Silver nitrate

Silver nitrate, AgNO3 (Lunar Caustic)

Physical Properties

(i) It is a colourless crystalline compound.

(ii) Soluble  in water and alcohol.

(iii) It melts at 212°C.

Chemical Properties

(i) It possesses powerful corrosive action on organic tissues, which at turns black especially in presence of light. The blackening is due to finely divided metalic silver, reduced by organic tissure It is therefore, stored in colored Bottles. 

(ii) Solutions of halides phosphates, sulfides chromates thiocyanates, sulphates and thiosulphates salt  with silver nitrate solution.

(iii) Ammonical silver nitrate is called as Tollen's reagent and used to identify reducing sugars and aldehydes.

It is known as silver mirror test of aldehydes and reducing sugars.

Ag2O + HCHO ¾¾®  2Ag + HCOOH.

Ag2O + C6H12O6 ¾¾®  2Ag + C6H12O7.

(iv)  Reactions with Iodine :

6 AgNO3 (excess) + 3I2 +3H2O ¾¾®  AgIO3 + 5AgI + 6HNO3

5 AgNO3  + 3I2 (excess) +3H2O ¾¾®   HIO3  + 5AgI + 5HNO3

Uses : It is used

  1. as a laboratory reagent for the identification of various acidic especially for Cl, Br and I
  2. Tollen's reagent is used in organic chemistry for testing aldehydes reducing sugars etc.
  3. for making AgBr, used in photography.
  4. in the preparation of inks and hair dyes.
  5. in preparation of silver mirror.

2. Photography :

(i) A photographic film consists of a light sensitive emulsion of fine particles (grains) of silver salts in gelatine spread on a clear celluloid strip or a glass plate. AgBr is mainly used as the light sensitive material.

(ii) The film is placed in a camera. When the photograph is exposed, light from the subject enters the camera and is focussed by the lens to give a sharp image on the film. The light starts a photochemical reaction by exciting a halide ion, which loses an electron. The electron moves in a conduction band to the surface of the grain, where it reduces a Ag+ ion to metallic silver.

 2AgBr(s)  2Ag + Br2

(iii) In modern photography only a short exposure of perhaps 1/100th of a second is used. In this short time, only a few atoms of silver (perhaps 10–50) are produced in each grain exposed to light. Parts of the film which have been exposed to the bright parts of the subject contain a lot of grains with some silver.

(iv) Next the film is placed a developer solution. This is a mild reducing agent, usually containing quinol. Its purpose is to reduce more silver halide to Ag metal. Ag is deposited mainly where there are already some Ag atoms. Thus the developing process intensifies the latent image on the film so it becomes visible. 

(v) If the film was brought out into daylight at this stage, the unexposed parts of the emulsion would turn black and thus destroy the picture. To prevent this happening any unchanged silver halides are removed by placing the film in a fixer solution. A solution of sodium thiosulphate is used as fixer. It forms a soluble complex with silver halides.

AgBr + 2Na2S2O3 ® Na3[Ag(S2O3)2] + NaBr

After fixing, the film can safely be brought out into daylight. This is called "negative".

Light is passed through the negative onto a piece of paper coated with AgBr emulsion. This is then developed and fixed in the same way as before.