errors and also regular positive H2S06-errors were found. To explain this experience we assumed that the errors arise from several sources : The rate of reaction between hydrogen peroxide and arsenious acid is rather low. According to Woods, Kolthoff, and Meehan (7) the second order rate coefficient is 1.O X 1. mole-lsec-l at 25 "C. In good agreement with this figure, an 8x conversion of hydrogen peroxide was observed at experimental conditions prescribed by Mariano (10-4M hydrogen peroxide, 1.6 X 10-2M arsenious acid, and 0.4M sulfuric acid). Previously it was pointed out (8) that arsenious acid reacts with peroxymonosulfuric acid fairly quickly. But the reaction is not quantitative within 5 min even at higher concentrations (0.1N) when the acidity is low. Consequently, in a very dilute solution the removal of peroxyrnonosulfuric acid will not be quantitative during a 10-min waiting time. On the other hand it is known (9-12) that at similar conditions a fast reaction takes place between cerium(1V) ions and peroxymonosulfuric acid resulting in the nearly quantitative disappearance of Caro's acid, while a part of cerium(1V) is also reduced. Through the disappearance of cerium(IV), this reaction may result in the reduction of the negative H202error. The reaction between arsenious acid and cerium(1V) is very slow in the absence of catalysts. In the presence of active impurities, however, this reaction could also become the source of positive HzOz-error. Concerning the negative H&08-error, it should be mentioned that according to Woods, Kolthoff, and Meehan (13) (7) R. Woods, I. M. Kolthoff, and E. I. Meehan, J. Amer. Chem. Soc., 86, 1698 (1964). (8) . , L. J. CsBnyi and F. Solymosi, Acta Chim. Hung. Acad. Sci., 17, 69 (1958). (9) L. J. CsBnyi and F. Solymosi, Acta Chim. Hung. Acad. Sci., 13. 19 (1957). (10) Ibid., 15, 501 (1956). (11) L. J. CsBnvi. F. Solymosi, and F. Sziics, Naturwiss., 46, 353 (1959). (12) L. J. Csdnyi and L. Domonokos, Acta Chim. Hung. Acad. Sci., 34, 383 (1962). (13) R. Woods, I. M. Kolthoff, and E. I. Meehan, J. Amer. Chem. SOC.,85, 2385 (1963). ~~
I
Sir: I was exceedingly surprised at the data plotted in Table I1 of Dr. Csanyi's paper and the interpretation which followed. In connection with his remarks on my work ( I ) , I would like to call your attention to the following points. 1. From the data given in such table it is inferred that, together with the [Hz02]and the [H2S208]values, those corresponding to the total oxidizing power of the solutions present, (1) M. H. Mariano, ANAL.CHEM., 40, 1662 (1968).
the reduction of peroxydisulfate by arsenious acid is induced by the reaction between iron(1I) and peroxydisulfate. In the presence of iron(II1) and copper(I1) the eq. of As(II1) oxidized can approach ineq. of Fe(II1) oxidized) finity, i.e, peroxydisulfate will be reduced by arsenic(II1) instead of iron(I1). Since the induced reduction of peroxydisulfate depends on the concentration ratio of [As(III)]/ [Fe(II)] and further, on the concentration of ferric ions, the magnitude of HzSz08-errorwill also depend on the concentration of hydrogen peroxide present as well as on the ferric content of the ferrous sulfate reagent. At low hydrogen peroxide concentrations more cerium(1V) ions remain in the solution after the oxidation; therefore more ferric ion will be formed in the reaction of ceric ions with ferrous sulfate. Consequently, in such cases a greater H2S208aTOr occurs. As according to Mariano's method, the peroxymonosulfate concentration is obtained by difference between the total oxidizing capacity and the sum of [H202] [HzSd&], it is understandable that the peroxymonosulfuric acid concentration can be obtained only with positive error. Disregarding the discrepancies observed, that may arise partly from the slight difference of reagents used, we are of the opinion that the first method is preferable, especially at kinetic runs, because the titanium sulfate is not only a reagent for the determination of hydrogen peroxide, but also a means for quenching the reaction. At the latter procedure, however, such a facility is not offered.
)
+
ACKNOWLEDGMENT
My gratitude is due to Mrs. M. Palotai for her technical assistance and to Laporte Chemicals Ltd., Luton, England, for providing the KHSOs preparation.
L. J. CSANYI Institute of Inorganic and Analytical Chemistry A. Jdzsef University Szeged, Hungary RECEIVED for review May 27, 1969. Accepted November 10, 1969.
also, regular negative errors (there is only one honorable exception, that of the second run where, apparently, an exact determination was performed). If we compare the well behaved T.O.P. values of Table I with those of Table I1 (obtained by exactly the same procedure), we arrive at the obvious conclusion that, in the latter case, between the moment of the preparation of the stock solutions and their measurement, some of the reactants have undergone a decomposition. Now, if the HzOzstock solution was not kept in a sufficiently
Table I. Values Obtained in Simultaneous Spectrophotometric Determinations of H202,H2S208,and H2S05
682
43.1
105.9
82.3
41.7
101.3
90.1
10.5 51.4 94.8
14.4 55.5 75.0
81.7 46.0 20.5
43.9 42.7 43.2 43.6 10.9 51.2 92.0
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
106.1 104.2 98.1 101.6 13.9 56.2 76.3
82.3 84.7 91.2 86.6 82.1 44.5 21.5
+1.9 -0.9 +3.5 +4.5 +3.8 -0.4 -2.9
+0.2 -1.6 -3.2 $0.3 -3.5 +1.3 +1.7
0.0 +2.9 11.2 -3.9 +0.4 -3.3 +4.8
acidic medium, it will have decomposed; moreover that perhydrol Merck is not free from organic and other impurities. On the other hand, peroxydisulfate hydrolyzes quite rapidlyin 0.4M H2S04, about 8 % of the peracid is decomposed, giving a corresponding amount of H2SOs, in a time interval of 24 hours. That something of this kind has happened with Dr. Csanyi’s solutions is inferred from the fact that, quite characteristically, for a given amount of H2S208, the “positive errors” in the H2SO6determinations are greater, the less the “initial” amount of HzS05 (compare runs 1 and 2 with runs 7, 12, and 13). For comparison, we give in Table I some of our determinations. 2. I wonder if the results obtained by Woods, Kolthoff, and Meehan (2) in studying the rate of reaction between H202 and arsenic(III), in a perchloric acid medium, can be extrapolated to H2S04 solutions. In particular, the possibility of the formation of the sulfate free radical through the reaction OH -/- HS04- + sodH20 [koH+H804-= 2 x 10’ 1. mole-l.sec-l (.?)I suggests the possibility of a quite different kinetics. In fact, the results plotted in Table I1 favor the hypothesis of a lower reaction rate in a H2S04medium. The finding of a 8 Z H202 transformation, pointed out by Csanyi, cannot also be extrapolated to our case since he uses perhydrol Merck, which contains some impurities, whereas we utilize very pure H202 solutions, obtained by radiolysis. 3. Against the assumption that a 10-min interval is not enough for the complete reduction of dilute Caro’s acid, by arsenite, we have found that under our experimental conditions all HzSOs is removed in about 3 to 4 minutes (see Table 111). 4. The interpretation given by Csanyi for the systematic negative error he claims to have found in the H&Os determinations, does not hold in our case. In fact, quoting Woods et al, ( 4 ) : “The induced oxidation of arsenic trioxide by the persulfate-iron(I1) couple, in the presence of an excess of iron(ZZZ) can be explained. . . according to the reaction mechanism
+
+ S20s2- Fe(II1) + sod2-+ SodAs(II1) + SOaAs(1V) + sod2As(IV) + Fe(II1) As(V) + Fez+
Fe2+
-+
-+
+
(1) (3) (4)
Iron(I1) and arsenic(II1) compete in reacting with the sulfate free radical Fe2+
+ SO4-
-+
Fe(II1)
+ SO4!+
(2)
In the absence of iron(III), the As(IV) produced is postulated to react with iron(I1) to re-form As(II1) As(IV)
+ Fe2++ As(II1) + Fe(II1)
(5)
.......... k 4 / k 5 = 1.4 and 2.3, respectively, in 0.01M and 0.005M perchloric acid-this ratio increases as the hydrogen ion concentration is decreased. . . e . . . . . . .
The induced factor, I.F.
=
equiv. As(II1) oxidized has a equiv. Fe(I1) oxidized
(2) R. Woods, I. M. Kolthoff, and E. J. Meehan, J. Amer. Chem. Soc., 85, 3334 (1963). (3) T. J. Hardwick, J. Phys. Chem., 66,2246 (1962). (4) R. Woods, I. M. Kolthoff, and E. J. Meehan,-J. Amer. Chem. SOC.,85, 2385 (1963).
Table 11. Effect of Arsenite on H201Solutions Sol. A. 10 cc (H202,2.1W5M) 1 cc (As3+) 2 cc (Ce4+)). Blank. 10 cc (HzS04,0.4M) 1 cc (As3+) 2 cc (Ce4+). The solutions were put into the spectrophotometer and the absorbances measured at given time intervals. Sol. B. 5 cc of As3+sol. were added to 50 cc of a 10-4M,H202 sol. At given time intervals this solution was tested for the HZOZ content, by taking 2-cc aliquots and adding 2 cc of ceric sulfate. The blank was treated similarly.
+
+
+
A A f (min.) 0 15
B
Abs. 0.171 0.169 0.170 0.170 0.170 0.170
20 25 30 35
+
Abs. 0.518
A f (rnin.) 0 2
0.515
4 7
0.505
0.510
0.502
10 13
0.500
19 0.499 Measured [H2O2]after 10 min of contact with arsenite: A, 1.98 X 1 0 - 5 ~ . B, 9.90 x I O - 6 ~ . Table 111. Time of Reduction of Caro’s Acid by Arsenite 1 cc of As3+sol. was added to 10 cc of H2S06 solutions. After
given time intervals, 2 cc of Fez+sol. were added and the absorbances measured relative to a blank without H2SO5. B(H2SO6, 5.10-5M) C(HzS05, 1W4M) A(HzS05, 10-5M) At At A f (min.) Abs. (min.) Abs. (rnin.) Abs. ~
~
1.5 3 5
10
0.000 0.000
I 3 5
0.081 0.000
0.000
7
0.008
0.022
3 5
0.000
0.001
7
0.001
0.000
10 15
0.000
0.001
value of 1.5 for solutions 0.50M in HClOa, 4.56.10-5M in Fez+and 5.10-4Min Fe3+,increasing to 4.6 when Fe3+attains 10-3M for the same Fe2-kconcentration’’-that is, the induced factor has but a value of 4.6 when [Fe3+][Fez+]1: 22. According to the mechanism given above it is easy to see that the reduction of perdisulfate would be induced by arsenite when, and only when, the number of Fez+ions initially present in the solution was insufficient for the complete reduction of S 2 0 ~ 2and - an excess of iron(II1) was present. In such case, the re-formation of Fez+ by Reaction 4 would determine a further reduction of perdisulfate. This is a set of conditions completely different from those encountered in my work since : (a). The initial concentrations of the reactants are: [HzS208] Q 10-4M; [Fez+] = 10-lM; (b). The iron (11) sulfate used (AR grade from Merck) contains but 0.002% of iron(II1) and 0.005 of copper(I1). Thus, even taking into account the ferric ions formed by reaction with excess of Ce(IV) present, they will never be in conditions to compete with Fez+ for reaction with As(1V) since k 4 / k 5< 1.4. (c). As for the competition represented by Reactions 3 and 2, the final result would be the same either by reaction of As(1V) with SO4-, according to Reaction 3 followed by Reaction 5 or by the direct reaction of SO4- with Fez+-+ each case there will be 2 equivalents of Fez+ oxidized per equivalent of perdisulfate decomposed. 5 . I agree with Dr. Csanyi in that the discrepancies observed may arise from differences in the reagents used. My technique was primarily intended for the use of radiation ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
683
chemists who utilize reagents of the greatest purity. However, the analytical procedure I described previously (1) can be used by any analytical chemist provided that A.R. grade chemicals are used. This technique presents, over that of Csanyi, the following advantages: it is simpler, it is faster, it uses only very common and cheap laboratory reagents and enables the analysis of solutions with very low concentrations of any of the oxidants present.
M.H.MARIAN0 Junta de Energia Nuclear DirecGBoGeral de Combust~veise Reactores Nucleares Industriais Av. da Repdblica 45-50 Lisboa, Portugal RECEIVED for review October 22, 1969. Accepted November 10, 1969.
I AIDS FOR ANALYTICAL CHEMISTS ~~
~~~
~~~~
Correct Procedures for Calibration and Use of Rotameter-Type Gas Flow Measuring Devices Claude Veillon and John Y. Park Department of Chemistry, Unicersity of Houston, Houston, Texas 77004
ROTAMETER-TYPE GAS FLOW meters are commonly used in flame emission, atomic absorption, and atomic fluorescence spectrometry to monitor the flow rates of the various gases used in the flame and sample introduction system. A typical flow meter is shown in Figure 1. This is the configuration in which these units are usually supplied and used by most workers, with the needle valve on the upstream side of the gas supply, Frequently the units are calibrated by connecting the outlet to a wet test meter, and then used by connecting the outlet to the burner. However, the pressure in the measuring tube is not the same in each case, rendering the calibration worthless. If the pressure in the measuring tube varies, such as might occur if the downstream restrictions change (different burner, clogging, etc.), the inlet pressure changes or the needle valve adjustment is changed to alter the gas flow rate, the calibration of the tube is no longer valid and one does not know the actual flow rate of the gas. In many cases, one does not know any longer even the relative gas flow rates. Changes in gas pressure within the measuring tube change the gas density and, consequently, alter the reading for a particular gas flow rate. One can easily demonstrate this by connecting two identical units in series. The downstream rotameter will have a slightly higher reading because of lower pressure in it, as compared to the upstream rotameter. While this might at first appear to be an obvious conclusion, it must be pointed out that most gas flow control systems in use today on flame spectrometers are incorrectly set up. N o means of providing constant pressure in the measuring tube are provided. As pointed out earlier, most flow meters are supplied and used in the configuration shown in Figure 1. The correct configuration is shown in Figure 2. Here, the needle valve used for flow control is placed on the downstream side of the rotameter. If gas is supplied to the rotameter inlet from a good two-stage regulator, then the pressure (and, hence, the gas density) within the measuring tube will be constant, regardless of the gas flow rate. Fortunately, most flow meters of this type can be inverted by inverting the measuring tube within the unit. This places the gas flow control valve on the downstream side of the measuring tube, as in Figure 2. Using this configuration, a Matheson No. 620 PBV flowmeter with Model 603 tube, No. R-2-15-B, was calibrated for air, argon, oxygen, acetylene, and hydrogen at various pressures. 684
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
Figure 1. Typical gas flow meter, as supplied by manufacturer and used by most researchers
G A S OUT
a
UEEDLE VALVE
Figure 2. Inverter gas flow meter. Inlet pressure constant. Flow controlled by valve on downstream side of measuring tube
Inlet pressures (hence, pressure within the measuring tube) were controlled by a VTS 400-B two-stage regulator (Victor Equipment Co., Denton, Tex.) in each case and the gas flow rate was measured using a precision wet test meter (Precision Scientific Co., Chicago, Ill.), Calibration data obtained for air at three pressures are shown in Figure 3. The rotameter used contained two spherical floats, one of glass and one of stainless steel, to increase the range of flow measurement. The data shown in Figure 3 are for the glass float. One can readily see the significant differences in the flow rates as the pressure is changed.
