Bruce 6. Johmon and John D. Wells Bendigo College of Advanced Education, Bendigo, Victoria 3550, Australia Direct-reading electronic analytical balances are likely soon to supersede one-pan suhstitution balances just as the larter sup&eded twwpan balances ( I ) . ~ d v a n t i ~of e srhe newer balances include speed, simplicity, portability, ease of calibration, and compatibility with data-handling systems. They do, however, have some little-advertized peculiarities which can significantly impair their performance. While these will not cause serious problems in most practical situations, a wise chemist will he aware of them. Common direct-reading balances operate on an electromagnetic servo principle (2). The weight of the pan and its load is balanced by the magnetic force from an electromagnet. actine either directlv on the pan supvort or via a lever system. when an ohject is placedbn thdpan the additional current reauired to restore the pan to its initial position is proportionHl to the added weight. Clearly anything which perturbs the field of the electromagnet will affect the current required to maintain the position of the pan, and hence the reading - of the balance. Schoonover (2) lists three potential causes of inaccuracy or imprecision: interference by ferromagnetic (and particularly magnetized) samples, interference by electromagnetic radiation, and dust (which may lodge between the pole pieces of the magnet). These are likely to he more important in the case of the more sensitive analytical balances. None of them were mentioned in the advertisine brochures or the instruction manuals supplied with ~ e t t i e or r Sartorius analytical balances which wibave purchased recentlv. A similar complaint, that manufacturers do not suppl; necessary information about buoyancy effects, has already been made (3). We became aware of the problem of weighing magnetized materials when one of us by chance used a slightly magnetized pair of scissors as a test ohject for an old equal-arm two-pan balance, The "control" weight shown by a new elec-
Table 1. Apparent Welght (g) of a Slngle Plece of Magnetite Ore
'
Journal of Chemical Education
Menler H I 0
Menler AEl8O
Orientation of sample Fw all positions Position of and orientations sample on pan "faceup" "face dawn" "on side" of sample from rear
A differenceof about 0.4 mg (7 parts in lo6) is expected between the weights indicated by the AE160 and the H10, because the former is referred to a calibration weight adjusted to match standard weights the latter has built-in weiahts adiusted to of densitv 8.0 a cm-"while the older.stan&rd of 8.4 g cmT3.Thus the balances disagreein effect by only 0.1 mg.
88
tronic balance not only differed by several milligrams from that measured on the two-pan balance. hut also varied with the position of the scissors on the pan. A one-pan mechanical (substitution) halance agreed with the two-pan balance. We decided to test the effect of a magnetic sample such as one might encounter in an analvtical lahoratorv. Accordinely, we weighed a single piece of magnetite ore (approximately 57 g) on a Mettler AE160 electronic balance and on a Mettler HI0 mechanical balance, the AE160 having been recalihrated and the sensitivity of the HI0 checked. The anvarent weieht indicated hv the electronic balance deuendedboth on the position of the sample on the pan and bn its orientation: the range of values was about 5 mg, or 0.01% of the weight of the sample (Table 1). By contrast, the weight indicated hv the mechanical balance was constant. reeardless of the position or orientation of the sample. 1; control experiments with a nonmagnetic 50-g "standard weight" (Table 2) we found that the apparent weight varied very slightlv with the position of the obiect on the AE160 pan, but only ifit were placed near the edgeof the pan. The difference between the weights indicated by the AE160 (center of pan) and H I 0 is within the combined tolerance limits for the two balances.'
centw left rinht
57.4862 76 65 65
57.4843 36 44 43
57.4834 59 39. 50
95
65
31
54.4843
.Other rtwdlngs. 57.4835 and 57.4859. were obtained in his position by rotatlng me a m p l e abom a vml~alaxis.
The results of further experiments with a range of ferromagnetic and nonferromagnetic samples are summarized in Table 3. We observed a systematic change in the indicated weight when a nonmagnetic object was moved from the center of the AE160 pan to the front or rear, the indicated weight being greater with the object a t the rear and less with it at the front. The small side-to-side variation in the case of a demagnetized soft iron sample was possibly associated with very slight residual magnetism. The magnetized samples caused much more dramatic weight variations (as high as 0.3%), although none were strongly magnetic. For example, in the case of a rubber-bonded ferrite sample (90% by weight Ba0.6Fez03) which appeared on casual inspection to be nonmagnetic, the indicated weight changed by about 0.1% when the specimen was inverted. Some of the same samples were alsousedas test objects for a different electronic balance which had similar specifications. Thevariations in the apparent weights of the magnetic samples were less pronounced in this case, suggesting that the second balance was better shielded from magnetic interference. Variations in the indicated weight as a nonferromagnetic sample was moved to different positions on the pan were also smaller. Our experiments show that small but significant errors
may he introduced if samples are placed off-center on the Dan of an electronic balance havine a '%OD-loadine"confie" uration, and that even slightly magnetic materials can cause large and erratic errors. Nonmaenetized ferromaenetics s e e k to be less of a problem. The effects of off-centerGading and interference by magnetic materials can both be reduced by use of a balance in which the pan is suspended below the weighing - - cell (2%but such balances are slower to use. less convenient, and not readily available. In any case the &e of an electronic balance cannot be recommended if stronalv magnetic materials are to he weighed, such as in Eaton a i d Eaton's method for measuring magnetic susceptibility (4). Schoonover (2) recommended testing for interference by ferromagnetic materials by "moving the material in and around the Dan area while lookine for lame chanees in the balance zero reading." We fonndtbis test to he h u c h less sensitive than lookine for chanees in the indicated weieht as " the sample is moved about on the pan, or its orientation changed. ~ i i a l l we y note that since an electronic balance measures the force (weieht) on the pan directlv. .. without the addition or removal of physical "weights," i t may be more sensitive to buoyancy changes brought about by the changes in the deusity of the air (2). The question of buoyancy effects in relation to the calibration of single-van balances has been discussed previously (3,5, 6). The calibration procedure for a direct-reading electronic balance is essentially the setting of its sensitivitv~sothat the aonarent mass in a& of a standard "weight" (oidensity 8.0 g Em-3, or adjusted (6) to be equivalent to such a weieht in air of densitv 0.0012 e cm-3) is indicated correctly.~ppropriatebuoyahcy corr&tions can then be applied if necessarv bv the use of standard eauations if the denskiof the air changes sign;ficantlY (5-9). between calibration and the weiehina of a samole. the change in the buoyant force on the itaniard weight iequires an adjustment of the sensitivity of the balance (i.e., recalibration). For most purposes the effect is negligibly small; a change of 1% in the air density requires a relative change in sensitivity of 1.5 X 10-? A major change in the weathermay cause the air pressure (and hence density) to change by about 4%; this would cause an error in sensitivity of 6 X Such a fluctuation could he important if day-to-day weight changes were being followed on a precision balance, but would probably not introduce serious error into most laboratory measurements. For precise measurements the problem is easily overcome by frequent recalibration against a standard weight, built in or external. T o summarize: electronic analytical balances represent a major technological advance over one-pan mechanical balanres, but m order t u make best useof them the user must be aware u i their perularities. I n particular, unr should
-
-
~~~~~
Table 2.
Apparent Welgnt (a) of a Nonmagnetic Standard Welght
Menlar AEt60 Position of sample on pan tront rear
center right
ien
Mettier HI0 ail positions of sample
FO,
49.9997 50.0003 50.0000 50.0000 50.0000
49.9995
Table 3. Observations on the Welahts lndlcated by a Mettler AEl6O Electronlc Balance for ~ l l f i r e n Classes t of~aterlals
Relative change indicated weight when sample moved: hom hont of pan to center,
Type 01 material Nonterromagnetic (glass: brass: austenitic stainlsss shreo Ferromagnetic low or zero magnetism (steel turnings: demagnetized
or from center
to rear 5-10 X lo-'
Comparison with weight indieatad from center
by H I 0
of pan to side
mechanical balance
-0
within
tolerance limits
lo-'
< zt5 X
10'
wimin tolerance limb
(1) be wary of ferromagnetic samples, and especially magnetized (2)
(3)
(5)
samples; be careful always to place the sample near the center of the pan of a "top-loading" balance; beware of stray electromagnetic radiation, such as that from large-scale electrical equipment; be aware of the danger of dusty environments; and note that a precision balance may require frequent recalibration merely because of changing weather conditions.
Uterature CNed
SOH imn)
magnetic (steelalloys: ferrite materials)
ow ever,
(4) 5-10 X
~~
> *lo
X lo-'
variable
> +5 X lo-@o~tsidetoiwan~e variable
limits;
indicated weight varies with Orientation 01 sample
(1) Rohrhach.D.F.;Pickering,M.J.Chsm.Edue.1982,59,118. (2) Schoonoucr,R. M. And. Chem. 1982.54.973A. (3) Bsftino,R.;Willismsan,A.G.J.Chem.Educ. 1984.61,51.
l979,56,170.
(4) Eatan.S.S.:Eaton.G. R., J. Chem. Educ. ( 5 ) Schoonover,R. Jones,F. E. A n d Chem. 1981.53, W. (6) Maeurdy. L. B. In "Treatise an Analytical Chemistry". Kolthoff. I. Eds.: Interscience New York, 1967:Vol7, p 4247.
M.:
M.: Elving, P. J.,
(7) Burg, W.%Veith,D.A.J. Chem.Edue. 1970.47,192. (8) Lewis, J. E.; Woo1f.L.A.J. Chem Educ 1971,48,639. (9) Winuiard,M. R.; Ww1ley.E.
M.:Butler.E.A.Anol.Chem.1977.49,2126.
Volume 63
Number 1
January 1986
87
