Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. The colloid is usually either an intense red colour (for particles less than 100 nm) or blue/purple (for larger particles). Due to their optical, electronic, and molecular-recognition properties, gold nanoparticles are the subject of substantial research, with many potential or promised applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, and materials science.

The gold sol changes colour from red to blue/purple when the gold particles coagulate, i.e. gold particles combine to become larger. The coagulation of gold particles happens when 10% sodium chloride (NaCl) is added to the sol.

Gold Number is defined as the minimum amount of protective colloid in milligrams which prevents a color change from red to violet of 10ml gold sol by the addition of 1 ml of 10%NaCl solution.

There are different colloids with different gold numbers:

Gelatin: 0.005-.01

Haemoglobin: 0.03-0.07

Egg Albumin: 0.15-0.25

Potato Starch: 25

Gum arabic: 0.15-0.25

Caseinate: 0.01

Sodium Oleate: 1-5

Dextrin: 6-20

Based on the gold number, smaller quantities of gelatin are required to prevent the gold sol from coagulating and turning purple.