Liebig’s method

Combstn of compressed MF in vacuo) J) Pepin Lehalleur(1935), p 141 (Structural formulas proposed for MF) 141-42(Reactions of formation MF from Hg, nitric acid alcohol 143-44(Liebig s method of manuf MF) 144—45 (Chandelon method of manuf MF) 148—49(Driers for MF) K) J.D. Hopper, PATR 480(1934)(Study of Pb salts of nitrocompds as substitutes for MF) L) J. D. Hopper, PATR 624(1935) (Development of detonating compds to replace MF) M) L. Majrich, SS 31, 147—48 (1936)(MF can be obtd in 99.6% purity by dissolving crude product in a mist of equal vols of monoethanolamine and ammonia and ppm with dil AcOH) N) Sancho (1941),... 

As far as the utilization of chlorine is concerned, Liebig s method is very uneconomic. As we can see from the equation (XVII-7), only one sixth of the chlorine introduced is utilized for the manufacture of chlorate, whilst the remaining five sixths are converted into almost worthless calcium chloride. Therefore this method is seldom used, as it cannot compete with the modern electro-lytical method of manufacturing chlorate the production costs of which are only one third of the cost of the Liebig method. 

Volumetric Methods, (i) Liebig s Method. This depends on the fact that when a neutral or alkaline solution of an alkaline cyanide is treated with silver nitrate solution drop by drop, a white precipitate of silver cyanide forms as each drop enters the solution. This disappears on shaking because the silver cyanide dissolves in the excess of alkali cyanide to form an argento-cyanide ... 

The triazine compound melamine (2,4,6-triamino-1,3,5-triazine) was first isolated, and named, in 1834 by Justus Liebig, who obtained it by reacting alkali with melam. A. Claus in 1875 improved on Liebig s method. S. Cloez and S. Cannizzaro in 1851, and E. Dreschel in 1875 and 1876, demonstrated that melamine was formed when cyanamide or dicyandiamide were heated at elevated temperatures (Scheme 32). In 1885, A. W. Hofmann made melamine from thiocyanuric esters by heating them in sealed tubes with concentrated aqueous ammonia101. 

QUANTITATIVE DETERMINATION OP CARBON AND HYDROGEN LIEBIG S METHOD... 

Liebig s method of determining the molecular weight of an organic acid by analysis of the silver salt (Berzelius used the lead salts, which are sometimes basic) is still in use. 

The principle of Liebig s method (1851) is as follows when a silver nitrate solution is added to a slightly alkaline solution of an alkaline cyanide, the soluble ion complex dicyanosilverate(I) ( argentocyanide ) forms according to the reaction... 

Liebig s method suffers from the following drawback the delay necessary for the dissolution of silver cyanide by cyanide ions still in excess (before the equivalence point). The persistence of the insoluble silver cyanide not yet dissolved may induce an error concerning the final point detection, which may be detected prematurely. This is the reason why it is recommended to carry out the titration very slowly. 

Deniges developed a very nice amelioration to Liebig s method. He proposed using an ammonia solution instead of pure water as solvent and, moreover, adding iodide ions as potassium iodide to the solution. The equivalence point is detected by the precipitation of silver iodide, which is very insoluble [X s(AgI) = 10 - ]. 

Now, the calculations are quite analogous to those encountered in Liebig s method. The three relations describing the titration before the equivalence point are... 

Wellings has shown that diphenylcarbazide can be used as an adsorption indicator for the estimation of cyanides by Liebig s method, and it gives a very sharp end-point especially for weak solutions (down to dilutions of 0-004M). By adding 2 or 3 drops of a 0 1 per cent ethanolic solution of diphenylcarbazide to the cyanide solution, the pink colour produced becomes pale violet at the end-point. The silver nitrate solution must be free from acid. 

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