White Zinc Oxide - Yellow

The absorption spectrum of this nonstoichiometric phase forms the basis for the formerly much-used qualitative test for zinc oxide yellow when hot, white when cold . Alternatively, anion sites can be left vacant, e.g. ... 

Pigments or dyes may be added at this stage and where clear water-white sheet is required a small amount of a soluble violet dye is added to offset the faintly yellow colour of the natural mix. Stabilisers such as zinc oxide, zinc acetate or urea may be added to prevent the composition from developing acidity. 

Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9 1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. 

The zinc oxide used in ZOE cements differs entirely from that used in zinc phosphate cements. Whereas the latter has to be ignited to a very high temperature to deactivate it, the opposite is true of the zinc oxides used in the ZOE cement, which are of an activated variety. They are normally prepared by the thermal decomposition of zinc salts at 350 °C to 450 °C such oxides are yellow. Zinc oxides prepared by oxidizing zinc in oxygen may also be used these are white. 

At room temperature, zinc oxide is white, but when heated it becomes yellow. A compound that changes color on heating is said to be thermochromic. 

Physical Properties. Zinc oxide is a fine white powder that turns yellow when heated above 300 °C. It absorbs UV light at wavelengths below 366 nm. Traces of monovalent or trivalent elements introduced into the crystal lattice impart semiconducting properties. The elementary particles of ZnO obtained by the thermal method may be granular or nodular (0.1-5 pm) or acicular (needle-shaped). Some physical properties are given below ... 

CUT SLIVER OF ZINC. HOLD IT IN FLAME. IT BURNS WITH BLUISH-GREEN FLAME TO ZINC OXIDE. ZnO IS YELLOW WHEN HOT, WHITE WHEN COOL. 

Lastly, a little of the substance is digested with ammonia and filtered, the filtrate being acidified with hydrochloric acid if this gives a white precipitate, zinc oxide is present, or if a yellow coloration, arsenic is present. 

A compound that has one color at room temperature and a different color at a high temperature is known as a thermochromic compound. Zinc oxide is such a compound because it is white at low temperature but yellow at high temperature. The structure of ZnO is like that of the wurtzite form of ZnS shown in Figure 3.5. 

Zinc oxide, ZnO, is a white powder (yellow when hot) rnade by burning zinc vapor or by roasting zinc ores. It is used as a pigment (zinc white), as a filler in automobile tires, adhesive tape, and other articles, and as an antiseptic (zinc oxide ointment). 

The supplier explained that zinc carbonate turned yellow when heated. Normally, a yellow color meant that iron was present as an impurity. The supplier found no iron in his zinc carbonate, but it was still yellow. Pharmacies would not buy yellow zinc oxide, so the supplier sold white zinc carbonate instead. 

Zinc reacts with sulfur in a strongly exothermic reaction to form zinc sulfide and with oxygen to give zinc oxide the latter is yellow when hot but becomes white on cooling (tbermochromism). 

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