W. R. ANGUS AND D. V. TILSTON 23 5

MAGNETOCHEMICAL INVESTIGATIONS. PART VI. ADDITIONAL NOTES ON STANDARDIZA- TION AND TECHNIQUE. BY W. ROGIE ANGUS AND (THE LATE) ~ O N A L D V. TILSTON.

Received 10th April , 1946.

1. Introduction. In Part I of this series 1 the magnet system and general technique

of measurement have been described in some detail. Resumption of

1 Angus and Hill, Trans. Farday SOC., 1943, 30, 185.

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236 MAGNETOCHEMICAL INVESTIGATIONS experimental work, after the lapse of some years, made it necessary to recalibrate the apparatus ; this paper deals with certain modifications of earlier methods which, it is believed, are more reliable than those formerly used.

The principle of the Gouy method may be fundamentally represented by the simplified equation w,y = c + orF . . . (I) in which w is the weight of an approximately cylindrical specimen of material of specific suscepti- bility' x ; c = KIA and a = 2Z/(H12 - H Z 2 ) are constants fcr a particular tube of cross-sectional area A , suspended in the pole-gap so that its bottom, at the centre of the gap, is in a field of strength H , and the top of the speci- men, I cm. above the centre, is in a field of strength H , ; K is the volume susceptibility of air ; and F is the force in mg. actually experienced by the specimen after applying the necessary correction for the fcrce experi- enced by the glass tube containing the specimen.

2. The Constants c and a. Since c = KIA = K'U (where o is the volume of specimen examined)

c may be evaluated precisely if 'U or its components I and A , can be deter- mined accurately. In the earlier standardisation A and I were measured directly, but there are inherent uncertainties in this method, e.g, possible lack of uniformity of bore, non-planarity of the sealed bottom end of the tube, and the need for applying a meniscus correction. The modification now used consists in filling the previously-weighed tube to the graduation mark with conductivity water or (preferably) benzene at a known tem- perature, weighing it (on the Gouy balance), and calculating v from density-temperature tables. This yields a value which not only includes the meniscus correction but has the advantage that from a series of values of v at different temperatures, the average value, v , can be calculated and the deviation, AV, of individual values of o from V can be derived. The fraction A%/:, which can be shown to be equal to Ax/x, is a measure of the accuracy of filling the tube and also a measure of the errors in x from this cause. It has been found that AEli is of the order of 0.02 % and, since the bore of the tube may be taken as sensibly uniform, this variation in volume corresponds with a variation in length; and, thus, for lengths of the order of 6 cm. amounts to 0.01 mm.

The constant a cannot be determined with the same precision as c. Experimental difficulties of determining I, H,, and H , accurately are great and are usually circumvented by the employment of a relative method in which the force exerted on a specimen of material of high purity and accurately known susceptibility is measured and a is cal- culated from equation (I). The choice of reference substance is of such great and fundamental importance in respect of subsequent measurements as to merit some general discussion here. Methods of purification are well established for numerous compounds but it is rare to find really concordant magnetic data for common substances. a n Three choices are available : (I) solid paramagnetic substances, (2) liquid paramagnetic substances, (3) diamagnetic substances.

(1) Solids must be finely ground, and homogeneously and tightly packed, and, since the choice is limited by available magnetic data, to hydrated salts of Ni, Cu, Fe, and Mn, it cannot be ignored that the use of such salts involves other possibilities of error, e.g. efflorescence during grinding, the presence of the metal in different valency states, non-uniform packing, filling " level " to the graduation mark on the tube and thus leaving out of account the meniscus. There is, however, the great advantage that the magnetic effect is large and that errors of weighing are extremely small.

Angus, Ann. Reports, 1941, 38, 27 . a Sugden, 9th Liversidge Lecture, J . Chem. Sot . , 1943, 328.

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W. R. ANGUS AND D. V. TILSTON 23 7 (2) Liquid paramagnetic standards are usually aqueous solutions of

strongly paramagnetic salts, the first choice being NiCl,.* The solution must fulfil certain requirements: i t must obey the additivity law con- necting the susceptibilities of solution, solute, and ~ o l v e n t , ~ it must be prepared from solute and solvent of high purity and known suscepti- bilities, must be relatively concentrated, chemically stable, and subject to precise chemical analysis. It has been noted that the magnetic be- haviour of the susceptibility tube is sometimes affected by the use of NiCl, solutions, but this is almost certainly a peculiarity of the particular glass used in making the tube.

(3) The use of diamagnetic substances has the disadvantage that the force experienced by the specimen is very much less (approximately 3 mg. as compared with approximately IOO mg. for solid paramagnetic substances6) and, consequently, cannot be measured with the same ac- curacy. Nevertheless, i t has the advantage, particularly if subsequent investigations are contemplated on diamagnetic substances, that the force exerted on the “ standard ” is of the same type and order as that experienced by the material being measured later.

All three classes of standard have been tested out and it may be succinctly stated (a) that with solid paramagnetic substances the un- certainties attaching to their use outweigh the advantages, and that the mean value of a derived has a probable error of 0.5 yo ; ( b ) that liquid paramagnetic substances involve tedious and frequent analysis ; (c) that diamagnetic standards are to be preferred.

as standard ; using this standard, Mr. G. Stott has carried out an extensive series of determinations of a and finds that the mean value has a probable error of 0.05 yo.

Recently Trew and French8 have advocated the use of diamagnetic standards, but it is felt necessary to enter a caveat against the use of water as the standard because of the difficulty of ensuring its purity, and also because of the existing uncertainty regarding the temperature coefficient of its susceptibility.

Like Selwood,7 we favour using benzene (x =- 0.702 x I O - ~ )

3. Calibration of the Magnetic Field. Repetition of calibration has established that, since the original

calibration, no appreciable decrease in field strength has occurred owing to a spreading breakdown of insulation of adjacent coils in the magnet. Periodic checking of the a values for a particular tube ensures that the field developed by the magnet does remain constant. The recorded values for field strength at different distances frcm the centre of the pole-gap (see Part I) have been confirmed by the calculated values given in Part VII.

The authors record their thanks to the Department of Scientific and Industrial Research for a maintenance grant to one of them (D. V. T.), and to Professor E. D. Hughes for his interest in the work.

Summary.

examined and certain modified procedures are described. Sources of error in calibration of the Gouy apparatus are critically

Nettleton and Sugden, Proc. Roy. Soc. A , 1939, 173, 313. Stoner, Magnetism and Matter (Methuen, London, 1g34), pp. 325-334. Ref. l, p. 189. Selwood and collaborators, J . Amer. Chem. Soc., 1939, 61, 3168 ; 1940,

62, 2765. 3055 ; 1941, 439 2509. 8 Trew and French, Trans. Faraday SOC., 1945, 41, 439.

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238 MAGNETOCHEMICAL INVESTIGATIONS

R6sum6. On examine d’un point de vue critique les causes d’erreurs dans

l’ktalonnage de l’appareil de Gouy et on dkcrit certaines parties de la technique que l’on a modifikes.

Zusammenfassung. Fehlerquellen in der Eichung des Gouy’schen Apparats werden kritisch

geprii f t und bestimmte Modifikationen der Vorgangsweise beschrieben.

Department of Chemistry,

Bangor. University College of North Wales,

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