— Phosphorus printing is carried out in order to find the amount and distribution of phosphorus in steel.
— The steel surface is prepared as in sulphur printing.
— Bromide paper is soaked in a solution of ammonium molybdate — five gms per 100 ml of water to which 35 ml of concentrated nitric acid is added.
— Thus soaked bromide paper is placed in contact with the surface of steel.
— After about five minutes, this paper is removed and developed in an aqueous solution of 35% hydrochloric acid to which a small quantity of alum and 5 ml of saturated stannous chloride solution is added. Developing time is about four minutes.
During developing, the complex phosphomolybdate areas formed on the bromide paper earlier are partly reduced. This changes the colour of the areas from yellow brown to blue. These blue areas on the paper indicate the phosphorus rich portions on the surface of steel.
Since with the increase in phosphorus usually the intensity of blue colour also increases, the amount of phosphorus can be estimated from the relative intensity of the blue colour on the areas of the paper.