58 THE ATOMIC WEIGHTS OF 



C. THE TITRATION OF BORAX. 



Rimbach dissolved known quantities of crystallized borax in 

 water and titrated the solution, using a weight-burette, with 

 dilute hydrochloric acid which had been standardized by pre- 

 cipitation with silver nitrate. The percentage of actual hydrogen 

 chloride was computed from the total silver chloride obtained 

 in three independent determinations. Methyl orange, said to be 

 indifferent to boric acid, was used as an indicator. With the 

 atomic weights which (in 1893) seemed most reliable to him, 

 Rimbach himself derived the value 10.945 for the atomic weight 

 of boron. Subsequent recalculations of his analyses point to a 

 much higher figure, that is to say, to the value now in use, 11.0. 

 It was not until the present investigation had practically been 

 completed that it was thought desirable to recalculate Rimbach's 

 analyses with the atomic weights used in the derivation of our 

 own value for boron. The figure thus found was 10.917. 12 That, 

 for present purposes, Rimbach should be credited with the latter 

 value is permissible. It thus appeared, quite unexpectedly, that 

 this recalculated value (10.917) agreed more closely with our own 

 (10.900) than any other previous determination. 



Although Rimbach's value, as recalculated by the writers, is 

 probably nearer the true atomic weight of boron than that found 

 by any of his predecessors in this field, the methods pursued by 

 him are open to a number of objections. We shall not dwell 

 upon those of a more general character : That a titration method, 

 in general, is inferior to a purely gravimetric method; that the 

 use of a hydrated salt is not advisable; or that the hydrochloric 

 acid might have been standardized, preferably, by a method 

 involving the use of the indicator to be used in the final titrations. 

 The chief errors which may have crept into Rimbach's work are 

 of a more serious nature and deserve attention here. 



In order to free borax from adherent moisture, Rimbach 

 exposed the finely powdered decahydrate to the air, merely pro- 



12 Rimbach, using the older atomic weights, finds his hydrochloric acid to contain 

 1.84983 per cent, of actual hydrogen chloride. This percentage has been retained, 

 inadvertently it would seem, both by Clarke and by Brauner. The present 

 writers, using the newer atomic weights, obtained 1.85095 for the percentage 

 strength of Rirnbach's acid. With this figure, and from the mean quantities 

 involved in the final titrations, the atomic weight of boron was computed and 

 found to be 10.917. The calculation was based on the atomic weights used 

 in the present work and, in addition, on 11 = 1.0076 and Ag = 107.880. 



