April
li, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
containing known amounts of magnesium. The data from several plates are averaged and used in the preparation of a calibration curve on semi-logarithmic coordinate paper, with linear measurements of differences between peaks on the direct scale and corresponding per cent magnesium on the logarithmic scale. This yields very nearly a straight-line graph which may then be used for interpolation in evaluating the results on analytical samples.
185
TABLE111. DATAOBTAINEDFROM FOURSPECTRA No.
OF
DIFFERENCE BETWEEN SPECTRUM PEAKS
Mm. 1 2 3 4
41 38 41 39
MAQNESIUM % 0.0097 0.0106 0.0097 0.0103 Av. 0.0101 f 0 0005
The maximum deviation of the analyses from the mean is about one part in twenty. I n routine practice the error TABLEI. MEASUREMENTS OF RECORD OF MAGNESIUM-BISMUTH ranges from one part in twenty to one part in ten. COMPARISON PAIR With less expenditure of time greater reliability can be DIFF.BETWEEN RECORDNo. MAQNESIUM PEAKS obtained with this method than with the other method deMm. % scribed by one of the authors (2). Thus, four check analyses 1 0.027 4 2 are taken on one plate consisting of four exposures. By the 0.009 44 3 84 0.003 older method four check analyses require four plates of five exposures each. ACCURACY OF METHOD The chief error in the method seems to be connected with The recording microphotometer has been carefully con- the excitation of the spectra. The arc between graphite structed and the technic of using it perfected so that the error electrodes is extremely variable and difficult to maintain to be expected from this step of the procedure is less than the centered on the slit. If, during the photography of one of the other errors present. Figure 3 shows five successive records four check spectra, the arc becomes most erratic while much of the same pair of lines from the same spectrum. The of the magnesium is being volatilized into the arc, and during the photography of another spectrum the arc remains steady differences between peaks are given in Table 11. or becomes erratic only after the magnesium is almost entirely expelled from the electrode, the two spectra may not check BETWEEN PEAKSFROM SAMEPAIR TABLE11. DIFFERENCE each other. It is wise, therefore, to take as many as four OF LINES check spectra in order to give a higher probability of correct(Five successive records) RECORDNo. DIFB.BETWEEN PEAKS ness to the average analysis obtained. Mm.
1 2 3 4 6
24.7 26.0 24.3 24.0 25.0 Av. 24.6 =t0.6
I n the routine practice of this method, four electrodes containing the sample to be analyzed are prepared. The four spectra are photographed on the same plate and a photometric record is made of each. ‘ The data obtained from a typical case are given in Table 111. The photometric record is shown in Figure 4.
ACKNOWLEDGMENT The authors are deeply indebted to M. L. Fuller for his painstaking efforts in editing the material. LITERATURE CITED (1) Gerlach, W., and Schweitzer, E., “Die Chemische Emissionsspectralanalyse,” Leopold Voss, Leipzig, 1930.
(2) Nitchie, C. C., IND.ENO.CHEM.,Anal. Ed., 1, 1-18 (1929). (3) Sohweitzer, E.,2.anorg. allgem. Chem., 165, 364 (1927). RECEIVED June 12, 1931.
.Analysis of Nitrous Oxide by Solubility in Water ALBERTL. CHANEYAND CHARLESF. LOMBARD, Los Angeles County General Hospital, Los Angeles, Calif.
A
LTHOUGH the U. S. Pharmacopeia does not specify a t present the percentage purity of nitrous oxide . suitable for anesthesia, nitrous oxide is only effective as an anesthetic a t concentrations of 90 per cent or higher, and it is therefore desirable a t times to ascertain the actual nitrous oxide content of such gas. As has been shown recently by Bennett (I), the principal impurity, nitrogen, may sometimes be found in concentrations of 10 per cent or more in the gaseous phase of full cylinders of commercial nitrous oxide. After some consideration of the various physical and chemical methods of determination of nitrous oxide, such as explosion with hydrogen, fractionation a t low temperatures, or “washing out” with water, as described by Bennett, a modification of this last method was found to be both accurate and rapid.
METHOD The modified apparatus for absorption of nitrous oxide in water is shown in Figure 1. The method differs from that described by Bennett in using water saturated with air, nitrogen, or oxygen in place of air-free water for absorption, and also by introducing a correction factor for the effect of this dissolved gas, instead of a graphical calculation of the percentage of nitrous oxide. A 10-cc. sample of gas is introduced through stopcocks 8, and 8 2 into the calibrated 10-cc. pipet P , and the pressure and volume adjusted by means of the mercury leveling bulb L. If the pipet is washed out with water before filling with the gas, sufficient water remains on the walls of the pipet and the mercury surface to keep the sample saturated with water vapor. Water saturated with air is then admitted from a flask and siphon, and is allowed to flow through the pipet at
ANALYTICAL EDITION
186
a rate of 10 to 30 cc. per minute. When water is first turned on, the leveling bulb should be lowered slowly so that all mercury may be displaced from the pipet, after which stopcock XS is turned so that the water continues to flow through the overflow tube TI. When solution of nitrous oxide is complete, which will be in from 5 to 10 minutes, the volume of undissolved gas is measured after adjusting the water levels in the pipet and overflow tube. The water above the gas bubble is readily 1 to the auxiliary overexpelled through stopcocks Xzand 8 flow tube T2. Further description of the apparatus is prob-
Vol. 4, No. 2
is to allow for the displacement of air from the water during the process of solution. The necessity of making a correction of this kind lies in the escape of air from solution, where it is in equilibrium with air at atmospheric pressure, into the gas phase, where the partial pressure of air is zero or small to begin with, increasing to atmospheric pressure at the end of the analysis. Bennett mentions the displacement of air from ordinary water when used in his method of determination of nitrous oxide, but he did not employ saturation of the water to correct the difficulty. This factor was first found empirically by using pure nitrous oxide prepared by condensation and fractionation a t low temperatures with liquid air. Using this nitrous oxide, the contraction of volume was about 97 per cent when solution was complete at a temperature of 25" C. The correction factor would therefore be 100/97 or 1.031. The magnitude of this factor suggests a relationship to the relative solubilities of nitrous oxide and air. Factors were then calculated on the assumption that when equilibrium is established, all of the air has been displaced from that volume of water which is just sufficient to dissolve the nitrous oxide. Stated another way, the ratio of solubilities, T , gives the volume of air displaced per cc. of nitrous oxide dissolved and the correction factor is represented by the expression 1/1 - T . Table I shows the ratio of solubilities and the calculated factors for obtainingthe percentage of nitrous oxide, calculated from data on solubility in the International Critical Tables (8)*
TABLEI. SOLUBILTTY RATIOSAND CORRECTION FACTORS RATIOOF SOLUBILITIES TEMP. Air:NzO Nz:NaO 0z:NzO
CORRECTION FACTOR Air Nz 0 2
c.
20 25 30
FIGURE1. MODIFIED APPARATUS FOR ABSORPTION OF NITROUSOXIDE IN WATER
ably unnecessary, except that the pipet was calibrated by weight of mercury, using a water meniscus, however, so as to simulate the conditions under which the final volume is read. The water is conveniently prepared by immersing an alundum extraction thimble in a flask of water and attaching the thimble to a source of compressed air or other gas. The porosity of the thimble produces many fine bubbles of gas, and saturation is rapidly accomplished. Care should be taken that the water is at room temperature during saturation and remains at the same temperature without appreciable change during analysis. It is probably more important to avoid supersaturation by a rise in temperature of the water than the reverse. CORRECTION FACTORS I n order to obtain the true per cent of nitrous oxide in the sample, the per cent of gas dissolved or per cent contraction must then be multiplied by a correction factor. This factor
0.0297 0.0313 0.0335
0.0254 0.0270 0.0292
0.0500 0.0524 0.0558
1.031 1.032 1.034
1.026 1.028 1.030
1.052 1.055 1.059
The data for nitrogen and oxygen are useful in case the water used is saturated with either of these gases instead of air. For the analysis of nitrous oxide-oxygen mixtures, as from an anesthetic machine, the use of oxygen for saturating water is preferable, indeed almost imperative. For the analysis of cylinders of nitrous oxide, the gaseous impurities, consisting largely of nitrogen and a tr&ce of oxygen, are sufficiently close to the composition of the atmosphere to permit the use of air for saturation quite satisfactorily. RESULTS The correctness of these calculated factors was tested by analysis of known mixtures of nitrous oxide with air, nitrogen, and oxygen under various conditions of temperature and absorbing liquid, The results of these analyses are presented in Table 11. The mixtures were prepared by thoroughly mixing the required amounts of gases in a 100-cc. gas buret, using nitrous oxide which had been carefully fractionated at low temperatures in their preparation. Table I1 indicates that results accurate within 0.5 per cent are readily obtained. Duplicate determinations were made for
TABLE11. ANALYSIS OF NITROUS OXIDE MIXTURES NzO % 100 95 90 85 80 75
A. N~O-ASRMIXTURES Vol. con- NzO by traction analysis Error % % % 96.6 91.6 86.7
81.8
77.0 72.2
99.8 94.6 89.6 84.5 79.6 74.6
0.2 0.4 0.4 0.5 0.4 0.4
B. NsO-Na MIXTURES Na0 by analysis Error
Vol. contraction
%
%
%
C.
NsO-Na MIXTUREE
Vol. contraction
NzO by analysis
Error
%
%
%
99.9 0.1 96.9 99.9 0.1 92.1 94.9 0.1 94.3 0.7 87.1 89.7 0.3 89.1 0.9 83.8 1.2 82.2 84.7 0.3 78.7 1.3 77.3 79.6 0.4 72.2 74.4 0.6 73.1 1.9 CONDITIONS OB ANALYSIS: A. Water saturated w,ith ajr; temp., 27O C.; factor, 1.033. B Water saturated with air' temp., 26' C . ; factor 1 0 3 2 . C.' Water saturated with nit;ogen; temp., 30' C.; bobor, 1.030. D. Water saturated with oxygen; temp., 24' C . ; factor, 1.055. 96.7 91.3 86.2 81.1 76.2 70.8
D. Nz0-02 MIXTURES VoI. con- Nz0 by traction analysis Error
%
% 94.7 89.8 84.9 80.3 75.5 70.3
99:s 94.7 89.6 84.7 79.7 74.2
0.1 0.3 0.4 0.3 0.3 0.8
April 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
all mixtures analyzed, and checked each other within 0.1 per cent, which is as close as the volumes may be read. The total errors in analysis of each mixture are also shown in Table 11. The errors involved in preparing the mixtures and also the residual impurities in the nitrous oxide are included in this total error. It will be noted that the error involved in series B of Table I1 is much greater than the other determinations. This is to be expected, since the gaseous impurity was pure nitrogen, whereas the water was saturated with air instead of nitrogen. I n this case there is a tendency for the more soluble oxygen in the water to be replaced by less soluble nitrogen, with a greater residual volume. Accordingly, for greatest accuracy the water should be saturated with gas having approximately the same composition as the gaseous impurities of nitrous oxide. This makes air a satisfactory gas for saturating the water in the case of commercial nitrous oxide, since the impurities are largely nitrogen and a small amount of oxygen.
187
The calculated factors of Table I seem to be accurate within the limits of error of the apparatus, since equally good results were obtained on the 100 per cent samples of nitrous oxide whether air, nitrogen, or oxygen was used for saturating the water. The apparatus described may be used for determining nitrous oxide by Bennett’s method by using air-free water, without any additional modification. It is difficult, however, to determine the absorption rate of a small residual bubble, especially on samples containing more than 98 per cent nitrous oxide. This absorption rate is the essential feature of Bennett’s graphical calculation. The use of water saturated with air or nitrogen eliminates this difficulty and considerably shortens the time of analysis.
LITERATURE CITED (1) Bennett, J. Phys. Chem., 34, 1137-57 (1930). (2) International Critical Tables, Vol. 111,pp. 255-7 (1928). R ~ C E I V EAugust D 28, 1931.
Deterrnination of Vanadium in Special Alloy Steels HOBART H. WILLARDAND PHILENA YOUNG,University of Michigan, Ann Arbor, Mich.
I
T HAS been shown in an
speed and simplicity to some of Vanadium is determined in the presence of earlier paper ( I d ) that sothe selective oxidation methods chromium by selective oxidation in a cold soludium azide is an excellent now in use for vanadium, the tion with excess of potassium permanganate, the differential r e d u c i n g agent for more important of these being: excess being reduced by sodium azide. All ceric sulfate in the presence of (1) direct titration of a vanadyl azide is removed by boiling and the vanadic chromic acid. A large excess of salt with permanganate ( 3 ) ; (2) azide acting for a considerable oxidation with boiling nitric acid acid titrated with standard ferrous sulfate, ditime had no reducing action on (4) to v a n a d i c acid which is phenylbenzidine being used as internal indicator. the chromium. It was pointed titrated with ferrous sulfate elecI n steels containing tungsten, the tungsten is out in the same paper that an intrometrically, a process in which kept in solution as a complex fluoride, and in this the oxidation is not quite comternal indicator such as diphenylform causes no interference. In such steels dia m i n e or diphenylbenzidine plete; (3) oxidation by permancould not be used in the titration ganate, removal of excess perphenylamine sulfonic acid is used as internal of chromic acid with ferrous sulmanganate with hydrogen peroxindicator in the titration of vanadic acid with fate if even a small amount of ide in acetic acid solution (8),or ferrous sulfate. This method for vanadium in azide were present, as the azide by nitrite, followed by urea ( 6 ) , tungsten steels is more rapid than any of equal prevented any color development and titration of vanadic acid with accuracy in use at the present time. with the indicator. The possiferrous sulfate electrometrically bility of removing azide by boilor with diphenylamine as indicaA rapid method of oxidizing tungsten in steel ing the solution and of titrating tor ( 2 ) ; and (4) oxidation with to tungstic acid is also described. the chromic acid by an indicator bromate in a solution containing method was not studied a t that ammonium salts and a definite time. concentration of hydrochloric acid, removal of bromate by It is obvious that vanadic acid, which is a weaker oxidizing boiling, and titration of vanadic acid with ferrous sulfate elecagent than chromic acid, should not be reduced by sodium trometrically or with an indicator ( I O ) . A method has been azide. Therefore, if hydrazoic acid can be removed easily developed for the determination of vanadium in steels containfrom a solution by boiling, there arises the possibility of ing not only chromium but tungsten, which is based upon its determining vanadium in the presence of chromium by selective oxidation by permanganate in cold acid solution, and oxidizing the former a t room temperature with excess of removal of excess permanganate by sodium azide. permanganate and removing the excess of oxidizing agent with The determination of vanadium in steels containing tungsodium azide. After boiling off all hydrazoic acid, the vanadic sten offers special difficulties. In most methods the tungsten acid could be titrated with standard ferrous sulfate, using is removed in the form of tungstic acid, this precipitate always diphenylbenzidine as indicator ( I O ) . Such a method for carrying with it some vanadium as an impurity. A number of vanadium would be of special value in that it would obviate methods have been suggested for the estimation of this small the difficulty experienced by many in determining accurately amount of vanadium in the tungstic acid, none of them being the end point in the direct titration of a vanadyl salt with entirely satisfactory ( I O ) . There is one method for vanadium permanganate (3) when a considerable amount of chromic in which the tungsten is kept in solution throughout the salt is present, and also in that it should prove superior in analysis (IO),but the end point in the vanadic acid titration in
