Gold element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides). Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of nitric acid and hydrochloric acid, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term "acid test". Gold also dissolves in alkaline solutions of cyanide,
Acetoacetic acid widely used in the production of arylide yellows and diarylide dyes. Although the esters can be used in this reaction, diketene also reacts with alcohols and amines to the corresponding acetoacetic acid derivatives in a process called acetoacetylation. An example is the reaction with 2-aminoindane: Acetoacetic acid is measured in the urine of people with diabetes to test for ketoacidosis and for monitoring people on a ketogenic or low-carbohydrate diet, This is done using dipsticks coated in nitroprusside or similar reagents. Nitroprusside changes from pink to purple in the presence of acetoacetate, the conjugate base of acetoacetic acid, and the