Silver chromate ( Ag2CrO4 ) is a brownish-reddish propolis crystal and is a chemical precursor to modern photography . It can be made by mixing silver nitrate (AgNO 3 ) and potassium chromate (K 2 CrO 4 ) or sodium chromate (Na 2 CrO 4 ). This reaction has been important in neuroscience, as it is used in the ” Golgi method ” to staining neurons for microscopy: silver chromate (Ag2CrO4) precipitates inside neurons and their morphology is visible.
preparation and properties
Silver chromate (Ag2CrO4) is produced by the salt metathesis reaction of potassium chromate and silver nitrate in pure water – silver chromate (Ag2CrO4) will precipitate out of the aqueous reaction mixture.
The solubility of silver chromate (Ag2CrO4) is very low ( a sp = 1.1×10 −12 or 6.5×10 −5 mol/L). Crystallization occurs in the orthorhombic space group Pmma , with two different coordination environments for the silver ions, one tetragonal bipyramidal and the other distorted trihedral.
The use of the compound itself in the laboratory is limited, although its formation is used to indicate the endpoint in the titration of chloride with silver nitrate in the Mohr method of Argentometry . The reactivity of chromate ions with silver is less than with halides (chlorides and others), so that only silver chloride will be formed in a mixture of both ions. Only when there is no chloride (or any halogen) left, will silver chromate (Ag2CrO4) (reddish-brown) form and precipitate out.
Before the endpoint the solution has a milky lemon yellow color, due to the color of the chromate ions and the precipitate of silver chloride already formed. When approaching the endpoint, the red pigments gradually disappear from the addition of silver nitrate. The endpoint of the titration is reached when a reddish-brown color remains (with brown spots of silver chloride). This method is only suitable for neutral pH: at very acidic pH, silver chromate (Ag2CrO4) is soluble, and at alkaline pH, silver precipitates as hydroxide.
This reaction is used, for example, to determine chloride levels in salt water pools.