1219
V O L U M E 2 5 , NO. 8, A U G U S T 1 9 5 3 Table 1 1 .
Calcium Oxide Values after Single Precipitation of Hydrated Manganese Oxides
Cement 62.72 62.72 62.90 62.86 62.88 62.72 63.00'' * A v e r a ~value f o r no removal. Table I!. 6 Aversqe t.ertificate value (CaO
Limeut o m 41.19 41.29
41.28 41.31 41.16
41.13 41 . 4 4 1
CaO corrected ior 3In10, (taken from
+ SrO).
difficult to know when sufficient ammonium hydro\i le h:iL been added t o make the solution ammoniacal, but the solution must be kept ammoniacal during a short period of boiling. This operation may be guesswork. Even when care has been taken to overcome the possible difficulties, in many instances very poor manganese removal is obtained with the bromine method (see tletermination I , Table I). The time involved for the two methods of separation iq comparable if evaporations can be arranged outside of actual working time. OtherM ise, the permanganate method will lengthen the ana1ytic:il procedure about 2 or 3 hours. LITERATURE CITED
I t is concluded that the permanganate method is better than bromine for manganese separation in cement analysis because the amounts of manganese left in the calcium oside and magnesium pyrophosphate precipitates are negligible and consistent results are obtained. I t is likely also that the permangn~iatemethod can be followed more easily than the broniine method by an inexperienced person. In following the bromine method, the acidit!. of the solution must be estimated before the bromine m t e r is added. because bromine hleaches the indicator. S o t only is it
( 1 ) Ani. Soc. Testing Materials, ".%ST11 Standards on Cemeiit," A p r i l 1952. (2) Federal Specification Sd-C-l58c, "Cemeiits, Hydraulic, lfethods for Sampling, Inspection and Testing," April 22, 1952. ( 3 ) Jensen. 33. -%., Z . anal. Chem., 86, 422-88 11931). (4) Sarker. P. B., and Dhar, S . R.. 2. anorg ollgem. C h m ~ .121, , 135-
55 (1922). 15) Volhard. . J . , Chem. N e w , 40, 207 (18T4). (6) Willard, H. H., and Greathouse. I,. H., .l. A m . C/woz. Soc., 39, 2366 (1917). RECEITEII f o r review February 7. 1953.
Accepted May 2 , 1933.
Preparation and Properties of 2-(2-Hydroxy-3,6-disuIfo-l naphthy1azo)-benzenearsonic Acid (Thorin) Analytical Reagent Jor Thorium
The increasing importance of 2-(2-hydroxy-3,6-disulfo-l-naphthj1azo)benzenearsonic acid (Thorin) as an analytical reagent for thorium has brought about a great need for more detailed inforniation as to its preparation and properties. A detailed procedure for the sjnthesis of Thorin is given. Work in these laboratories concerning the nature of the thorium-Thorin complexes had led to the determination of the acid dissociation constants of Thorin. These constants, pK3 = 3.7 and pK4 = 8.3 for the arsono hydrogens and pK, = 11.8 for the naphtholic hydrogen, were determined by potentiometric titration and spectrophotometric means. This information should he useful i n more extensive applications of this sensitive reagent.
T
HE reagent 2-(2-hydroxy-3,6-disulfo-l-naphthylazo)-ben- titration aaq performed using glass and calomel electrodes and zenearsonic acid (Thorin) ( I ) , also known as l-(a-arsonohence the dissociation constants reported here may be considered benzeneazo)-2-naphthol-3,6-disulfonicacid and under the synoapprovimate since they are subject to the inherent error of liquid nyms Thoron and Iiaphtharson, has recently attained considerable importance in the colorimetric determination of microgram quantities of thorium ( I , 8, 9, 14). Difficulties were encountered in this laboratory in attempting to use the brief procedure of Kuznetsov (9) for the synthesis of Thorin. The procedure given is intended to replace that previously reported ( 1 2 ) which carried over some errors of Kuznetsov's work. This procedure is more accurate and detailed. The separation of 2-aminobenzenearsonic acid is eliminated, thereby giving higher yields. I n the course of the authors' studies of the thorium comples with Thorin the equilibrium constants for the dissociation of the arsono and naphtholic hydrogens of Thorin were determined. The dissociation constants of the first hydrogen of the arsonic acid group, Ka, and of the second hydrogen of the arsonic acid group. KI, Were obtained by potentiometric titration. The
junction potential ( 2 ) . The dissociation constant of the naphtholic hydrogen, K6, was determined spectrophotometrically through the alteration of the ultraviolet spectrum of Thorin with variation of pH. The attribution of this spectral change to the removal of the naphtholic hydrogen was verified by comparison with 2-naphthol-3,6-disulfonic acid (R-acid) for which the dissociation constant was also obtained. Although the monosodium salt of 2-(2-hydrosy-3,6-disulfo-l-naphthylazo)-benzenearsonic acid may be readily isolated by recrystallization of the compound from dilute acid solutions, the dissociation constants of the sulfonic acid groups, K, and Kf, could not be obtained either potentiometrically or spectrophotometrically. Thorin may be obtained as the mono-, di-, tri-, tetra-, or pentasodium salt and mixtures of these salts, depending upon the conditions of recrystallization. For this reason the poten-
AN ALYTICAL CHEMISTRY
1220
-
80 grams of sodium hydroxide in 1 liter of aqueous solution. The reaction mixture was well shaken and allowed to cn I I / \ stand a t room temperature for several hours after which t \\ it was warmed to 60" C. and J was maintained a t this tem0 perature for 2 hours. The warm solution was then acidim fied to pH 6 with acetic a 0 acid trrated with decolorizing VJ rarbon until a light yellow m filtrate resulted. The filtrate a 4 \\\ I was then made acid to congo ad I I I red with hydrochloric acid 260 280 300 320 and chilled. The yellow, needlelike crystals of 2-nitroWAVELENGTH, m p benzenearsonic acid were Figure 1. Absorption Spectra of R-4cid with Varj-ing pH filtered and washed with 150 ml. of cold water; yield 115 grams (64%), melting point 194' to 6'. Analysis: calculated for CeHeN06-4~~ As, 30.4%; found, As,30.5%. 2-Aminobenzenearsonic Acid (6, 7). An excess of saturated ferrous chloride solution [6.3 equiv. as determined by titration with cerium(1T') sulfate] was added to a solution of 50 grams (0.2 mole) of 2-nitrobenzeneSOLN. pH arsonic acid in 550 ml. of 6 ill (3.3 mole) sodium hyI 168 droxide contained in a 3-liter flask. The mixture was 6 11.08 shaken vigorously for 10 minutes, filtered, and washed with two 100-ml. portions of Karm dilute sodium hydroxIO 11.82 ide solution. I t is important that the ferric hydrox18 13.52 ide precipitate be thoroughly washed as it tends to adsorb the product. The filtrate was brought to pH 7 viith hydrochloric acid arid then an additional 60 ml. of concentrated hydrochloric acid were added. 2-(2-Hydroxy-3,6- disulfo-1-naphthy1azo)Benzenearsonic Acid (Thorin). Technical grade disodium salt of 2-naphthol-3,6-disulfonic acid (R-salt), 100 grams, was purified by dissolving it in 350 ml. of hot saturated sodium chloride solution which was subsequently cooled, and the R-salt residue was filtered and washed with water. Seventy grams (0.2 mole) of the purified R-salt and 65 grams of anhydrous sodium carbonate were dissolved in 650 ml. of water, and then filtered and placed in a 4liter Erlenmexer flask. The 2-aminobenzenearsonic acid solution was cooled to -5' C. and diazotized by the a01 I I I 1 1 slow addition of 100 ml. of 2 .If sodium nitrite. The 280 300 320 340 360 resulting diazonium solution was then added to the WAVELENGTH, mp R-salt in the &liter Erlenmeyer flask and the red product was shaken until u ell mketi, being careful to avoid posFigure 2. Absorption Spectra of Thorin with Varying pH sible ewessive foaming. The heavy red precipitate was filtered and then recrystallized from 800 ml. of water and washed with 100 ml. of cold ~ a t e r .The finely divided Thorin tiometric titration, rather than analysis for sodium or arsenic, is was allowed to stand under acetone to speed its drying and was recommended as a test of purity of the reagent. subsequently filtered, air dried, and pulverized giving 60 grams of bright red product Yield 52y0. Over-all yield 33%.
c
c
I
IO
I
-
PREPARATION OF THORIN
The synthesis is performed in three steps: (a)the coupling of 2-nitroaniline with disodium hydrogen arsenite by the Bart reaction to form 2-nitrobenzenearsonic acid ( 6 ) , ( b ) the reduction of 2-nitrobenzenearsonic acid to 2-aminobenzenearsonic acid with ferrous chloride ( 7 ) , and ( c ) the coupling of the 2-aminobenzenearsonic acid with the disodium salt of 2-naphthol-3,6-disulfonic acid (R-salt) by the Griess reaction (9, 14). 2-Nitrobenzenearsonic Acid. A mixture of 100 grams (0.73 mole) of 2-nitroaniline and 500 ml. of 1 to 1 hydrochloric acid was ground thoroughly in a mortar and the suspension was transferred to a 3-liter, 3-necked, round-bottomed flask equipped y i t h a thermometer, stirrer, and dropping funnel. The suspension was stirred 1 hour and then cooled to 0" C. Diazotization was performed by the slow addition of 250 ml. of 3 M (0.75 mole) sodium nitrite solution. The diazonium solution was then cooled below -10" C. and slowly decanted from any unreacted 2-nitroaniline into a 5-liter flask equipped with a stirrer and containing 500 ml. of 6 iM sodium hydroxide also cooled to - 10" C. The sodium hydroxide solution was stirred rapidly with 100 grams of crushed ice during this addition which should not exceed 15 minutes owing to the instability of the diazonium hydroxide formed. The solution was immediately poured into a Pliter flask containing 1 liter of chilled 1 V disodium hydrogen arsenite. The latter was prepared from 99 grams of arsenous oxide and
The product prepared by this procedure was primarily the trisodium salt and was better than 99% pure. The color of Thorin depends upon the number of replaced hydrogens and varies from orange for the monosodium salt to brilliant red for the trisodium salt to a rust red for the pentasodium salt. All the sodium salts may be oven dried without decomposition. Thorin is stable in acid solutions but undergoes slight decomposition on standing in basic solutions. DISSOCIATIOR' CONSTANTS OF THORIN
Dissociation Constants by Potentiometric Titration of Thorin.
A 0.01 M solution of Thorin was prepared by accurately weighing
the trisodium salt, adding 2 equiv. of carbonate-free, 0.1018 M sodium hydroxide and sufficient sodium chloride to give an ionic strength of 0.10 upon dilution to a total volume of 250 ml. A 100-ml. aliquot of this solution was then titrated lvith 0.1012 J l hydrochloric acid; the pH v a s measured Lvith the Beckman Model G pH meter. Values for the dissociation constants were obtained from a graphical plot in the usual manner. Correction was applied for dissociation of the first arsonic hydrogen and for hydrolysis of the weaker acid hydrogens (11). Correction was also applied for the
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1221
ionic strength in order to give a better approximation of the thermodynamic dissociation constant. This correction is +0.12 for an ionic strength of 0.1. The graphical and corrected pK values are recorded in Table I; pKs is an approximation included only for purposes of comparison with the more accurate spectrophotometric results. Bates et al. ( 2 ) discusses the accuracy of dissociation constants obtained in this manner. Spectrophotometric Determination of Dissociation Constants of Thorin (23). The dissociation of the naphtholic hydrogen of Thorin, which was not observable in the potentiometric titration, causes a significant change in the ultraviolet region of the spectra for both Thorin and R-acid. The similarities of these spectra are quite apparent (Figures 1 and 2 ) leaving no doubt that they both result from the dissociation of the naphtholic hydrogen. R-acid, the simpler molecule, has definite isosbestic points in its spectra with changing pH while those of Thorin shift slightly, prior to and following the dissociation of the naphtholic hydrogen. The pK' values obtained from Figure 3 a t both wave lengths of 310 and 370 mp. for R-acid
-1
t 1.0
310 rnp
0
13 0
01
''
J
,9.08
W 0
z a m
/-
/ I
370 rnp
K
5: m a
0 . o L 5
Figure 3.
6
I
I
I
7
8 PH
9
I
IO
I
II
I
Graphical Determination of the Naphtholic Dissociation Constant of R-Acid
Table I.
Dissociation Constants
RIethod of Determination Potentiometric Titration Spectrophotometric PK PK'! PK, PK.', graphical corrected graphical correcied 3.63 3.70
__
Compound Thorin pKa D Ki
are in excellent agreement. Similar agreement was obtained for the pK' values of thorin at wave lengths of both 295 and 345 mr. Absorbance (5) measurements were made with the Cary recording spectrophotometer, Model 12, using matched 5-cm. silica cells. pH readings were taken with the Beckman Model G pH meter. R-Acid. Solutions were 2.5 X M in R-acid, 0.1 M in ammonium chloride-ammonium hydroxide buffer, and 0.4 M in sodium chloride t,o give an ionic strength of 0.5. Absorbance measurement,s were taken on ten solutions varying in pH from 5.00 to 11.19 of which four are shown in Figure 1. The absorbance versus pH plots from which the pK' value (4)was takrn are shown in Figure 3. Thorin. Type E electrodes were used with the Beckman pH meter. Correction was made for thr high pH readings due to the M in sodium ion concentration. Solutions were 2.5 X Thorin. The p H was adjusted with a sodium hydroxide solution and constant ionic strength was maintained a t 0.5 by the addition of sodium chloride. A4bsorbancemeasurements were made immediately after preparation on a series of 18 solutions varying in p H from 7.68 to 13.52. The absorption spectra of four of t,hevo solutions are shown in Figure 2. The absorbance versus pH plots from which the pK' value was taken are shown in Figure 4. Correction was made for ionic strength, adding 0.21 unit t o the p K ' values for p = 0.5. The difference in the value of pKa for R-acid and pKb for Thorin is close t,o that expected from purely electrostatic effects of the di-ionized arson0 group. Using Bjerrum's approximation ( 3 ) and a dielectric constant of 80, a difference of 1.i to 2.1 pK units would be expected depending upon the assigned orientation of hydrogen-oxygen bond. The observed difference of 2.5 units might be readily accountable as due to intrahydrogen bonding. Such hydrogen bonding has been observed in other azo compounds and in 2-carhoxyazo dyes ( 10). LITERATURE CITED
g t J
s /I
1.5
290 mp
( 1 ) Banks, C. V., and Byrd, C. H., .$SAL. CHEM.,2 5 , 4 1 6 (1953). (2) Bates, R. G., Siegel, G. L., and Acree, S. F., J . Research NatE. Bur. Standards, 3 0 , 3 4 7 (1943). (3) Bjerrum, Ii.,Z . p h y s i k . Chem., 1 0 6 , 2 1 9 (1923). (4) Desha, L. J,, Sherrill, R. E., and Harrison, L. M,, J . Am. Chem. Soc., 4 8 , 1 4 9 3 (1926). (5) Hughes, H. K., et al., ANAL. CHEM.,24, 1349 (1952).
1.57
(6) Jacobs, W. d.,Heidelberger, M., and Rolf, I. B., J . Am. Chem. SOC., 40, 1582 (1918).
( 7 ) Johnson, J. R., and Adams, R., Ibid., 45, 1307 (1923). (8) Kronstadt, R., and Eberle, A. R., Atomic Energy Commission, Rept. RMO-838 (1952). (9) Kuznetsov, V. I.,J . Gen. Chem. ( U . S . S . R . ) ,14, 914 (1944). (10) Ospenson, J. N., Acta Chem. Scand., 4 , 1 3 6 1 (1960). (11) Pressman, D., and Brown, D. H., J . Am. Chem. Soc., 65, 570
11.55
345mp
0.5 7
8
9
IO
II
12
13
I
PH Figure 4.
Graphical Determination of the Naphtholic Dissociation Constant of Thorin
(1943). (12) Reed, S. A , Byrd, C. H., and Banks, C. V., Atomic Energy Commission, R e p t . AECD-2565 (1949). (13) Stenstrom, W., and Goldsmith, K.,J . Phys. Chem., 30, 1683 (1926). (14) Thomason, P. F., Perry, M. A . , and Byerly, W. M., ANAL. CHEM.,21, 1239 (1949). RECEIVED for review January 17, 1953. Accepted M a y 14, 1953. Contribution KO.237 from the Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa. Work performed in the Ames Laboratory of the Atomic Energy Commission.
