Лако-красочные материалы — производство

(a) Nitrite Value. (Calculated in terms of the acid sodium salt, mol. wt. 341.) A solution of 3.41 grams of H acid in 5 cc. 10 per cent soda ash solution is diluted to 250 cc., precipitated with 25 cc. concen­trated hydrochloric acid, and diazotized at 5°C. with 1N nitrite solution. H acid should give a yellow diazo compound which can be salted out as beautiful crystals. The number of cubic centimeters of nitrite multiplied by 10 gives the per cent purity.

(b) Coupling Value. To a solution of 3.41 grams of H acid in 50 cc. 10 per cent soda solution, diluted to 300 cc., is added at 0°C. enough phenyldiazonium solution to give a minimum excess of diazo compound. The endpoint is determined by placing a few drops of the red reaction

mixture on a small heap of salt on filter paper. After 5 minutes, the color­less outflow is tested with diazotized aniline solution, and if H acid is present, a red ring is formed immediately. If the diazo solution is in excess, a red ring is formed with H acid solution. The last traces of H. acid often react very slowly, and hence the last test should be delayed for 15 minutes or so. At the end of the reaction, more or less strong “after-coupling” always ocurs, being weaker the purer the H acid being analyzed. The nitrite value of a good sample of H acid is about 0.3 per cent higher than the coupling value. The number of cubic centi­meters is equal to the per cent purity.

All aminonaphtholdisulfonic acids, as well as the monosulfonic acids, are determined in this way. The diazonium solution is added from a 100-cc. graduate and the per cent purity is read off directly. Many laboratories use elegant, but complicated, ice-cooled burettes. The solu­tion is stirred with a glass rod bent at the end to form a large loop. Cou­pling is conducted in a clean porcelain dish.