CORRESPONDENCE The Spectrophotometric Determination of Manganese , i n Steel
liter in order to conform t o the original data.) Figure 1illustrates several important points. 1. Beer’s law is obeyed over the concentration range employed. 2. Deviations from the straight line are plainly revealed, which IS not t h e case in a smooth empirical curve. 3. T h e average slope of the straight line is more reliable than any individual calibrdtion point. 4. A fe.w accurate determinations yielding a straight line provide a reliable value for e and no reference t o a curve is necessary. A11 subsequent measurements can be computed by means of Equation 2.
SIR: In a recent article [IsD. ENG.CHEM.,Anal. Ed., 7, 27 (1935)l J. P. Mehlig suggested the use of the spectrophotometer for the colorimetric determination of manganese. He proposed to eliminate entirely the use of permanent standards in this way. Nineteen standard solutions of permanganate were measured a t four different wave lengths t o furnish empirical calibration curves relating transmission to concentration. I t is the object of this brief note to show that if the simple, fundamental optical principles of colorimetry had been employed in this case, very little calibration would be necessary. For a system which obeys the Lambert-Beer law we may write: I
=
Io 10
ecd
An added advantage which is obvious from Equation 2 is the possibility of computing the results for any thickness d . The empirical calibration method would require a different curve for every thickness. The results which are reported on manganese determinations in steel show excellent agreement with other methods. It is felt, however, that the procedure is needlessly laborious, in that separations are not absolutely necessary in the photometric method.
(1)
Conclusion
where Io is the incident light intensity I is t h e emergent light intensity c is t h e concentration in molea per liter d is the thickness of t h e solution layer in om. e is t h e molecular extinction coefficient
IO I
or
log-
= ecd = E
h few precise measurements of transmittancy over the concentration range will reveal the reliability of a spectrophotometric method, if the results are computed or plotted in accordance with the Lambert-Beer law. For many substances the extinction coefficients may be found in the literature, in which case, if they are sufficiently reliable and exact for the purpose, no calibration of the spectrophotometer is necessary. I t is understood, of course, that the instrument must be calibrated in the physical sense. This is especially true with respect to the wave-length calibration [Gibson, K. S., J . Optical SOC.Am., 21, 564-87 (1931)1. The author’s statement that “the spectrophotometric method has no advantage over the volumetric bismuthate method” and that “they are to be regarded as alternate procedures” is unfortunate. The spectrophotometer is a costly but very precise instrument if it is used properly. If its employment invariably involved laborious empirical calibration as described, this would undoubtedly be a strong deterrent t o its wide use. Such is rarely the case. RALPHH. MULLER
(2)
where E is t h e extinction.
Some spectrophotometers have a portion of the scale calibrated Z directly in E. I n any case the transmittancy can be read dia
rectly and, if e is known, c can be calculated. values for X = 520 mp yield a value of e = 2.24 X
Mehlig’s nineteen
lo3
il single precise measurement of a standard permanganate solution, protected against fading by a small amount of periodate, was made on a Bausch and Lomb s p e c t r o photometer and gave the value: e
FIGURE1
=
2.23 X
WASHISGTOS SQUARE COLLEGE SBWYORKU N I V E X ~ I T Y Xwiv YORK,N. Y. May 17, 1935
lo3
The agreement is well within the ave r a g e deviation of the e values calculated from Mehlig’s data. Figure 1 shows a straight line passing through the origin and having a slope computed from o u r v a l u e of e = 2 . 2 3 X l o 3 . The circles indicate M e h l i g’ s observations. (The abscissas are expressed in millig r a m s per liter ins t e a d of moles per
SIR: In the a-ork recently reported under the title “Spectrophotometric Determination of Manganese in Steel,” the writer was especially interested in the establishment of a reference curve correlating transmittancy a t a given wave length with manganese concentration and preferred to use such a curve rather than the method suggested by R. H. Miiller in his recent note. If the most careful work is done, the amount of data required for constructing the curve is little more than is necessary for calculating the average slope of a straight line. Once constructed, the curve is just as easy to use in determinations as is a straight line and it is even simpler to read off from the curve the manganese concentration for a given transmittancy than to solve the equation log T = ecd for c in a given determination. I t was already evident from the results obtained by the writer that Beer’s law is obeyed over the concentration range employed: 361
362
IKDUSTRIAL AND ENGINEERING CHEMISTRY
In constructing the straight line in his Figure 1 Muller should have used an average value for e instead of a value calculated from a single measurement supposed to be precise, since an analyst can never be certain that a single observation is precise. How many atomic weight values have been based upon a single determination? The empirical calibration method does not require a different curve for every thickness d, since the transmittancy for the desired cell length may be calculated from data secured with another length by means of the fundamental Lambert’s law equation
where T1and Tzare the respective transmittancies for thicknesses dl and dz. This calculation is very easily made by means of the special K. and E. slide rule. The writer felt that it would be safer as a general policy to separate the manganese in his procedure, since such separation makes the method universally applicable without question. As
VOL. 7, NO. 5
yet it has not been proved that the separation of manganese is not absolutely necessary in either spectrophotometric or photoelectric methods which are to have general application. So far as the determination of manganese in steel is concerned, the spectrophotometric method has no advantage over the volumetric bismuthate method, because the latter gives just as good results without the necessity of separating the manganese and rarely requires any dilution. The spectrophotometric method, however, does have a decided advantage over colorimetric methods which require a series of standards. The strongest deterrent to the wide use of the spectrophotometer is the cost of the instrument rather than any so-called laborious, empirical calibration that may be involved. It is naturally understood that the instrument, if used a t all, is used properly.
J. P. MEHLIG OREGONSTATECOLLEGE CORVALLIB, OREGON June 28, 1936
Note to Authors
I
N THE preparation of manuscripts authors should address themselves to specialists in their particular fields, rather than to the general reader. If the article describes a new method, the author should endeavor to tell the complete story, so that the reader will not have to wait for succeeding contributions or duplicate the unpublished tests in order to find out whether he can apply the method in his own work. The following is suggested as a general outline to be followed in preparing analytical methods for this edition: 1.
2.
3.
Preliminary statement or introduction, in which the need for the method should be stated, brief reference t o other methods or literature given, etc. Experimental: Outline of proposed method Description of apparatus and reagents Procedure Data: Interfering substances or conditions Concentration range through which the method is applicable Accuracy of the method Precision of the method Discussion and summary
The author should state a t the outset why he thinks the paper is worth publishing. If it deals with EA method of analysis, he should give some comparison with established methods in point of speed, applicability, accuracy, and cost. Extensive reviews of the literature should not be given and such references as are cited should be carefully checked. Incorrect references are inexcusable and cast doubts on the author’s reliability. The theoretical considerations on which the method is based should be clearly set forth. In the experimental part, previously published or wellknown procedures which have been followed should only be designated or references given to them. If, however, the method is new, the data upon which it is based should be presented but in no greater detail than is necessary to prove its soundness. New procedures should be clearly described, that readers can easily duplicate the work. Loose directions should be avoided, unless
the author knows that no possible harm can result from the most liberal interpretation that can be made of such expressions as “to the faintly acid solution,” “wash the precipitate,’’ “ignite,” etc. If new or uncommon reagents are needed, the author should state their probable cost, where they can be purchased if rare, or how they can be prepared, if not on the market. The author should distinguish carefully between precision and accuracy. Briefly but somewhat roughly stated, accuracy is a measure of degree of correctness; precision is a measure of reproducibility. The precision of a result does not necessarily have anything to do with its accuracy; it serves merely as a measure of the duplicability of the procedure in the hands of a given operator. No claim for accuracy should be made unless the author believes that he has satisfactorily established the correct result. The author should be frank and define the limitations of the method. Tests dealing with the effects of foreign compounds should be made on mixtures in which the ratios of the compounds sought to the foreign compounds are varied and simulate conditions that are likely to be encountered in practice. If the author has made no such tests, he should state that he has no knowledge of the effects of foreign substances. It is desirable that possible applications of methods should be stated. A summary or prefatory abstract should acquaint the reader with the main points of the article. This should give concisely where possible the substances determined, nature of material t o which determination is applicable, interfering substances, range of concentration to which method is applicable, whether or not a sensible constant error is involved-that is, the accuracy of the method-and its precision. Either the summary or the prefatory abstract is so often used by abstractors that the author may well spend considerable time in their preparation, in order to be certain that proper emphasis is given to the main features of the contribution. Our “Suggestions to Authors” is available to those unfamiliar with the form of manuscript and illustrations preferred by INDUSTRIAL AND ENGINEERINQ CHEMISTRY.
