ANALYTICAL EDITION
JULY 15, 1939
the separation of tin and antimony (6, 6) on the possible interference of other metals with tin and on the effect of phosphate in the subsequent analysis of the other elements present. Lastly, bronzes of higher tin content seem to settle so rapidly after nitric-phosphoric acid treatment that sufficient phosphoric oxide is not withdrawn from solution.
Summary A study of the tin-phosphorus precipitate in the analysis of bronze has shown that the amount of phosphorus present must equal in weight the tin present t o get a precipitate of constant stannic oxide-phosphorus pentoxide ratio. Whereas
371
phosphorus will remove all the tin from solution, the reverse is not true.
Literature Cited Testing Materials, B28-36T (1936). (2) Bornemann, Z. angew. Chem., 12, 635 (1899). (3) Friend, J. A. N., “Textbook of Inorganic Chemistry”, Vol. 5, p. 360, London, Griffon and Co., 1917. (4) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel”, p. 212, New York, John Wiley & Sons, 1931. (5) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. VII, p. 482, New York, Longmans, Green & Co., 1927. (6) Prescott, A. B., and Johnson, 0. C., “Qualitative Chemical Analysis”, New York, D. Van Nostrand Co., 1917. (1) Am. Soc.
A Photometric Method for the Determination of Carbon Dioxide RICHARD J. WINZLER’ AND J. PERCY BAUMBERGER, Stanford University, Calif.
I
X MANY chemical and biological studies it is convenient to make very frequent determination of the carbon dioxide content of gases. This paper describes a simple photometric method for the rapid determination of carbon dioxide by measuring the change in light transmission of a solution of pH indicator through which the gas is bubbled. Only one gas of known carbon dioxide content is reauired in order to be able to apply a simple equation to the determination of the carbon dioxide content of gases of unknown composition. Several workers-e. g., Parker (7) and Fegler and Modze(@-have used pH indicators to determine carbon ,dioxide. Most carbon dioxide methods make use of basic 1
Present address, Sterling Hall of Medicine, S e w Haven, Conn.
FIGURE 1. DIAGRAM OF APPARATUS
solutions and the consequent accumulation of carbonate in the solution. The present method depends, as did that of Higgins and Marriott (4), upon the equilibrium established between the indicator solution and the gas to be measured. The use of this method in closed and open circuit indirect calorimetry will be described in other papers.
Apparatus and Method Methyl red is the pH indicator used in this method. Its pKa is 5.1, being red on the acid and yellow on the alkaline side of this pH. A stock solution is prepared by neutralizing the acid indicator with sodium hydroxide, and dilutions are made from this stock solution to final concentrations ranging from 0.22 X to 1.5 X gram of methyl red per liter. To these are added a few drops of dilute sodium or potassium hydroxide to bring the excess alkali concentration to something in the neighborhood of to 10-3 molar. These dilutions are made up in liter flasks and calibrated as described below. They were found to remain unaltered over several months. Soft-glass cuvettes with depths ranging from 10 to 25 mm. are used in the apparatus. The arrangement of apparatus is shown in Figure 1. The photometric colorimeter consists of a wooden box with light-tight hinged lid. Light from a flashlight bulb, L, operated at 4 volts from a storage battery illuminates the entire sensitive area of a General Electric blocking layer photoelectric cell, A . Between the light source and the photoelectric cell are suitable light filters (Wratten No. 74 and Corning No. 397) and a rack bearing two cuvettes which may be moved into the light path. One cuvette, S , is a reference standard and contains methyl red in alkaline solution. The other, T , contains methyl red in very dilute bicarbonate solution and is equipped with a bubbling arrangement to permit passage of the unknown gas through it. The solutions are moved into position before the filters by means of the slider, H . The practice is to adjust the position of the standard cuvette by means of set screws, ss, so that the photoelectric current developed by the light passing through either of the two vessels is the same when the test solution is brought into equilibrium with gas containing no carbon dioxide. The ratio Io/I when no carbon dioxide is present, then, is unity.
INDUSTRIAL AND ENGINEERING CHEMISTRY
372
Gas Mixtures
3
In order to test the relation between Io/I and carbon dioxide content it was necessary to make many gas mixtures of known carbon dioxide tension. This was done by mixing air with gas from a cylinder containing 5 per cent carbon dioxide in nitrogen. These mixtures were usually stored in small cylinders which were evacuated beforehand and made up to a pressure of two atmospheres. The final carbon dioxide content of the mixture was determined indirectly by analyzing the oxygen content of the gas mixture by means of the dropping mercury electrode oxygen method to be described by Baumberger and Mueller. This method of oxygen determination permits of a very accurate and rapid determination of the oxygen per cent of a gas mixture. From the following equations and a knowledge of the oxygen contents of the beginning and final gases, the carbon dioxide content of the final mixture can readily be calculated.
-lo 2
-.8
-.6 -.4
1
T
T
1 0 s y
1‘1 0
0
VOL. 11. NO. 7
I
-coo,-
L
I
I
I
2
3 +.2
4
5
+.4
+, 6
o
t.8
Then ‘v”
MEASUREMENT OF CARBON DIOXIDE FIGURE 2 . PHOTOMETRIC
*
0 . Io/I plotted against percentage of carbon dioxide 0. log CO? plotted against log
Both lines come from data where mixtures of known carbon dioxide content were bubbled through a methyl red solution in which pH and Io/I were observed. Log l-a was calculated from pH.
The photoelectric current is measured by observing the deflection of a d’Arsonval galvanometer (Leeds & Northrup, type 2420-C) which has a sensitivity of 1.8 X 10-8 ampere, a resistance of 913 ohms, a critical damping resistance of 18,000 ohms, and a period of 3.2 seconds. A light lever of about 3 meters is employed and the maximum deflection with the standard solution is about 1000 mm. The following precautions are taken: The galvanometer deflection is kept within the range where the deflection is directly proportional to the current. Large temperature changes are avoided. The photoelectric cell is constantly illuminated. A low-resistance galvanometer is used, so that the photoelectric current is linear with the light intensity. Very high concentrations of methyl red are not used because its solubility is less in acid than in alkaline solutions. The pH is kept from falling below 5.1, the pKa of methyl red. I n making the measurements the flow of gas through the test solution must be momentarily stopped, and in this sense the method is not a continuous one. However, this stoppage is only long enough to permit the gas bubbles to rise to the surface, and the observation is actually made in a very short time. Therefore, the authors feel justified in calling the method a continuous one. The solution serves for repeated analyses over many hours. The length of time required for the gas to come into equilibrium with the test solutions depends upon the efficacy of the bubbling arrangement. The slow reaction of carbonic acid to carbon dioxide is also involved. Three minutes’ bubbling with a single stream of bubbles from a fine capillary will bring 10 cc. of solution into equilibrium with a gas of carbon dioxide content which is widely different from the original. Any acid- or alkali-forming gas-e. g., sulfur dioxide or ammonia-will interfere with the determination. Such cotnpounds may, however, be removed by suitable absorbing agents before the gas reaches the methyl red solution.
=
VOZ
v + Ti’
= 100
+ V’02’ = V”0,”
(1) (2)
Subtracting 1 from 2
(4)
co2
=
V’C02‘
+ v”C0,” V
(5)
Equations 3, 4, and 5 are readily solved to give the carbon dioxide tension of the final gas mixture. The indirect determinations of carbon dioxide by the oxygen dilution method, described above, were checked by means of the Fenn (3) barium hydroxide electrical conductivity method for carbon dioxide and were found to give good agreement. The oxygen dilution method has the advantages of speed and simplicity over the Fenn method. Often it was convenient to make the gas mixture up quickly in a rubber bag by inflating the bag with compressed air and then adding an amount estimated by the eye from the tank of 5 per cent carbon dioxide in nitrogen. The carbon dioxide content of the gas in the bag was determined by rapidly analyzing for oxygen and making calculations as above. The whole process could be carried out in 5 to 10 minutes. The authors have also allowed the gas to flow continuously from a compressed air system and from the 5 per cent carbon dioxide in nitrogen tank through a Y-tube, through the dropping mercury electrode where the oxygen determination is made, and through the test solution where the ratio Io/I is observed. By properly adjusting the flows a mixture of any carbon dioxide content up to 5 per cent can readily be obtained. This is the more convenient method when a series of mixtures is desired.
Theoretical Basis of the Method CARBONATE EQUILIBRIUM. The method described depends on the carbonate equilibria, which according to Byck (1) are as follows:
ANALYTICAL EDITION
JULY 15, 1939
373
where k = extinction coefficient d = depth of cuvette 1 - 01 = red form of methyl red This equation applies rigidly only when monochromatic light is used. I n this method a rather narrow range of wave length is obtained by passing the light through a Corning No. 397 and a Wratten No. 74 filter. Figure 3 shows the region of maximum absorption of both the yellow and the red form of methyl red, and also the absorption of the two filters. It is clear from the figure that the requirement of the Beer-Lambert law is fairly well met. This equation was checked and found to hold by a dilution technique and also by calculating 1- a from the pH measured by the glass electrode.
[EInC03] = ncq nc = Bunsen coefficient = 0.0393
atmosphere For pressures of carbon dioxide over 1 X several of these constants may be neglected, and the equation becomes
100
80 These equations apply only to pure water. When the salt of an indicator is present, more and more base is made available for bicarbonate formation as the tension of carbon dioxide increases. The situation is comparable to the twoacid problem of Michaelis (6) which he states is difficult to solve. The pH, observed with the glass electrode, of conductivity water in Pyrex glass in equilibrium with various carbon dioxide mixtures agrees well with the pH calculated by Equation 13. Since some base is always present-e. g., from the indicator, impure distilled water, or alkali of soft glass-it is convenient to use as simple a system as possible to avoid laborious calculations. This is done by adding enough base so that the total HC0,- concentration is high enough to make negligible both the additional HCOa- from carbonic acid dissociation and the base freed through methyl red association. The buffer equation, 13, satisfies our needs, provided free bicarbonate is a t least ten times as large as the two factors mentioned above. pH
-log k i - log[HzCOs]
+ log HCOs-
= k
['io -
11
Z
kd (1 - or)
a > 0.45 the hyperbola, when plotted against the logarithm, yields a straight line, Therefore, it is necessary to work with carbon dioxide tensions low enough so that the p H will not fall below 5.1, the pKa of methyl red, in order that 1-a never exceeds 50 per cent of the total concentration of methyl red. For higher tensions of carbon dioxide more free base may be added.
dpH X Con
Sensitivity = 3= CO, 0.4343 X COa Sensitivity = 0.0210.4343 = 2.3% The error of the photometric method may be determined by calculating the effect on the ratio Io/Iresulting from the inaccuracy of the measurement of these two factors. The error in the ratio resulting from an error in reading the value can be calculated by Equation 19.
The authors have made calculations of error in the case where I o equals 1000 and I is measured to within *1, as in these experiments. These calculations are given in Table I and show that the error resulting from an error in measurement of I is a t a minimum when I is one half of IO. The absolute error in measuring carbon dioxide resulting from this
VOL. 11, NO. 7
1
-O
I
=
error in COz
(20)
Dividing Equation 20 by Equation 14 and multiplying by 100 we obtain the error in terms of percentage of carbon dioxide present. This error holds for all concentrations of carbon dioxide and dye for any depth of cuvette. It is also the percentage error of the ratio, and is therefore generally applicable to all cases of photometric measurement, provided I o equals 1000 times the smallest significant scale division. When the accuracy of measuring I and Io is changed, the error in percentage of ratio changes.
8
&
I-d 4
4
2
lot42 4 6 8 BETWEEN COz AND I o / I FIGURE4. LINEARITY 1. !L2 plotted against a 0 . lo'-" plotted against a 1 - a 0 . 7 plotted against lo'-".
IO
This curve shows that in the
interval 1 > a > 0.45 a straight line results from plotting the hyperbola against the logarithm. This accounts for the convenient linear relationship between COSand I Q / I .
Table I shows that the error is fairly constant for a wide range of ratios and has a minimal value of *0.4 per cent when Io equals 1000 and I equals 500. Compared with the pH method, the photometric method is about six times as accurate with the authors' setup.
Summary
A method is described which permits the continuous photometric determination of carbon dioxide. A method of making gas mixtures of known composition is described. A mathematical relationship is derived that simplifies the determination of the concentration of acids by direct measurement of Io/I. A method of calculating the reliability of photometric measurements is developed and applied to the case of carbon dioxide. Literature Cited (1) Byck, H. T., Science, 75, 224 (1932). (2) Fegler, J., and Modzelewski, T., Compt. rend. soc. biol., 116, 244, 248 (1934). (3) Fenn, W. O., Am. J . Physiol., 84, 110 (1928). (4) Higgins, H. L., and Marriott, W. M., J . Am. Chem. SOC.,39, 68 (1917 ) .
JULY 15, 1939
ANALYTICAL EDITION
( 5 ) Kauko, Y . ,Angew. Chem., 47, 164 (1934); 48, 539 (1935); Acta Chem. Fennica, 5B,54 (1932). (6) Michaelis, L., "Die Wasserstoffionen-Konzentration", 2nd ed., p. 40, Berlin, Julius Springer, 1922. (7) Parker, G . H., J . Gen. Physiol., 7, 641 (1925). (8) Thiel, A,, Dassler, H., and Wulfken, F., Fortschr. Chem. Physik physik. Chem., 18, 1 (1924).
375
(9) Wilson, F. W., Orcutt, F. S., and Peterson, W. H., IND.ENO. CEBM.,Anal. Ed., 4, 357 (1932). CONTRIBUTION from the Physiology Laboratory, Stanford University. Work supported in part b y a grant from the Rockefeller Foundation and by the National Youth Administration.
Volumetric Oxidation of Iodide to Iodate by
Sodium Chlorite L. F. YNTEMA AND THOMAS FLEMING, St. Louis University, St. Louis, Mo.
T
H E iodide ion may be oxidized to iodate by a chlorite when the iodide solution is buffered with sodium acetate and acetic acid ( 1 ) . The potential of the couple HCIOz
+ 3H+ + 4e = C1- + 2H20
is given by Latimer (4) as 1.56 volts, and that of the couple IOs-
+ 6H+ + 6e = I- + 3Hz0
as 1.085 volts. There is enough difference in the EOvalues so that a quantitative oxidation of iodide to iodate by chlorous acid may be expected. This investigation was undertaken to ascertain under what conditions the reaction may be use3 for a quantitative volumetric determination of the iodide ion and to examine some of its limitations. Obviously, other oxidizing and reducing agents would interfere. According to the equation 3HC102 21- = 21033C13H+
+
+
+
it is apparent that six equivalents of chlorous acid, measured as an oxidizing agent, will be required to oxidize one mole of iodide. Accordingly, the ratio of equivalents of sodium
chlorite to moles of potassium iodide should be 6 to 1, if the method is valid. There are few references in the literature to the use of sodium chlorite as a volumetric reagent. Levi and Ghiron (6) employed it for the volumetric determination of permanganate, and Jackson and Parsons (3) developed a method in which it is used in the estimation of sulfite.
Reagents The sodium chlorite was obtained from The Mathieson Alkali Works, Inc., by whose analysis it consisted of 98.5 per cent sodium chlorite, with traces of sodium chlorate and sodium hydroxide. All other reagents used were A. R. quality. Distilled water for preparation of solutions was freshly boiled. The sodium chlorite solutions, about 0.13 N , were prepared and standardized essentially by the method described by Jackson and Parsons (5). Their observations and those of Levi and Natta (6) concerning the stability of the standardized chlorite solutions were confirmed in this investigation. The potassium iodide solutions, about 0.02 M , were standardized by precipitating and weighing the iodide as silver iodide.
Experimental
Two buffer systems, acetic acid-sodium acetate and monosodium orthophosphatedisodium orthophosphate, were investigated in detail (Tables TABLEI. ACETATEBUFFERSOLUTIONS I and 11). The pH measurements were made with a comRatio of M1 Moles of Equiv. of NaClOz Equiv. of 2 M HCzHsO; H ~ O , KI x 103 Ratio, Moles of XI Average Ratio NaClOz x mercial glass electrode aspH 108 Amber Blue Amber Blue Amber Blue 2 M NaCrHaOz M1. sembly. All titrations were 0.5 carried out a t room tempera9.5 25 .. 4.70 0.784 0.786 6.00 ( 8 ) 5 5 . 9 7 (8) 6.00 5.99 ture. A measured volume of 0 . 5 __ the standardized sodium chlo,. 3.69 0.618 0,620 5.98 5.96 5,98'(5) 5 . 9 6 (5) 9.5 50 rite, about 30 ml., was run from 0.5 5.80 3.71 0.620 0.622 5 . 9 8 (5) 5 9 5 (5)b 100 5.98 5.96 9.5 a buret into a flask containing measured volumes of buffer "5 5.95 (2), 5 . 9 0 (2)b)C .. 3.66 0.616 0,620 9.5 200 5.94 5.89 solutions and water. Starch I solution was added and the .. 3.69 0,614 0.617 9 25 6.00 5.99 6 . 0 0 (4) 5 . 9 9 (4) standardized potassium iodide 1 added from a buret. The blue .. 3.71 0.618 0.620 5 . 9 9 (8) 5 , 9 7 (8) 5.99 5.97 9 50 color of the iodine-iodide-starch 1 __ complex appeared first, and 9 100 5.37 4.48 0.746 0,747 6.00 5.99 6 . 0 0 (10) 5.98 (IO) then disappeared as the iodine -.I 9 200 ,. 4.25 0.708 0.714 6.01 5.96 6 . 0 0 (3) 5 . 9 6 (3) was oxidized to iodate. It was 2 found that, upon the first ad__ 8 25 .. .. ... ... .. .. .... ..... dition of iodide, there was an 2 appreciable interval of time be.. 3.68 0.603 0.611 6.11 6.03 8 50 6 . 0 8 (2) 6.02 (2)c)e fore any reaction took place. 2 __ However, when the first few 5.02 4.25 0.697 0.708 6.10 6.01 S 100 6 . 1 1 (2) 6 . 0 4 (2)C milliliters of iodide had been 2 oxidized, the reaction pro8 200 *. .. ... .. .... .... d ceeded rapidly, provided the a Number of experiments on whioh averages are based indicated b y digit in parenthesis. 1, Reaotion slow. End point not sharp. d No end point. e C1Oz evolved. acidity was high enough. According to Bray (1)the reaction I
...
..
