In some studies, E171 was given to animals in drinking water without the stabilizers that keep E171 suspended in the liquid. Without stabilizers, E171 can settle and prevent the ingredient from combining with surrounding ingredients.

4.Used as a white pigment, the hiding power is second only to titanium dioxide, but stronger than zinc oxide. The hiding power increases as the ZnS content increases, and the light resistance also improves, but the acid resistance decreases.

2. Barite calcination method A solution of barium sulfide is prepared. The sulfuric acid and zinc oxide are reacted, purified by adding potassium permanganate and zinc powder, and compressed to obtain a zinc sulfate solution. Then, the prepared barium sulfide solution is mixed and subjected to a metathesis reaction to obtain a mixture of zinc sulfide and barium sulfate, and then the precipitate is subjected to pressure filtration, calcination, wet grinding, drying, and pulverization to obtain a zinc white product.

As mentioned above, these oxide NPs are harmful in part because both anatase and rutile forms are semiconductors and produce ROS. Particularly, P25 kind has band-gap energies estimated of 3.2 and 3.0 eV, equivalent to radiation wavelengths of approximately 388 and 414 nm, respectively. Irradiation at these wavelengths or below produces a separation of charge, resulting in a hole in the valence band and a free electron in the conduction band, due to the electron movement from the valence to conduction bands. These hole–electron pairs generate ROS when they interact with H₂O or O₂ [43,44]. It was described that they can cause an increase in ROS levels after exposure to UV-visible light [45]. The NBT assay in the studied samples showed that bare P25TiO₂NPs produce a large amount of ROS, which is drastically reduced by functionalization with vitamin B2 (Fig. 5). This vitamin, also known as riboflavin, was discovered in 1872 as a yellow fluorescent pigment, [46] but its function as an essential vitamin for humans was established more than sixty years later, and its antioxidant capacity was not studied until the end of the XX century [47,48]. This antioxidant role in cells is partially explained because the glutathione reductase enzyme (GR) requires it for good functionality. This enzyme is the one in charge of the conversion of oxidized glutathione to its reduced form which acts as a powerful inner antioxidant and can quench the ROS [49,50]. The cost of this action is that the glutathione is converted to the oxidized form and needs to be recovered by the GR. Consequently, the cells need more vitamin B2. Another glutathione action is the protection against hydroperoxide. This activity is also mediated by riboflavin. Therefore, local delivery of this vitamin seems to significantly help the cells in their fight to keep the oxidative balance, once they are exposed to high levels of ROS.

Lithopone is a white pigment (PW5) co-precipitated from solutions of zinc and barium salts according to the following reaction: