V O L U M E 26, NO. 4, A P R I L 1 9 5 4 thiourea solution in 1 to 1 ethyl alcohol-hydrochloric acid. The perchloric acid was fumed for 5 minutes and the permanganate trap brought to boiling. The two vessels u-ere heated for 10 minutes further after which time the current of air was discontinued and water baths initially a t 85" C. were placed about the receivers for 15 minutes. The receiving solutions were transferred to a 50ml. volumetric flask; 1 to 1 ethyl alcohol-hydrochloric acid was used for rinsing and making up to volume. The resulting solution was filtered through a large Whatman's S o . 42 filter paper into the transinittancy cell. A solution of 1 to 1 ethyl alcoholhydrochloric acid was used as a blank solution. Samples distilled into 3% hydrogen peroxide in the manner described previously, then distilled into thiourea from the peroxide solution to which 5 ml. of concentrated sulfuric acid were added, gave results in agreement n i t h the standard curve. h standard curve was prepared by distilling aliquots of the ruthenium solution from perchloric acid. Twenty milliliters of perchloric acid were added to the solution in the distillation flask and brought to the fuming state. -4further 5-ml. portion of acid was then added. The vapors lverc drawn through a permanganate trap and were absorbed in two receivers containing 15 and 5 ml., respectively, of 1 to 1 ethIl alcohol-hydrochloric acid at room temperature. The pot and trap \?ere heatrd for the same periods as in the case of osmium. Eight milliliters of 5 % thiourea in 1 to 1 ethyl alcohol-hydrochloric acid were added to the first receiver and 2 ml. to the second. The blue color developed almost immediately but not completely until the receivers had been heated in water baths as with osmium samples. The remainder of the treatment was identical to that used in determining osmium except for the filter employed in the transmittancy measurements. Ruthenium in samples distilled into hydrogen peroxide m-adistilled into alcohol-hydroc,hloric acid upon addition of 5 ml. of concentrated sulfuric acid and excess sodium bromate. Most of the bromine formed upon adding bromate to the hydrogen peroxide solution was discharged from the receivers by bringing them to boiling for a feiv seconds. The remaining bromine was reduced by the thiourea. Attempts to oxidize the ruthenium quantitatively with permanganate were unsuccessful. Solutions containing aliquots of both the osmium and ruthenium solutions were treated as described and the results of the
74 1
Table 111.
so.
Separations of Osmium and Ruthenium by Colorimetric Determination Metal Ru
Metal Taken, 101
zoo
os
T98
Ru OS
Ru OS Ru
os
Ru Os
Error.
Y
os
Ru OS Ru
hfetal Recovered,
002 200
101
a02 a99
201 200
201 "00
201 200
99
Y -
3
202 507
202 106
788 503 599
201 197 201
+ 2 + 5 - 10 + 1 0 0
- 3
0 0
200
202 206
+ I
+ 6
colorimetric determinations are given in Table 111. The original sample solution in S o s . 5 to 7 contained 5 mg. of each of platinum, palladium, rhodium, and iridium as well as osmium and ruthenium. LITERATURE CITED
(1) .kllan, W. J., and Beamish, F. E., AINAL. CHEM.,24, 1569 (1952). (2) Ibid., p. 1608. (3) Ayres, G. H., and Toung, F.. Ibid.,22, 1277 (1950). (4) Gilchrist, R., J. Research S a t l . Bur. Standards, 6, 421 (1931). (5) Ibid., 9, 279 (1932). (6) Hoffmann, I.. Srhweitzer. J. E., Ryan, D. E., and Beamish, F. E., ASAL. CHEM.,25, 1091 (1953).
Rogers, W ,J., Beamish, F. E., and Russell, D. S., IND.ENG. CHEM.,ASAL. ED.,12, 561 (1940). (8) Sandell, E. B., Ibid., 16, 342 (1944). (9) Sauerbrunn, R. D., and Sandell, E. B., Anal. Chim. Acta. 9, 8 6 (7)
(1953). (10)
Thiers, R., Graydon, IT., and Beamish, F. E., ANAL.CHEM..20, 831 (1948)
RECEIVED for review October 2 3 , 19.53.
Accepted December 2 2 , 1953
Purification and Properties of Disodium Salt of Ethylenediaminetetraacetic Acid as a Primary Standard W. J. BLAEDEL and H. T. KNIGHT' Department o f Chemistry, University o f Wisconsin, Madison, W i s .
C
Ol\IPLESO31CTRIC titrations have increased in popularity during the past few years. Especially, ethylenediaminetetraacetic acid, [(HOOC CH2j2N CH, 12, or HIT, and and its sodium salts have been used for titrating directly various di- and trivalent cations, such as the alkaline earths (f-3,5, 6, 11, 13-15, 18, fQ), zinc ( 3 , f2), nickel (lo),cadmium (S), bismuth (I?'), lead ( 8 ) , and iron (5, 1 6 ) . Most of these titrations were performed on the 1 to 2% error level, but some authors reported errors as low as 0.2% (6, 8 ) . Only Schwarzenbach (3, 18, 19) and Flaschka ( 7 ) have used the dihydrated disodium salt of the acid (Ka2H2Y2H20) as a primary standard a t the 1% error level, but have given no instructions for its preparation prior to use. All of the other investigators standardized the disodium salt of ethylene diaminetetraacetic acid solution by titrating against a standard solution of the sought-for substance I t is the purpose of this study t o show that the dihydrated disodium salt may be conveniently used as a primary standard for the titration of several common metals a t the 0.1% error level. 1
Present addre-
ran-tee1 Metnllurgical Corp , h'orth Chicago, Ill
The stability of its =ohtion*and its high equivalent weight are particular advantages. PURIFICATION OF DIHYDR4TED DISODIUM SALT
Of five manufacturers of the dihydrated disodium salt, none would guarantee the purity or assay value of their product mithin 99.9 t o 1 0 0 . l ~ o . When three different lots of analytical reagent dihyclrated disodium salt (Bersworth Chemical Co ) Rere analyzed by titration against reliable standard solutions of copper nitrate and by m-eight 10s- upon heating, the assay values ranged from 98.0 to 100.0%. The impure samples contained more IT ater-insoluble material than the pure one, and diqcolored on heating a t temperatures considerably below 150" C. I t is therefore necessary to purify the available commercial product- before using them as primary standards. A single recrystallization from alcoholic solution served for purification of the poorest of the three lots of reagent grade product manufactured by Brrsworth Chemical Co. A nearly saturated aqueous solution of the impure reagent was a t room temperature (about 10 grams of the d i h y d r a t e c ? ~ ~ ~ ~ ~ ~
ANALYTICAL CHEMISTRY
742 100 ml. of water). Alcohol was added slowly until a permanent precipitate a eared which was filtered o f f and discarded. The filtrate was c&ted with an equal volume of alcohol, and the resulting precipitate was filtered, washed with acetone and then with ether, and then air-dried a t room temperature overnight. The yield was around 75%. The roduct contained excess water, the assay value being about 99.5$Na2H2Y .2Hz0. The product was stable to heat and could be dried to the anhydrous form a t temperatures up to 150' C. without appreciable decomposition. All the following work was performed using several lots of the reagent grade product (Bersworth Chemical Co.) purified in this way. SOLUBILITY AND VAPOR PRESSURE OF DIAYDRATED DISODIUM SALT
To aid in selecting conditions for purification and drying of the dihydrated salt, rough solubility and vapor pressure data were obtained. I n Table I are given solubilities and densities of saturated solutions at various temperatures. . At each temperature, the solubility was determined by withdrawing a measured volume of the saturated solution, weighing, diluting quantitatively, and determining the dihydrated salt content by titration of an aliquot with standard 0.01000F calcium chloride solution. The vapor pressure-temperature data were obtained for several systems of varying compositions. -4static method was used (bo), and the pressure of the vapor phase was measured a t several different temperatures for each system. The results of these measurements are shown in Figure 1. Mercury was used in the manometer for systems I and V, while butyl phthalate was used for the other systems. A plot of the solubility-temperature data of Table I indicates no breaks or discontinuities. Also, within experimental error, systems ranging in composition from 75 mole % NazHzY 25 mole % NazHzY.2H20 to 10 mole % Na2H2Y 90 mole % Na2H2Y.2Hp0 all fall on the same vapor pressure-temperature
+
+
plot. It therefore seems probable that there are no stable hydrates other than the dihydrate within the range of conditions studied. DRYING OF SALTS
Elevated temperature is required to dry the purified, wet dihydrate to a composition corresponding to the formula Na2H;Y.2Hz0, for the rate of loss of excess water from the recrystallized preparation is too low t o be practical a t temperatures of 60" C. and below. At 50% relative humidity (25' C . ) , it is felt that 80' C. is an optimum temperature for safe air-drying of the purified, wet dihydrate to its stoichiometrical composition. The data in Table I1 show that constant weight is attained within 4 days, and that no further losses occur up to a t least 2 weeks. The assay values of two different preparations dried in this way were 100.03 and 100.05% NazH2Y.2H20. At about 50% relative humidity (25" C.), air drying a t 100" C. causes slow conversion to the anhydrous form.
Table I.
Solubility of Na&Y .2H20 in Water at Various Temperatures
Temp.,
c.
98.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 21.0 0.5
iTazHaY.2Hz0, Grams/ 100 Grams of S o h . 27.0 24.3 22.2 20.0 17.0 15.5 14.2 12.8 11.1 10.6
Table 11. Change i n Weight of Moist, Air-Dried Na2HzY .2HzO at 80' C. Wt., Grani 0,8725 0.8677 0.8667 0.8656 0,8669 0,8657
1.01
-I
W'
a 2 v)
cn
W
EO I
20
I
I
40
I
I
I
~
1
1
60 80 100 (Go) Figure 1. Vapor Pressures of NaSH2Y-HsO Systems a t Various Temperatures System I I1 I11 IV
V
Code Mark 8
H Q
~
Mole Percentage Composition XaaHzY SazHzY .2Hz0 HtO 5 95 90 10 25 75 67 33 75 25
..
.. .. . I
. I
1
Density of Satd. Soln., G./Ml. 1.09 1.09 1.08 1.08 1.07 1.07 1.06 1.06 1.05 1.05
Time, Days 0 1
2 4 7 15
It is also possible to dry the disodium salt quantitatively to the anhydrous form. An inspection of Figure 1 shows that on a humid summer day temperatures above 120' C. are necessary to air-dry the salt to the anhydrous form. At 150' C., conversion to the anhydrous form is rapid and quantitative, but a t temperatures over 150" C., charring may occur. The most satisfactory procedure is to heat the disodium salt in a vacuum oven overnight a t 120" C. The assay values of two different preparations dried in this way were 100.00 and 99.99% NanH2Y. From Figure 1, it may be seen that the dihydrate is stable over a wide range of humidities, while the anhydrous form is hygroscopic under ordinary atmospheric conditions. The dihydrate is therefore the preferred form for use as a primary standard, as no particular precautions need be observed in its use. It may be weighed in open air and may be stored in a closed container or over sulfuric acid or calcium chloride desiccants for several weeks with no significant change in weight. However, a slow loss in weight is observed if the dihydrate is stored over phosphorus pentoxide. If the anhydrous form is used as a primary standard, it must be weighed in a closed container and stored over phosphorus pentoxide. STABILITY OF SOLUTIONS
Betz ( 3 ) could detect no significant change in titer of 0.01M solutions after 8 months. This is confirmed by the authors' findings that 0.01M Na2H2Y changed less than 0.05% in titer after etorage in borosilicate glass or polyethylene bottles for 5 months.
743
V O L U M E 26, NO. 4, A P R I L 1 9 5 4 However, a decrease in titer of 1% in four months on storage in soft glass bottles has been reported by Goeta, Loomis, and Diehl (9).
Bets, J. D., and Noll, C. A , , J . Bm. Water Works Assoc., 42, 49,749 (1950).
Biedermann, W., and Schwaraenbach, G., Chintia (Switz.), 2, 56 (1948).
TITRATIONS WITH PRIMARY STANDARDS
Blaedel, W. J., and Knight, H. T., ANAL.CHEY.,26,743 (1954). Cheng, K. L., Kurta, T., and Bray, R. H., Ibid., 24, 1640 (19.52). ,----,-
Solutions of 0.01F NazHzYwere prepared from the dihydrate and anhydrous disodium salt as primary standards, and were titrated against standard solutions of copper nitrate, calcium chloride, and zinc chloride, using the high frequency technique to avoid indicator errors and corrections. These titrations are described in another paper (4). The results of 20 titrations gave an assay value of 100.00% Na2H2Y for the primary standard anhydrous form, and five titrations gave an assay value of 100.05% Na,HZY. 2H2O for the primary standard dihydrate. The relative standard deviation of a single titration was around 0.1%.
Diehl, H., Goetz, C. A., and Hach, C. C., J . Am. Water Works Assoc., 42,40 (1950). Flaschka, H., Mikrochemie w r . Mibrochim. Acta, 39, 38 (1952). Ibid., p. 315. Goetz, C. A., Loomis, T. C., and Diehl, H., ANAL.CHEM.,22,
ACKNOWLEDGMENT
Pribil, R., and Koudela, Z., Collection Czechoslou. Chem. Corn-
The authors wish to thank E. I. du Pont de Nemours B: Co. for a grant-in-aid, and the Atomic Energy Commission for a research grant in support of this work.
Pribil, R., and Matyska, B., Ibid., 16, 139 (1951). Schwarsenbach, G., and Biedermann, W.3 Helv. Chim. Acta,
798 (1950).
Harris, mi. F., and Sweet, T. R., Ibid., 24,1062 (1952). Hernandez, H. R., Biermacher, U., and Mattocks, A. JI., Bull. Natl. Formularv Comm., 18, 145-52 (1950). Keihei, U., ANAL.CHEM.,24, 1363 (1952). Langford, K. E., Electroplating, 5 , 41 (1952). Mattocks, A. M., and Hernandez, H. R., J . Am. Assoc. Pharrn. (Sci. Ed.). 39.519 (1950). h l k g e r , J.’R., kippier, R’. W., and Ingols, R. S., rlx.4~.CHEX., 22,1455 (1950). mum., 16,80 (1951).
31,459 (1948).
Schwarsenbach, G., Biedermann, W., and Bangerter, F., Ibid., 29,811 (1946).
LITERATURE CITED
(1) Ranewicz, J. J., and Kenner, C. T., ASAL. CHEM.,24, 1186 (1952).
Weissberger, A,, “Physical Methods of Organic Analysis,’* New York, Interscience Publishers, 1949. RECEIVED for review July 13, 1953. Accepted December 2, 1953.
Stoichiometry of Titration of Metal Ions with Disodium Salt of Ethylenediaminetetraacetic Acid Using High Frequency Technique W. J. BLAEDEL and H. T. KNIGHT‘ Department o f Chemistry, University o f Wisconsin, Madison, Wis.
I
T IS the purpose of this study t o show that direct titrations of some typical metal ions [copper(II), zinc, calcium, and magnesium] with the disodium salt of ethylenediaminetetraacetic acid (NazH2Y) are stoichiometrical over rather wide ranges of condit ons, within relative errors of 0.1%. The titration of many kinds of metal ions with the disodium salt of ethylenedianiinetetraacetic acid is described in the literature (1) usually a t error levels of 0.5 to 2%, though in some cases greater accuracy is claimed. Very recently, it has been shown that the disodium salt of ethylenediaminetetraacetic acid standardized against copper as a primary standard reacts stoichiometrically with nickel and iron, a t the 0.1% error level ( 5 ) . I n most of these works, the titrant is standardized by titration against known solutions of the sought-for metal, or of another primary standard metal. If properly performed, this method is acceptable for circumventing errors due to reagent impurities, indicator blanks, side reaction, and nonstoichiometrical reaction between the reagent and sought-for substance. However, this method does not easily reveal the magnitudes and sources of the errors, nor does it allow the conclusion that the reaction on which the titration is based is stoichiometrical. In proving the stoichiometry of any titration, an instrumental method of following the course of the reaction is preferable to the use of chemical indicators. Elimination of the indicator error allows easier resolution of the other errors mentioned above. In the particular case of the direct titration of the alkaline earths in ammoniacal solution, the differential high frequency titration method (.2) is very advantageous, since the endpoint break is quite 1
Present address, Fansteel Metallurgical Corp., North Chicago, Ill.
indistinct for the ordinary type of conductometric titration curve obtained by plotting conductance against volume of standard solution. TITRATION OF COPPER
-40.0094331 solution of the disodium salt of ethylenediaminetetraacetic acid was prepared from the dihydrate as a primary standard (1). A standard solution of copper nitrate was prepared from copper foil (assay value, 99.98% copper). The foil was cleaned first with ether, then with dilute nitric acid, then rinsed off with distilled water and air-dried before weighing. The weighed foil was dissolved in 5 ml. of redistilled water and 2.5 ml. of concentrated nitric acid and diluted to 1 liter in a volumetric flask, giving a solution which was 0.01000M in copper nitrate and about 0.02.V in nitric acid. Titrations of aliquots of the standard copper nitrate with the standard disodium salt were performed in end-point volumes of 80 to 100 ml., since the titration cell required 50 to 100 ml. of solution. Also, the concentration range for adequate sensitivity of the 30-megacycle instrument lies between 0.0001 and 0.01M sodium chloride, or solutions of other electrolytes with the same conductances. It was for this reason that the investigation was confined to titrations with 0.01M, rather than 0.1M solutions. The titration technique was quite similar to that described in an earlier publication ( 9 ) . In the region of the equivalence point, standard disodium salt was added a t a uniform rate (about 0.5 ml. per minute), and the rate of change of frequency was followrd on a recording milliammeter, giving a recorded differential titration curve. Results of titrations of different-sized aliquots of standard copper nitrate a t different acidities are shown in Table I. Differential curves for some of the titrations of Table I are shown in Figure 1. In Figure 1, abscissas represent volume of standard disodium salt added, and ordinates represent a current which is proportional to the rate of change of conductance produced by
