Chemical Properties
Pure cadmium yellow is chemically composed of cadmium sulfide or a solid solution of cadmium sulfide and zinc sulfide. Its color is vibrant and rich (saturation can reach 80% to 90%), and its color spectrum ranges from pale yellow through true yellow to reddish yellow. Cadmium yellow containing zinc sulfide decreases in yellowness, down to pale yellow, as the amount of zinc sulfide dissolved in the solid solution increases. Industrially produced cadmium yellows include pale yellow (primrose yellow), bright yellow (lemon yellow), true yellow (medium yellow), dark yellow (golden yellow), and orange. Cadmium sulfide can also form an orange mixture with cadmium carbonate, but it is not a solid solution. The color of cadmium yellow is determined by secondary factors related to the pigment's colloidal aggregation, largely depending on the valence of the coagulating anion. Cadmium yellow particles are spherical or aggregated. Yellow-type cadmium yellow has a particle size of 0.04 to 0.4 μm, a pore radius of 20 to 200 nm, and a specific surface area of 7 to 8 m??/g. Cadmium sulfide yellow has two room-temperature stable forms: β-CdS, a cubic crystal, and α-CdS, a hexagonal crystal. The β-CdS is known as the low-temperature stable form, heat-resistant up to 500°C; the α-CdS is known as the high-temperature stable form, with a melting point of 1405°C and a heat resistance of 600°C or higher. Both forms can coexist stably between room temperature and 500°C. The individual grains of the cubic form are ≤100 mm, while the grains of the hexagonal form range from 100 to 280 nm. Cadmium sulfide yellow turns brown above 700°C. It sublimes but does not decompose in a nitrogen atmosphere at 980°C. In the presence of air, it completely oxidizes to CdSO4 and CdO. In the presence of moisture and air, cadmium yellow can oxidize to toxic cadmium sulfate. Cadmium yellow is insoluble in water, alkalis, organic solvents, and oils. It is slightly soluble in 5% dilute hydrochloric acid and soluble in concentrated acid, dilute nitric acid, and boiling dilute sulfuric acid (1:5). Cadmium yellow is unaffected by hydrogen sulfide. Its relative density ranges from 4.5 to 5.9, with lighter yellows having a lower density than darker yellows. The β-type is more lipophilic than the α-type. Cadmium yellow has good abrasive properties and blends easily with adhesives, but suffers from poor abrasion resistance. Prolonged abrasion in a dry state can darken the color, but heat treatment can restore discolored cadmium yellow. Cadmium yellow has strong tinting strength, excellent light and weather resistance, and does not migrate or bleed. While its insolubility makes it safe for coloring products, it is still a toxic pigment.
