V O L U M E 22, NO. 7, J U L Y 1950 The extent of such agreement is shown graphically on the frequency distribution plots by reversing this correction and translating the spectrometric values to correspdnd to the combustion values. These translated values are indicated by the arrows marked “MS Values.” I t is seen how nearly the chemical Method B is in agreement with the spectrometric method, nhile Method A is not. Calculated Heating Value. The frequency distribution plot for the calculated heating value (Figure 4, left section) shows a greater reproducibility and perhaps a slightly greater accuracy yielded by Method B. Highest frequency for Method B lies between 1105 and 1110 B.t.u. per cu. foot, while that for Method A is centered a t 1105. However, the median for Method A is 1100, and that for Method B is 1105. The mean for Method B is 1107 and that for Method A is 1097. Thus the most probable value for Method B lies nearer the measured value, 1105, than does the most probable value for Method A . [Heating value was measured by J. H. Eisman and Ralph Jessup, National Bureau of Standards (see 6, p. 30).] Calculated Specific Gravity. The calculated specific gravity is an over-all measure of the accuracy of the analysis, and is accordingly of interest. The known value, 0.68200 * 0.00005, is neatly matched by the highest frequency of Method B, while the highest frequency of Method A, 0.672, is considerably to the left of this value. The mean and median of Method B are 0.680. The mean of Method A is 0.671; the median is 0.670. Thus, although Method A has yielded better reproducibility in this instance, the accuracy obtained is relatively poor. [The specific gravity was measured by Carroll Creitz, Sational Bureau of Standards. The method used is described in (2).] Comparison of New with Old Results Obtained by Method A. Because sample A.S.T.M. D 3-VII-4 was the same as A.S.T.M. D 3-lrI1-2, the opportunity occurs of comparing the analyses of the same sample made by the same laboratory several years apart. The laboratory averages have been used for this purpose and are tabulated (Table I ) to show the old and new results and the difference between the two, for each component and for calculated heating value and gravity. Differences greater than 1.0% or greater than 10 B.t.u. and 0.007 specific gravity, are indicated. On the whole the earlier results are in agreement with the later
885 ones. There are only two notable disagreements in the determination of nitrogen, four in the determination of methane, three for ethane, three in calculated heating value, four in calculated specific gravity, and none in the determinations of carbon dioxide and oxygen. REVIEW
Method B, which takes account of the oxygen consumed during a combustion as well as the contraction and carbon dioxide prcduced, gives a better analysis than Method A, which takes account of only contraction and carbon dioxide. In particular, Method B gives greater accuracy in the determinations of carbon dioxide, oxygen, nitrogen, methane, and ethane and in calculated heating value and calculated specific gravity. Method B yields greater reproducibility in the determination of carbon dioxide, oxygen, and nitrogen and in calculated heating value; Method A gives better reproducibility in the determination of methane and in calculated specific gravity. In the determination of ethane, reproducibility appears about equal. The results from Method B can be made to agree with those obtained by the mass spectrometer, but this is not true for Method A. Thus, in this instance, two totally different analytical methods, one chemical and the other physical, can be brought into agreement. ACKNOWLEDGMENT
The author is grateful to Jean Doyle, who checked the computation of heating values and specific gravities. LITERATURE CITED
(1) Kilday, M. V.,J . Research Nall. Bur. Standards, 45,43 (1950); R P 2113. (2) Natl. Bur. Standards, Misc. Pub. M177 (1947). (3) Shepherd, Martin, ANAL.CHEM.,19,635 (1947). (4)Ibid., 22,23 (1950). (5) Shepherd, Martin, J. Research Wail. Bur. Standards, 36, 313 (1946);R P 1704. (6)Ibid., 38, 19 (1947); RP 1759. (7)Ibid., 38,491 (1947);R P 1789. (8)Ibid., 44,509 (1950); R P 2098. (9) Shepherd, Martin. and Branham, J. R., Ibid., 11, 783 (1933); R P 625. RECEIVED February 3, 1950.
Analysis of a Carbureted Water Gas By Volumetric Chemical Methods and by Mass Spectrometer MARTIN SHEPHERD, National Bureau of Standards, Washington, D. C. Fifty-one laboratories throughout the country have analyzed a standard sample of the carbureted water gas type by the conventional volumetrio chemical methods and the mass spectrometer. The analytical data are compared in a series of frequency distribution plots which show the strong and weak points of these totally different methods.
T
HIS is the sixth of a series dealing with the cooperative analysis of standard samples of fuel gases. (These samples are not to be confused with the regular standard samples prepared and offered for sale by the National Bureau of Standards. Instead, they are mixtures prepared especially for the laboratories cooperating on this project with the American Society for Testing Materials.) The analyses mere conducted to furnish basic information for the preparation of standard methods by Subcommittee VI1 of Committee D-3, American Society for Testing Materials. The procedure for the development of these standards has been outlined in previous reports ( 3 , 4),and includes the evalua-
tion of apparatus and methods by means of performance with standard samples. The 5 1 laboratories cooperating in the analysis of this sample are widely distributed throughout the United States, and represent gas, oil, steel, and various chemical industries as well as college and federal research groups, Their work is typical of the better grade of gas analysis by laboratories using modern equipment. The preparation and distribution of this standard sample of the carbureted water gas type have been previously described (3), and its analysis by the conventional chemical methods (3) and by
ANALYTICAL CHEMISTRY
886 the mass spectrometer (6) has been reported. These reports give a general account of the cooperating laboratories, apparatus, and specific methods of analysis employed-information which is desirable in fully evaluating the data presented in this paper. The analytical data from both of these cooperative analyses are assembled here in such a manner that comparisons and contkasts between these widely different chemical and physical methods are brought into sharp focus. These data again show the real state of gas analysis in this country, and are worth a few moments’ reflection by those who analyze gases or use gas analyses.
I MS
MANNER OF PRESENTING ANALYTICAL DATA
Although the usual tabulation of the data has been made, prclonged contemplation of the tabulated version does not disclose what may be seen in a moment’s study of frequency distribution plots of these same data. The plots are really actual pictures of the analytical data and make most of the conclusions self-evident. Thus, little of the usual discussion is necessary. In each plot the results of the chemical analyses have been given in the lower or left section, and are marked C. The results of the mass spectrometric analyses are marked MS, and appear in the upper- or right-hand sections. Each circle of a plot represents a single determination of the gas indicated in the legend. The circles are plotted equidistant on the ordinates. Thus the abscissas are values derived from the analyses, and the ordinates indicate the frequency with which they occur. For example, in the plot for the chemical determination of carbon dioxide (Figure 1, left section), the following values for carbon dioxide have been reported: 2 determinations of 3.7’30, 1 of 4.0%, 5 of 4.2%, 20 of 4.3%, etc. [In reporting the arithmetical mean of chemical determinations, the values given are for the preferred chemical method unless otherwise noted. This is designated as Method I1 in the complete report of the chemical analyses (S),and involves a combustion over copper oxide for hydrogen and carbon monoxide rather than the absorption of carbon monoxide in cuprous chloride.] DISCUSSION OF ANALYTICAL DATA
Determination of Carbon Dioxide. Continuing the study of the frequency distribution plot for carbon dioxide (Figure l ) ,the highest frequencies for both chemical and spectrometric methods occur a t 4.5%. Mean and median of the chemical method are both 4.5, and these values are neatly matched by the spectrometric approach. Everything is in agreement. The spread of values is slightly greater for the spectrometric method, although the Gaussian distribution is evident in both sections of the plot. The chemical method appears to give somewhat greater reproducibility. Determination of Oxygen. The oxygen originally present in this sample, when it was analyzed as No. A.S.T.M. D 3-VII-1 by chemical methods, waa 0.5%. Most of this was removed before the sample waa analyzed aa No. A.S.T.M. D 3-VII-5 by the mass spectrometer, leaving less than 0.05%. When rounded off, the spectrometric values should indicate 0% if the determination is correct; the chemical values should indicate 0.5%. Thus, if the abscissa 0.5 on the C plot is changed to 0, the two plots (Figure 2) can be directly compared. It is seen a t once that the chemical method found considerably more oxygen than was present, and the spectrometric method did not have this fault. This duplicates the history of the analysis of a natural gas by these two methods ( 2 ) . These analyses give convincing evidence of the superiority of the mass spectrometer with respect to the determination of small amounts of oxygen in fuel gases of these types, and clearly demonstrate the need of improved chemical methods. Determination of Nitrogen. The values for nitrogen obtained by both methods are rather diffused over the frequency distri-
-I , 45
40
PERCENT
CARBON
DiOXlOE
Figure 1. Frequency Distribution Plot of Carbon Dioxide
bution plots (Figure 3). The chemical plot s e e m more to suggest the Gaussian pattern, but not overstrongly. The chemical mean is 6.3 * 0.3 (average deviation), and the median is 6.3. The spectrometric mean is 5.9 * 0.8 (including values from 4.2 to 8.2 only), and the median is 5.8 (dropping the high ones). These values are more closely in agreement with the best chemical value, 5.74 0.06. The 12 high rogues noted on the MS plot attest to some confusion of nitrogen with carbon monoxide in the mass analysis. Determination of Hydrogen. The frequency distribution plot (Figure 4) reveals the fact that neither the chemical nor the mass spectrometric method is altogether happy with this detci,niina-
+
I
C
I
S
1 1 PERCENT
.i
-
OXYGEN
Figure 2. Frequency Distribution Plot of Oxygen
V O L U M E 2 2 , NO. 7, J U L Y 1 9 5 0 tion. Of the two, the spectrometric values are more widely scattered over a range of 7.4y0and the plot shows no really significant highest frequency. The chemical values are somewhat more clustered, the total spread being 5%, with the majority of the determinations grouped between 34 and 35.5’%. The mean of the chemical values is 34.4 * 0.25 and the median is 34.8. The mean of the spectrometric values (33 to 37.8, inclusive) is 35.1 * 0.8, and the median is 35.1. These values are not in disagreement but their derivation is from data too widely scattered to warrant dependence upon the data of any laboratory selected at random.
887 monoxide to determine the resulting difference in the sensitive 28 peak, or a better method of computation to unscramble the overpopulous 28 peak. Determination of Methane. With the first of the hydrocarbons in this sample, the mass spectrometer is obviously competent to deal with the situation, and the chemical methods are relatively less adequate (see Figure 6). In the nature of things this must be so; for, as is well known, the chemical methods cannot (as a matter of simple algebra) determine more than two of a related series of hydrocarbons. Thus, if propane is present together with methane and ethane (as it almost invariably is), it is calculated from the combustion data as 2C1H6 CH,, and would never be identified in the actual analysis to compensate in any way for the error its presence had caused. The spectrometric mean is 8.0 * 0.4 (with three low values dropped); the chemical mean is 8.6 * 0.3. The spectrometric median, 8.0, is in agreement with the highest frequency, 8.1; the chemical median is 8.7; the highest frequency is 8.6. The preferred chemical method, using a combustion procedure rather than absorption for the determination of carbon monoxide prior to the combustion of the hydrocarbons, gave a mean of 8.4.
-
Figure 3.
Frequency Distribution Plot of Nitrogen
Determination of Carbon Monoxide. The frequency distribution plot (Figure 5) clearly shows the effect of two independent chemical methods [one absorption, the other combustion ($1, with two high frequencies apparent a t about 29.5 and 30.0%. Even so, all the chemical values are more closely grouped than the spectrometric values, which are indeed inconveniently various. The spectrometric values show no adherence t o the Gaussian pattern, and the wide dispersion causes the uncomfortable feeling that, if the results of but a single laboratory were available, they might be 6 or 7% away from the results of another, 3% away from the mean of, say, 30 laboratories, and probably no closer to the mean than 1%. (These observations are based on the laboratory averages.) In spite of all this, the mean of the spectrometric values (28.4 to 32.3, inclusive) is 30.3 * 0.9, which is in reasonable agreement with the (mean of all chemical values, 29.8 * 0.2. The median of the spectrometric values is 30.4; that of the chemical values is 29.7. From the chemist’s viewpoint, the problem is to determine whether absorption or combustion is the preferable method. From the physicist’s viewpoint, the direction of required improvement should be decided. This may involve increasing the sensitivity of the relatively .weak 12 peak of carbon monoxide, or the chemical removal of carbon
1
PERCENT
Figure 5.
Y I
29.0 30.0 CARBON
31.0
320 MONOXIDE
33.0
34.0
Frequency Distribution Plot of Carbon Monoxide
. ..
P E R C E N T
METHANE
Figure 6. Frequency Distribution Plot of Methane
P E RGENT
Figure 4.
HYOROG E N
Frequency Distribution Plot of Hydrogen
Determination of Ethane. The mass spectrometer offers a clean-cut determination of ethane with values well grouped between 2.7 and 3.7% (see Figure 7). The highest frequency occurs at 3.3, the median a t 3.3, and the mean at 3.3 * 0.15. In contrast, the chemical values are more scattered between 0.5 and 3.7. The highest frequency is about 2.8, the median is 2.8, and the mean is 2.9 * 0.17-all lower than the spectrometric values.
ANALYTICAL CHEMISTRY
888 Determination of Ethylene. If two outside values are eliminated, the mass spectrometer indicates from 11.8 to 13.7% of ethylene; eliminating three values from the chemical determinations leaves a range of 12.1 to 15% (see Figure 8). However, almost all the chemical values lie between 12.1 and 13.7%. The chemical plot is more nearly along the Gaussian lines, with two peaks, the lesser of which is caused by absorption in bromine, the greater by absorption in fuming sulfuric acid. The highest frequency (chemical) is a t 13.0%, and the mean is 12.9 * 0.10, corresponding to the median, 12.9. The highest frequency of the spectrometric plot is not so apparent, although the blocklike structure may be estimated to center around 13%. The mean is 12.9 * 0.4, and the median is 12.9. Agreement between the physical and chemical methods is good.
r
1i I I
.1
:
!
... pERIEL C,*
?ERCENT
I
. i
'os
I
T
PERCENT I
BUT N E
i !
PERCCNT I U T A N E ( N
I
I
i
. I
i
if.. I 1
PERCENT
I
PROWLEWL
!/!(.: I 11.0
120
PERCENT
13.0
140
15.0
ETHYLENE
QQ3l 01 03 0.5 PERCENT PROPANE
Figure 8. Frequency Distribution Plot of Ethylene, Propane, and Other Hydrocarbons
c PERCENT
Figure 7.
ETHANE
Frequency Distribution Plot of Ethane
Other Hydrocarbons. The right section of Figure 8 gives a record of the small amounts of other hydrocarbons reported by a few of the spectrometric laboratories. Out 01 138 analyses, propane was reported 15 times, propylene 18 times, n-butane 6 times, butenes 6 times, "C3"three times, C: 12 times, and acetylene 3 times. One of the delicate decisions sometimes to be made in mass spectrometric analyses is illustrated by these minority reports; small or trace amounts of such components may actually be present in the sample, or may be contributed by desorption of lubricant, glass, or metal within the spectrometer or sampler. Extremely careful evacuation, flushing, and equilibration of the spectrometer and sampler are always necessary if such difficulty is to be avoided. CALCULATED HEATING VALUE
The heating values calculated from all the analyses are plotted in Figure 9. The spectrometric values show no decisive Gaussian pattern. The mean of values from 529 to 566 is 550 * 7; the median is 551. The chemical plot also shows widely scattered values, but the approximately central portion is well shaped, with a high frequency at 545, and a median of 545. The mean of the chemical values is 545 =t 3.3. The measured heating value is 549. [This measurement was made by John Eiseman and Ralph Jessup of the National Bureau of Standards (9, p. 327).] Thus, it appears that the spectrometric approach may give the greater accuracy, and the chemical approach the greater reproducibility, although the matter is somewhat vague.
.... -
! I'
560
520
0 T U
Figure 9.
PER
CUBIC
FOOT
565
Calculated Heating Values
CALCULATED SPECIFIC GRAVITY
The calculated specific gravity is an over-all measure of accuracy, inasmuch as the specific gravity of this standard sample had been carefully determined to a significance at least an order of magnitude greater than the values calculated from the analyses. The known value was 0.6475 [measured by Carroll Creitz of the National Bureau of Standards; for the method used, see ( 1 ) and (3, p. 324j1. The calculated values obtained from both chemical and spectrometric analyses are somewhat scattered, although in both case5 the greater proportion lies between 0.640 and 0.660 (Figure 10). The high frequency for the chemical method appears at 0.648, the median at 0.650. The mean is 0.648 * 0.002. These values agree well with the known. The spectrometric mean (0.626 to 0.677) is 0.650' * 0.007. The median is 0.651, and the Gaussian pattern is too distorted to permit the clean selection of a high frequency. Thus, although greater accuracy may have been expected from the spectrometric method because it discloses more complete composition than does the chemical
I
I
MS
.. :. .. ......................................... i, : : ........ : ::::::..i ............ :.:::::::::::: ................. .................. ....... : :.: 1 . :. .: .. .:.:...................... I
.
I
............ .I
SPECIFIC
;
GRAVITY
Figure 10. Calculated Specific Gravity
approach, it has not been attained. Improvements in the determinations of hydrogen and carbon dioxide are needed to correct this situation. TABULATED DATA
KO attempt has been made to set forth all the statistical inferences that can be derived from the data given in this report. The whole object of the report is to present a general integrated picture of the real state of analysis of this kind without, for the moment, distracting particulars. This is not to say that a statistical study will not prove of equal or greater value, but it should be presented separately. To tempt someone to do this, the tabulated data have been given in two other papers ($, 6). Without invoking a formal statistical study, a few observations may be briefly made. In the instance of calculated specific gravity, which is a good over-all measure of the success of the analysis and involves all the components determined, these facts are evident in glancing at the spectrometric data: The differences between two calculators using the same spectrogram in the same laboratory were small. The average difference is about 0.001 The differences between two operators using the same instrument in the same laboratory were small. The average difference is about 0 001. The differences between spectrograms were larger than between operators and calculators. The average difference is about 0.0025. These observations agree with those made previously in the analysis of a natural gas by the mass spectrometer ( 5 ) .
0
.
Youden ( 7 ) remarks: “It is a fact of experience that a set of measurements made by different operators a t different times or in different localities is subject to greater variation than a set of measurements made by one operator using the same apparatus on the same day.” The data presented here offer no exception to this fact of experience. Thus, for example, the mean of all spectrometric values of calculated specific gravity (between 0.626 and 0.677) is 0.650 * 0.007; the mean of the laboratory averages is 0.648 * 0.009; but the reproducibility of each laboratory of 0.0017-three times better than the reproducibility obtained from the laboratory averages. REVIEW
Although the superiority of the mass spectrometer over volumetric methods was clearly demonstrated in the analysis of a natural gas (b), the same superiority is limited to the hydrocarbon fraction of the carbureted water gas discussed in this report. The full significance of this is not indicated by the data presented, for the sample itself contained few of the heavier saturated, unsaturated, and ring hydrocarbons that are ordinarily present in fuel gases of this type, and the spectrometer is capable of analyzing a mixture of 20 or more components that would prove a vexing and rather durable problem to one employing conventional chemical methods. As in the analysis of a natural gas (b), oxygen was correctly determined in the present sample by the spectrometer and not by the volumetric chemical method. However, the spectrometric method needs considerable improvement in the determination of hydrogen and particularly carbon monoxide. It may be that a combined physical-chemical analysis will offer the best solution for gases of this type. LITERATURE CITED
(1) Xatl. Bur. Standards, Misc. Pub. M177 (1947). (2) Shepherd, Martin, ANAL.CAEM.,19,635 (1947).
(3) Shepherd, Martin, J . Research Natl. BUT.Standards, 36, 313 (1946); R P 1704. (4) Ibid., 38, 19 (1947); R P 1759.
(5) Ibzd., p. 491; RP 1789. (6)Ibid., 44,509 (1950); RP 2098. (7) Youden, W. J., paper presented at 109th annual meeting, Am. Statistical Assoc., Dec. 27, 1949. RECEIVED February 3, 1950.
Coulometric Titrations with Chlorine Titration of Arsenic and Use of an Amperometric End Point PAUL S. FARRINGTON AND ERNEST H. SWIFT California Institute of Technology, Pasadena, Calif.
T
HE secondary type of coulometric process involves an intermediate half-cell reaction which is caused to take place with 1 0 0 ~ ocurrent efficiency a t a generating electrode. Szebelledy and Somogyi (6) and Meier, Myers, and Swift (2)have discussed the advantages of this type of process and the characteristics of suitable intermediate half-cell reactions; the latter discussion has particular reference to titrations with amperometric end points. Until the present work only three intermediate half-cell re-
actions had proved satisfactory for coulometric determinations. For the titration of reducing agents, bromine has been employed rather extensively (1, 3, 5, 7 , 8) and recently Ramsey, Farrington, and Swift (4)have shown that iodine can be used for the titration of arsenic. Cuprous copper has been used for the coulometric titration of certain oxidizing agents (5’). In order to increase the number of substances which could be titrated by such processes, a stronger oxidizing intermediate than bromine was sought. Because the halogen half-cells are
