A novel volumetric method for quantitation of titanium dioxide in cosmetics

Nowadays there are many sun‐protection cosmetics incorporating organic or inorganic UV filters as active ingredients. Chemically stable inorganic sunscreen agents, usually metal oxides, are widely employed in high‐SPF (sun protection factor) products. Titanium dioxide is one of the most frequently used inorganic UV filters. It has been used as a pigment for a long period of cosmetic history. With the development of micronization techniques, it has become possible to incorporate titanium dioxide in sunscreen formulations without the previous whitening effect, and hence its use in cosmetics has become an important research topic. However, there are very few works related to quantitation of titanium dioxide in sunscreen products. In this research, we analysed the amounts of titanium dioxide in sunscreen cosmetics by adapting redox titration, reduction of Ti(IV) to Ti(III), and reoxidation to Ti(IV). After calcification of other organic ingredients of cosmetics, titanium dioxide is dissolved by hot sulfuric acid. The dissolved Ti(IV) is reduced to Ti(III) by adding metallic aluminum. The reduced Ti(III) is titrated against a standard oxidizing agent, Fe(III) (ammonium iron(III) sulfate), with potassium thiocyanate as an indicator. In order to test the accuracy and applicability of the proposed method, we analysed the amounts of titanium dioxide in four types of sunscreen cosmetics, namely cream, make‐up base, foundation, and powder, after adding known amounts of titanium dioxide (1 ~ 25 w/w%). The percentages of titanium dioxide recovered in the four types of formulations were in the range between 96% and 105%. We also analysed seven commercial cosmetic products labelled with titanium dioxide as an ingredient and compared the results with those obtained from ICP‐AES (inductively coupled plasma‐atomic emission spectrometry), one of the most powerful atomic analysis techniques. The results showed that the titrated amounts were well in accord with the analyzed amounts of titanium dioxide by ICP‐AES. Although instrument‐based analytical methods, namely ICP‐MS (inductively coupled plasma–mass spectrometry) and ICP‐AES, are best for the analysis of titanium, it is difficult for small cosmetic companies to install such instruments because of their high cost. It was found that the volumetric method presented here gives quantitatively accurate and reliable results with routine lab‐ware and chemicals.