A method of determining trace vanadium oxytrichloride in anhydrous titanium tetrachloride was required, which would be simple, rapid, and sensitive to vanadium-(V) at the I-p.p.m. level. Several simple methods were considered and evaluated.
A strong fundamental stretching band for V= 0 has been reported to occur at 9.66 prnl. Titanium tetrachloride also absorbs in this region, hence the determination of vanadium oxytrichloride is difficult. A weaker overtone band occurs at 4.54 ,uml and is frequently used for quantitative analysis 2. Even though titanium tetrachloride has no significant absorption in this region, the 4.84 pm band is not especially sensitive. Also, carbonyl sulfide absorbs at 4.Sg ,uml and therefore interferes.
A simple wet chemical method for determining vanadium in titanium tetrachloride involves hydrolysis with sulfuric acid and subsequent evaporation of the solution in the presence of nitric acid 3~4. Vanadium imparts a yellow color to the resulting solution. The evaporation step makes this method somewhat time-consuming and the resulting solutions were found to be difficult to handle because of their high viscosity.
Transition metal chlorides and oxychlorides form colored complexes with many organic compounds. A particularly simple method for determining vanadium oxytrichloride requires only the mixing of equal portions of the.titanium tetrachloride sample with thiophene-free benzene 6. The resulting yellow-orange solution can be measured spectrophotometrically.
However, titanium tetrachloride reacts with benzene to form an intensely colored yellow blank. This reduces the accuracy of the method.
Several redox indicators have been reported to respond to oxidizing agents such as vanadium(V) with a high degree of sensitivity. The sensitivity of one such indicator, diphenylbenzidine, relative to the methods mentioned so far is compared in terms of the absorbance produced by IO p.p.m. vanadium(V) in Table I. The superior sensitivity of the redox indicator is clearly evident. The details of the procedure in which this indicator was employed are discussed below.