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SCIENCE. 



[N. S. Vol. IV. No. 92, 



the different indicators. The dissociation 

 factor is much higher for methyl orange 

 and for cochineal than for litmus and 

 phenol phthalein, and while the dissocia- 

 tion factor of hydrochloric and similar acids 

 is so high that a very small excess will 

 cause the change in color, even of methyl 

 orange, the dissociation factor for many 

 acids, and especially for most organic acids, 

 is so low that a quite appreciable excess is re- 

 quired, and the change in color will be slow 

 and uncertain. Hence methyl orange and 

 cochineal are entirely unsuited for the titra- 

 tion of weak acids, and litmus or phenol 

 phthalein must be used. For weak bases, and 

 notably for ammonia, the conditions are re- 

 versed. The salts of such bases with phenol 

 phthalein, or with litmus, undergo hydroly- 

 sis in dilute solutions, and a considerable 

 excess of the base will be required before 

 the ions characteristic of the indicator will 

 appear. The salts of the same bases with 

 methyl orange or cochineal are not so 

 readily hydrolyzed, and these indicators are 

 more suitable. 



A practical complication arises from the 

 presence of carbonic acid in most of the 

 solutions which we titrate. I will not take 

 the time here to discuss the details of the 

 theory which points out very clearly that, 

 for accurate results, carbonic acid must be 

 excluded from solutions in which litmus or 

 phenol phthalein are employed, while, if 

 concentrated, methyl orange or cochineal 

 may be used satisfactorily for strong acids. 



The other illustration of the application 

 of the principles of physical chemistry to an 

 analytical problem is one recently given by 

 Stefan Bugarsky.^^ A great many methods 

 for the separation of bromine and chlorine 

 have been developed, but nearly or quite all 

 of them rest on a purely empirical basis. 

 Bugarsky has studied the subject from an 

 entirely different point of view. Sometime 

 since Bancroft ^* determined the electromo- 

 tive forces developed between oxidizing and 



oxidizable solutions connected by an indif- 

 ferent electrolyte, and with a platinum 

 electrode immersed in each. The results 

 may be considered as giving a quantitative 

 expression for the relative oxidizing and 

 reducing power of the various substance& 

 studied. Among other things it was found 

 that, no matter what substance was oxi- 

 dized, iodic acid with sulphuric acid de- 

 velops a greater electromotive force than 

 bromine with potassium bromide and less 

 than chlorine with potassium chloride. It 

 appears, therefore, that iodic and sulphuric 

 acids together should liberate bromine, but 

 not chlorine, from a solution containing 

 bromides and chlorides. The practical ap- 

 plication of this theoretical conclusion ap- 

 pears to have been entirely successful. 



It is not alone in chemistry that the theo- 

 ries of osmotic pressure and of electrolytic 

 dissociation have proved of practical value. 

 Nernst has developed from these theories a 

 theory for the cause of the electromotive 

 force in batteries, which, while it may not, as 

 yet, have received general acceptance, is a 

 more useful expression for our present 

 knowledge than any previously proposed. 

 The most important conception at the basis 

 of this theory is that of what may be called 

 a solution pressure for metals, correspond- 

 ing in some sense to the vapor pressure of 

 liquids. When zinc, for instance, is in con- 

 tact with water, or an aqueous solution, 

 this solution pressure is a force impelling 

 the atoms of zinc to pass into solution. In 

 order that they may do so, however, each 

 atom miist pass over into the state of an 

 ion ; that is, it must receive a charge of 

 positive electricity which is carried with it 

 into the solution. But only a very few 

 atoms can pass into solution before the 

 negative charge left in the mass of the zinc 

 in proportion as the positive ions separate 

 from it will cause such an accumulation of 

 zinc ions in proximity with the zinc as to 

 balance the solution pressure. If, however, 



