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CHEMICAL NEWS, July 10, 1903.

THE CHEMICAL NEWS.

VOL. LXXXVIII., No. 2276.

ON THE ATOMIC WEIGHT OF LANTHANUM. By HARRY C. JONES.

In a recently described determination of the atomic weight of lanthanum, Brauner and Pavlicek (Amer. Chem. Journ., 1902, xxviii., 23) found the value 139 04. A careful examination of their paper shows that a large amount of painstaking work was done, and great care was exercised in certain directions. Their value, however, differed from my own by a little more than a quarter of a unit (Fourn. Chem, Soc., Ixxxi., 1243).

It was obvious, therefore, that there must be some hidden source of error in at least one of the series of determinations, and possibly in both.

Work was undertaken with the object of determining, if possible, the cause of the difference between the values found by Brauner and those obtained in this laboratory. An examination of the paper by Brauner and Pavlicek, which was not published in full until after the appearance of my own, convinced me that, notwithstanding the fact that they had undoubtedly taken very great care in certain directions, in other directions their work contained possible sources of error. Their own rather sweeping criticism of a large number of investigations by careful workers, near the close of their paper, viz., "all atomic weight determinations of the rare earth elements made by the synthetical sulphate method during the nineteenth century are vitiated by an error which tends to lower the atomic weight and diminishes as the basicity of the earth decreases," seemed to me not to be sufficiently substantiated by experiment. This would probably depend upon the temperature to which the sulphate had been heatad, and also the time during which it was heated, free access of air, &c. This point was very carefully tested in my earlier work, using methyl orange as the indicator, and not the slightest trace of acidity could be detected.

Quite recently a criticism of my work by Brauner has appeared, in which a number of points are raised (Zeit. Anorg. Chem., 1903, xxxiii., 317). He thinks that my material was not sufficiently free from cerium, after I had stated that "The oxalate was then decomposed to the oxide and analysed spectroscopically by Mr. L. E. Jewell, whose work in this field is so well known. The only impurity which could be detected was a trace of cerium, and this was not more and probably much less than o'01 of 1 per cent." This analysis was made by means of the large Rowland spectroscope, by one of the most expert spectroscopists living, and I therefore think the results can be accepted, notwithstanding Brauner's opinion to the contrary. A moment's thought would, however, convince any one that traces of cerium would produce a negligible influence, since the atomic weight of cerium is so close to that of lanthanum.

Brauner's suspicion that in my determinations some of the oxide was lost by spattering, would have been removed by carefully reading what was stated on this point.

The various other points referred to by Brauner are matters which would be looked after and guarded against by any one in an ordinarily careful quantitative analysis, and still more, of course, in an atomic weight determination, and there seemed to be no occasion on my part to refer to them in print.

Possible Sources of Error in the Work of Brauner. I was particularly impressed in reading the paper by Brauner and Pavlicek with the fact that they heated the oxide of lanthanum in a platinum crucible placed inside

the oxide in

13

another platinum crucible. They themselves state that the oxide in "contact with the hot walls of the platinum crucible assumed an extremely slight pale buff tint. This tint was slightly more prominent in the higher fractions."

In my own work, I at first had used this same method for heating the oxide to constant weight. I had also observed that the oxide next to the platinum became coloured, and immediately abandoned the platinum crucibles altogether, and adopted porcelain crucibles in their stead. I supposed that the change in colour indicated a change in the composition of the oxide-a supposition which we shall see later is made highly probable.

Brauner determined the amount of acid sulphate in his product by titration with a standard alkali, and introduced the corresponding correction. It is difficult to see how he knew the composition of the acid sulphate which was present in such very small quantities. This, of course, could not be assumed from the composition of the acid sulphate prepared under different conditions.

This is

One reads Brauner's paper in vain for an account of a thorough spectroscopic study of the material which he used in his work. This does not refer to a study of the absorption spectrum, since many substances do not yield characteristic absorption spectra, but to a careful examination of the emission spectrum. Certainly no material should be used to-day for atomic weight work, whose emission spectrum had not been carefully photographed, and the proper comparisons made to determine the presence or absence of foreign substances. especially desirable in the case of the rare earths, where, on account of the unusual similarity in the chemical properties of the substances, chemical methods are not capable of detecting traces of impurities, if these impurities are the other rare earths. I do not doubt that the Rowland spectroscope would have convinced Brauner that his lanthanum contained at least traces of impurities, and that he was not dealing with absolutely pure material. One other point in connection with the work of Brauner and Pavlicek seems to call for special comment. They state (Fourn. Chem. Soc., 1902, lxxxi., 1252): “On heating the salt (lanthanum sulphate) to a temperature which in some cases may have finally exceeded 600°, that part which adheres to the platinum walls of the crucible may become partly converted into the basic salt, that part which lies nearer the centre may consist of the normal salt, and the uppermost inner layer may consist of some incompletely decomposed acid sulphate."

This would show that the sulphate was not heated uniformly throughout; the outer layer being heated to the highest temperature, the middle layer less highly heated, while the innermost layer was still less strongly heated. This is quite explicable when we consider the arrangement used by Brauner and Pavlicek for heating the sulphate. They state (Fourn. Chem. Soc., 1902, lxxxi., 1250) that "the platinum crucible was fastened in the centre of a large porcelain crucible, having in its lid a thermometer graduated up to 550°, by which, at least, the order of the temperature was indicated. The large crucible fitted into a larger plate of asbestos cardboard, in order to exclude the products of the combustion of coal."

Under the above conditions, with such a small air-space around the platinum crucible, it is obvious that the sulphate would be heated to a temperature which grew less and less as the distance from the platinum walls increased, and the temperature indicated by the thermometer suspended in the air in the innermost crucible may have borne no very close relation to the temperature of the lanthanum sulphate in contact with the platinum. The thermometer would, obviously, have indicated a considerably lower temperature than that to which the outer layer of lanthanum sulphate was subjected, and have given no clue to the correct temperature of the sulphate in contact with the platinum. This is shown by the fact that the outer layer of sulphate was partly decomposed into basic sulphate, while the inner layer still contained some acid sulphate.