitex OB is less than 3% of the optical density due to interference at 378 nm by all the other migrated polymer additives present. Uvitex cannot be determined under these conditions. 

Figure 5.1 shows ultraviolet and visible fluorescence spectrograms of a solution of Uvitex OB in the 50% wiv ethyl alcohohwater extract. This substance absorbs ultraviolet energy at 360 and 378 nm and re-emits a strong fluorescent light at 415 and 435 nm (i.e., about 55 nm higher in each case). Figure 5.4 shows a calibration curve for the direct spectrofluorimetric determination of up to 25 ppm Uvitex OB in the 5% sodium carbonate extractant at 415 and 435 nm. 

In many instances, visible fluorescence methods are less subject to interference by other polymer additives present in the extractant than are ultraviolet methods. Thus, Uvitex OB has an intense ultraviolet absorption at a wavelength high enough (378 nm) to be outside the region where many interfering substances in the extractant would be excited to fluoresce. Therefore, in some instances visible fluorimetry offers a method of determining an extractant constituent without interference from other constituents when this would not be possible by ultraviolet spectroscopy. 

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