Conductometric Titrations with Organic Reagents Determination of Copper with Cupferron and Iron with Sulfosalicylic A c i d J. F. CORWIN'
AND
H. V. MOYER, Chemistry Department, O h i o State University, Columbus, O h i o
F,
The use of certain organic analytical reagents in conductometric titrations has been studied and cupferron found suitable for titrating copper in a sulfuric acid solution. The method determines copper rapidly in brass without removing tin and lead. Sulfosalicylic acid and its ammonium salt were used in the volumetric determination of ' iron. These compounds illustrate the use of organic reagents in titrations in which the change in conductivity i s due to the formation of a soluble metallo-organic complex:
w a b obtained by multiplying the slide-wire ratio,
M
Some increase in precision'was attained by calculating the end point according to the method described below.
OST of the analytical methods involving organic reagelit are either colorimetric or gravimetric. I n this work a study has been made of the use of organic reagents in conductometric titrations. Van Suchtelen and Itano (21) in 1914 titrated nitrate with nitron, calcium n-ith oxalic acid, and a number of organic acids with barium acetate. Kolthoff and others (6-8, 10, 16) have used the amperometric titration method as a means of utilizing organic reagents in volumetric analysis. Cupferron (ammonium nitrosophenylhydroxylamine) has been found satisfactory for the conductometric titration of copper. When a standard solution of cupferron is added to an acid solution of copper the very insoluble precipitate of coppercupferron is formed and ammonium ions replace the copper ions in the solution. The reaction is
+ 2CsHaN(NO)OiYHc
CU++
=
+
CU[CP,H~K((NO)O]~ 2XH4+
The release of the fast-moving ammonium ions causes a rapid increase in conductivity before the equivalence point is reached. After the end point, the combination of hydrogen ions with the excess cupferron causes a &arp decrease in conductivity. Typical titration curves are shown in Figure 1. Copper in brass can be titrated with a fair degree of accuracy without the removal of tin, provided the percentage of tin does not exceed 5%. Lead, zinc, nickel, and cobalt do not interfere but iron is precipitated with copper and causes high results. The solution of cupferron is stable for a t least 3 days if kept in a dark bottle. If allowed to stand for longer periods an appreciable quantity of nitrobenzene is formed which can be removed by pouring the solution through a filter paper. Germuth (4) has pointed out that the addition of 0.3 to 0.4 gram of p-acetophenetide to each liter of cupfeiron solution aids in its preservation.
by the ratio of the final volume of the solution over the original volume. The calculation of t.he actual conductance was eliminated by 1000- a , as the relative conductiviby. using the slide-wire ratio, Six readings, 3 before and 3 aft'er the end point, were plotted against the number of milliliters of titrant' used, and straight lines were drawn through the points bcfore and after the equivalence point. The intersection of these two lines represents the equivalence point on the volume coordinate. x
-
END POINT BY CALCULATION
The inconvenience of determining the end point graphically can be avoided by a calculation similar to that of Boulad (9).
+
The equation for a straight line, y = ax b, is used with the number of milliliters of titrant represented by x and the corre1000 - a sponding slide-wire ratio, , represented by y. The slope, a, is obtained by picking two sets of readings before the equivalence point, which are far enough from the region of curvature t o assure a straight-line relationship. The value of a is then used to calculate the constant, b, for each reading. An aveonge of the b values is used with the value of a to make an equation for the line before the equivalence point. Similar procedure with readings after the end point results in another equation. Simultaneous solution of these two equations for x gives the volume of titrant a t the equivalence point. TITRATION OF COPPER WITH CUPFERRON
SOLUTIOKS. Cupferron solutions were prepared by dissolving 30 grams of reagent quality salt in 500 ml. of water. The solution was kept in a dark bottle and its strength was checked against a standard copper sulfate solution about every 3 days. Standard copper solutions were made by dissolving accurately weighed samples (about 4 grams) of electrolytic copper foil in nitric acid. After the addition of 4 ml. of concentrated sulfuric
APPARATUS
The apparatus consisted of an alternating current Wheat'slone bridge arrangement with an electrically driven tuning fork and a Kohlrausch slide-wire. A Knight 7-watt amplifier, modified so that crystal earphones could be used, gives sufficient sensitivity of sound balance. The bridge is connected to the primary of a T-58A37 Thordarson transformer and the secondary of the transformer is connected to the phonograph plug in the amplifier. Apparatus similar to this is used for student conductivity experiments. A more precise Leeds & Sorthrup conductivity measuring apparatus built around the Campbell-Shackelton shielded ratio box was used in checking the precision of the method, and a Coleman pH meter was used in following the change in hydrogenion concentrations in certain reactions. The conductivity cell which easily accommodated 200 ml. was large enough so that no correction for dilution was necessary unless more than 10 ml. of titrant was used. Where it was necessary to add more than 10 ml., the correct'ed conductivity 1
0
1
2
9
4 5 6 7 8 9 1 0 1 1 1 MILLILITERS O F CLJPFERRON
2
Figure I. Cupferron vs. Bureau of Standards Bronze 5Pa
Present address, Antiooh College, Yellow Springs, Ohio.
302
ANALYTICAL EDITION
May, 1946
1 I
3.51
1
,
,
,
0
1
2
3
,
4
,
5
6
,
,
7
8 9 OF
MILLILITERS
Figure 2.
,
,
,
,
,
,
1 0 1 1 I 2 CUPFERRON
/
I
3
Cupferron vs. Bureau of Standards Bronze, Showing p H Change
acid the nitric acid was removed by evaporation to fumes, and the solution was diluted to 1000 ml. PROCEDURE. Solutions of brass and bronze were prepared by dissolving weighed samples of William S. Murray brass samples and a bronze Figure 3. from the National Bureau of Standards. No. 52a. in nitric acid and proceeding in the same manner as with the standard copper solution. If the tin content of the brass or bronze was below 5y0,no constituent or precipitate was removed. The bronze solution of Bureau of Standards 52a, which contains 14% tin, was filtered after the sample had dissolved in nitric acid. Other constituents of the brass or bronze were not removed. A 50-ml. portion of the standard copper solution or of the brass or bronze was transferred by pipet to the titration cell and sufficient water was added to make approximately 200 ml. The cell was placed in a constant-temperature bath and the conductivity was measured. The standard cupferron solution was added from a microburet in about 2-ml. portions and the conductivity was measured after each addition. As the reaction between cupferron and copper is practically instantaneous, very little time is required for a titration.
303
good curve after the end point and not so much that the conductivity became too high for measurement. The 4 ml. of concentrated sulfuric acid used in the preparation of the ,Sample, with the subsequent dilutions, made a satisfactory arid concentration. In accuracl- the rcsult,s shoTv that, the largest deviation is 0.8 mg. of copper per 100 nig. of copper taken, and t,hat the average deviation is 0.3 mg. per 100 mg. of copper. An attempt to analyze a bronze that contained iron, nickel, and antimony gave consistently Sulfosalicylic A c i d vs. Iron and Iron high results. Ore The conditions of the analysis allow it to be fitted into the standard gravimetric procedure for brass SULFOSALICYLIC ACID AND ITS AMONIUM SALT AS REAGENTS FOR IRON
Sulfosalicylic acid and its ammonium salt were found to produce usable curves in the titration of ferric iron (Figures 3 and 4). This acid has been used for separating iron from elements such as aluminum, manganese, magnesium, and others (12-16, 17') and in colorimetric determination of iron (1, Z?, 6,9, 11, 18, 19, $0).
I
h
i
Typical titration data, obtained by calculation, are plotted in Figure 1. The pH change a t the end point is indicated in Figure 2. The appearance of this curve suggests that accurate pH measurements to three or four significant figures might be nsed to follow this reaction. DISCUSSION
The time required for a titration was about It? minutes and the calculation required about 5 minutes with a calculator, less time than the plotting of curves which would give the same accuracy. The acid concentration is not a factor in the reaction; it was regulated to the extent that there was enough present to give a
Table No.
Samples Taken
33
2
35 36
2
1
I.
William S. Murray Samples Cupferron Cu Titer of
Cu Taken Gram 0.1450 0.1452 0.1245 0.1415 0.1416
Used M2. 9.15 9.39 8.56 9.78 9.82
Cupferron
Cu Found
0.01583 0.01551 0.01453 0.01453 0.01453
0.1449 0.1456 0.1243 0.1421 0.1427
7.06 7.02 7.02 7.02 7.93 7.89
0.01258 0.01258 0.01258 0.01258 0.01115 0.01115
0.0888 0.0883 0.0883 0.0883 0.0884 0,0880
Gram
Bureau of Standards cast bronze 52a '4
2
0.0883 0.0883 0.0883 0.0883 0.0882 0.0882
0.950 I 2 3 4 5 6 7 8 9 10 M L OFAMMONIUM SALICYLSULFONATE
Figure 4.
Iron
VI. Ammonium Salicylsulfonate
Figurc 3 shon-s the results when a lfi70solution of sulfosalicylic acid, standardized conductometrically against iron wire, was used in determining iron in iron ore. Figure 4 shows tho results when a standardized 10% solution of the ammonium salt of sulfosalicylic acid was used in place of the acid. The standard salt solution was prepared by dissolving the solid acid in ammonium hydroxide until acid t o methyl red and titrating it conductometrically against standard iron wire. The calculated results in Table 11show the accuracy of the methods. The iron solutions were prepared by the usual methods except that instead of reduction to the ferrous state, the iron was oxidized with either bromine water or Perhydrol to the ferric state. SUMMARY
A method for the conductometric titration of copper in brass and bronze with cupferron is described, the accuracy of which
INDUSTRIAL AND ENGINEERING CHEMISTRY
304
Table II. Sulfosalicylic A c i d and Ammonium Salt of Sulfosalicylic A c i d VI. Thorn Smith and Bureau of Standards Iron Ores Bample
Iron Taken
Titrant
Gram
lM1.
Iron Found Grdm
Difference Gram
15% Sulfosalicylic Acid, Iron Titer 0.0380 Gram per M1.
T.S. 72 T.S. 7 1 B.S. 26 B.S. 27 B.S. 29
0.0927 0.1048 0.1072
2.44 2.83 2.85
0.0927 0.1075 0.1083
0.0000
+0.0027
$0.001 1
10% Ammonium Salt, Iron Titer 0.0247 Gram per M1. 0,0813 3.36 0.0828 +0.0015 0.1126 4.57 0.1128 +0.0002 0.1126 4.56 0.1127 +0.0001 0.1356 5.47 0.1351 0.0005 0.1356 5.46 0.1349 0.0007 3.66 0.0902 -0.0003 0.0905
-
varies from 0.2 to 0.8 mg. of copper per 100 mg. of copper taken. The method is fast and may be used without separation of certain constituents. The conditions of the titration allow it to be fitted into the usual gravimetric procedure for brass or bronze. Methods for determining iron in iron ores are described which use both sulfosalicylic acid and its ammonium salt as titrating agents. The accuracy varies from 0.1 to 1.5 mg. per 100 mg. of iron taken. The conductometric method may be used in applying the
Vol. 18, No. 5
precipitation and colorimetric reaction of organic reagents t o volumetric analysis. LITERATURE CITED
Alten, Weiland, and Hille, 2. anorg. allgem. Chem., 215, 81 (1933). Bauer and Eisen, Angew. Chem., 52, 459 (1939). Boulad, J. H., Bull. S O C . chim. [5], 3, 408-12 (1936). Germuth, F. G., Chemist-Analyst, 17, 3 (1928). Gutzeit, Helu. Chim. Acta, 12,713, 829 (1929). Kolthoff, I. M.,and Langer, A., J . Am. Chem. Soc., 62, 211 (1940). Ibid., 62, 3172 (1940). Kolthoff. I. IM., and Lingane, J.. “Polarography”, New York. - . . Interscience Publishers; 1941. Korenman, Mikrochemie, 15, 315 (1934). Lingane, J., J . Am. Chem. Soc., 65, 866-72 (1943). Lorber, Biochem. Ztg., 181,391 (1927). Moser, L., Monatsh., 44, 91 (1923). Moser, L., Brukl, A, and Ven, I., Ber., 58B,380 (1925). Moser, L., and Iranyi, E., Monatsh., 43, 673 (1922). Moser, L., Neumayer, K., and Winter, K., Ibid.,55, 85 (1930). Neuberger, A4., Arch. Eisenhiittenw., 13, 171 (1939). Remsen, I., Ann.: 179, 107 (1875). Theil and Peter, 2. anal. Chem., 103, 161 (1935). Theil, and Van Hengel, Ber., 70B, 2491 (1937). Urech, Helv. Chim. Acta, 22, 322 (1939). Van Suchtelen and Itano, A., J . Am. Chem. SOC.,36,1793-1807 (1914).
Determination OF Tetraethyllead in Gasoline by X-Ray Absorption MILES V. SULLIVAN AND HERBERT FRIEDMAN United States Naval Research Laboratory, Washington,
D. C.
R
IGID control of the antiknock rating of gasoline and other motor fuels for use by the armed forces is of utmost importance in maintaining high standards of performance. The use of tetraethyllead to bring various base stocks up to a set standard of quality requires the addition of varying amounts of lead to each lot of fuel. Continuous control of this process by the manufacturer and continuous checking by the user may involve the determination of the lead content of hundreds of samples per day in one laboratory. As a result, chemical methods of analysis for lead in gasoline have received a great deal of attention. No single chemical method has been proposed, however, which is generally applicable to the analysis of lead in chemically different base stocks, and which does not require an excessive amount of time for reasonably accurate results (3). Aborn and Brown (1) employed an x-ray absorption technique for the quantitative analysis of lead in gasoline, but their method was limited in sensitivity by the use of an ionization chamber to detect the radiation. In the present work a Geiger counter was employed which had a quantum efficiency of roughly SOY0 for the radiation used. The result is a method for the quantitative determination of lead which is equivalefit t o the chemical analysis in accuracy, but may be completed by an unskilled worker in 5 minutes. X-RAY ABSORPTION
Under the conditions necessary for the production of x-rays one observes a spectrum of continuous radiation upon which is superimposed a series of monochromatic characteristic radiations. The continuous spectrum (Figure 1) is composed of all wave lengths down to a certain minimum wave length given by AO = 12395/V Angstroms, where V is the peak voltage applied. ”he intensity of wave lengths longer than this minimum passes
Figure
1. X-Ray
Spectrum of M o l y b d e n u m Bombarded with 35-Kv. Electrons
through a maximum a t about X = 1.3 XO and then gradually approaches zero. The intensity of hard radiation in the continuous spectrum increases with atomic number of the target material. The characteristic K series radiations appear only when the peak voltage applied to the tube exceeds the K shell ionization potential of the target atoms. By the proper choice of target, voltage, filters, and possibly a crystal monochromator, almost any range of x-ray wave lengths or monochromatic radiation may be obtained. When x-rays impinge on matter, the manner in which they are absorbed may be expressed by the relation:
I
= Iec-rP2
(1)
