Лако-красочные материалы — производство

Many amines cannot be analyzed directly by diazotization, either because they form diazoamino compounds or because they give diazo­nium compounds which color starch-iodide paper just as free nitrous acid does. These amines, exemplified by the nitroanilines, dichloro — anilines, etc. must be determined indirectly.

The amine (0.01 mole, for example) is dissolved in concentrated or partially diluted acid, and the solution, after dilution with water and ice, is diazotized with an appreciable excess of sodium nitrite. The clear diazonium solution is made up to a known volume in a calibrated flask and is then added, from a burette or a graduated cylinder, to a carbon­ate solution of /З-naphthol of accurately determined strength. The end point is taken as the point where a spot test with diazonium solution on filter paper shows the absence of /З-naphthol. Usually, the proportions of the reactants are so chosen that the number of cubic centimeters used divided into 100 gives the per cent of amine present in the original sample.

For example, 3.45 grams of p-nitroaniline (2.5/100 mole) is dis­solved in 10 cc. 30 per cent hydrochloric acid and 10 cc. water. The clear solution is poured into 50 grams of water and 50 grams of ice and treated with a 20 per cent solution containing 2 grams of pure sodium nitrite. The clear solution, containing about 0.2 gram of excess NaN02, is made up to 250 cc.

100 cc. of this solution is measured out in a graduate and added portionwise with thorough stirring to a solution containing 1.44 grams of 100 per cent /J-naphthol, 2 cc. 30 per cent sodium hydroxide, and 20 grams of soda ash in 300 cc. ice water. Spot tests are made on filter paper to determine the point where the reaction mixture no longer gives a reaction for /З-naphthol with diazonium solution. The number of cubic centimeters of nitroaniline solution divided into 100 gives the per cent purity. If the nitroaniline is 100 per cent pure, exactly 100 cc. is required. Usually, 101 to 102 cc. will be used.