Chelometric Titrations Using an Azoarsonic Acid Indicator JAMES S. FRITZ, RICHARD T. OLIVER, and DONALD J. PIETRZYK Institute for Atomic Research and Deparfment of Chemistry, Iowa Stafe College, Ames, Iowa
b Arsenazo [3-(2-arsonophenylazo)4,5 - dihydroxy-2,7- naphthalenedisulfonic acid, trisodium salt] i s an excellent indicator for the EDTA titration of rare earths and yttrium in weakly acid solution. In many cases, use of masking agents or a simple preliminary separation permits determination of rare earths in samples containing foreign metal ions. Aluminum is masked b y sulfosalicylate, and interference from calcium and magnesium i s avoided b y proper pH control. Small amounts of uranium, iron, and most divalent metals are masked b y diethyldithiocarbamate; larger amounts of these metals are removed by extraction of their diethyldithiocarbamate complexes. Thorium interferes with the titration of rare earths, but can b e titrated a t a more acidic pH. Titration of thorium in the presence of uranium i s possible.
T
paper is concerned with the use of the trisodium salt of 3-(2-arsonophenylazo) - 4,5 - dihydroxy - 2,7 - naphthalenedisulfoiiic acid as a n indicator for the titration of certain metal ions with EDTA, (ethylenedinitrilo) tetraacetic acid. The name “arsenazo” was proposed for this compound by Kuznetsov, who reported color reactions with rare earths (9) and m-ith several other nietal ions. Arsenazo is a chroniotropic acid derivative, similar in some of its propcrties to other chroniotropic acid derivatives that have recently found analytical use. Banerjee used p-sulfophenylazochroniotropic acid (SPADSS) as an indicator in the direct E D T A titration of zirconiuni (4) and thorium ( 5 ) and for the spectrophotometric determination or thorium, zirconium, and fluoride ( 2 , 3 ) . Datta used p - sulfonaphthylazochromotropic acid ( S S A D S S ) and some related compounds for similar purposes (6). Arsenazo forms violet or red-violet romplexes in acid solution with thorium, zirconium, rare earths, aluminum, and qome bivalent metals such as copper, lead, and beryllium; in basic solution calcium, magnesium, and manganese(I1) form red-violet complexes. Uraniuni(VI) forms a blue complex over a very wide pH range. The free indicator is HIS
orange-pink in neutral and acidic solutions, and rose in alkaline solutions. As an indicator, arsenazo is useful for E D T A titration of calcium and magnesium in alkaline solutions, for titration of thorium a t p H 1.7 to 3.0, and for titration of yttrium and rare earths in the approximate p H range of 5 to 10. Through the use of masking agents, it is possible to achieve excellent selectivity for the titration of yttrium and rare earths (as a group) in the presence of other metal ions. REAGENTS AND SOLUTIONS
Ammonia buffer. Mix 67.5 grams of aninionium chloride with 570 ml. of concentrated ammonium hydrouide and dilute to 1 liter. ilrsenazo, 0.5%. Dissolve 0.5 gram of arsenazo [trisodium salt of 3-(2arsonophenylazo) - 4,5 - dihydroxy - 2,7 iiaphthalenedisulfonic acid] in 100 ml. of water. Calcium perchlorate, 0.05M. Dissolve 17.4 grams of reagent grade Ca(C10a)2.6H20 in 1 liter of water. Standardize b y titration with E D T A using o-cresolphthalein complexone (metallphthalein) indicator (1). Chloroform. reagent grade. Diethyldithiocarbamate, 0.5M. Mix equal volumes of 1X carbon disulfide in ethyl alcohol and 2M diethylamine in ethyl alcohol. For best results use only reagent grade chemicals and prepare the diethyldithiocarbamate solution fresh each day or two. The ethyl alcohol solutions of carbon disulfide and diethylamine are more stable and can be stored. EDTA, 0.05M. Dissolve 20 grams of reagent grade disodium (ethylenedinitri1o)tetraacetate in 1 liter of lvater. Standardize by titration of zinc nitrate (prepared from pure zinc metal), using naphthyl azoxine (8) or Eriochrome Black T indicator. Magnesium perchlorate, 0.05M. Dissolve 16.6 grams of reagent grade ilIg(C104)2.6H20 in 1 liter of water. Standardize b y titration using Eriochrome Black T indicator. Rare earth solutions, 0.05J1, rrere prepared from Ames Laboratory rare earth oxides stated to be 99.9% pure. Dissolve the calculated weight of oxide in Derchloric or nitric acid and dilute to voiume with rvater. Sulfosalicdic acid. 0.1M. Dissolve 21.8 grams Gf reagent’grade sulfosalicylic acid in 1 liter of rrater. Thorium nitrate, 0.05-11. Dissolve
27.6 grams of reagent grade Th(N03).4 H 2 0 in 1 liter of Tater. Standardize b y titration with EDTA using Alizarin Red indicator ( 7 ) . DETERMINATION OF CALCIUM AND MAGNESIUM
Procedure. T a k e a sample containing 0.5 t o 1.0 mmole of calcium or magnesium and dilute t o approximately 50 nil. Add 5 ml. of ammonia buffer and check t o see t h a t t h e pH is 10.0 =I= 0.1. Add 3 drops of arsenazo indicator and titrate with 0.05111 E D T A , taking t h e disappearance of t h e last violet tinge (the solution should be orange or rose-orange) as the end point. K h e n calcium is titrated by the above procedure, the end point is sharp and compares favorably with that obtained in the direct E D T A titration of calcium with “metallphthalein” indicator (1). The end point in the magnesium titration is less sharp, but can be located well enough for good accuracy. Addition of 20 or 30 ml. of acetone near the end of the titration sharpens the end point somewhat. Traces of metal ions such as iron(II1) and copper(I1) d o not destroy the end point, as they d o in titrations of calcium and magneqiuiii using Eriochrome Black T indicator. Arsenazo does not seem well suited, however, for the microtitration of calcium and magnesium with 0.01M EDTA. Data for titrations of calcium and magnesium are given in Table I. DETERMINATION OF THORIUM
Procedure. T a k e a sample containing 0.25 t o 0.5 mmole of thorium a n d dilute t o 100 t o 200 ml. Add dilute acid (perchloric, nitric, or hydrochloric) or base until t h e p H is in t h e range 1.7 t o 3.0. Add 2 drops of Table I.
Ion
Titrated Ca++ llg++
Titration of Calcium and Magnesium EDTA, 111. DifferTheory Actual ence 7.87 7.88 +0.01 7.86 -0.01 7.87 10.00 7.57 7.54 -0 03 7.56 -0.02 7.54 -0.03
VOL. 30, NO. 6, JUNE 1958
1 1 11
arsenazo indicator and titrate v i t h Table
II.
Titration of Thorium
EDTA, M1. Ion Added
0
Theory 10.24 10.24 4.80 4.80 2.87 1.91 1.91
PH 2.5 2.5 So;-- ( i : i j 2.7 uoz++(2:l) 2.1 uo*++(7: 1) 2.2 uoz++(10: 1)5 2.4 uo2++(20:1)0 2.4 Titrated with O.005M EDTA.
Table 111.
Difference t o ,02
Actual 10.26 10.28 4.80 4.80 2.87 1.91 1.90
+0.04 =tO.OO
fO.OO dZO.00 =to. 00 -0.01
Titration of Rare Earths
EDTA, M1. Ion Titrated La + 3
PH Theory 5.6 22.20 6.1 22.20 Ce +3 6.2 13.73 Sm +3 5.8 8.35 5.8 8.35 9.10 16.05 Yb + 3 6.2 14.37 Excess EDTA4added and back-titrated with Sm+3.
Table IV.
Difference
Actual 22.20 22.22 13.70 8.34 8.35 16.01 14.38
=t 0.00
+o. 02 -0.03 -0.01
fO.OO -0.04 $0.01
Titration of Rare Earths in Presence of Other Metal Ions
EDTA, MI. Ion Titrated Dy +a
Pr +3
Ion Addeda
Ca +2
Theory 4 64
Mg +?
4 64
Ca+2 f Mg+2
4 64
Ca + 2
3 50
hig +2
3 50
Ca'2 Sm + 3 Y+3 Er + 3 Y+3
Masking Agent
+ Mg+2
Ca +2 (4: 3) Ca + 2 Cu +2 cu+2 COf2, Ni Cz (1:3)
F?22
3 50
Cyanide Cyanide
16 06 18 75 2 98 14 10
14 10 Iodide Dithiocarbamate 3 20 Cd +% Dithiocarbamate Pb +1 Dithiocarbamate Hg +2 Dithiocarbamate Dithiocarbamate Y +a Cd +2 4 97 Dithiocarbamate 22 42 Dithiocarbamate 22 42 c o +a Dithiocarbamate Hg +2 4 97 Dithiocarbamate Dithiocarbamate Pb +2 Dithiocarbamate Dithiocarbamate 9 39 Dithiocarbamate Pb+a(3:5) 14 10 Dithiocarbamate UOzA2(1:7) 22 42 Zn Dithiocarbamate 4 97 Dithiocarbamate Dithiocarbamate 14 10 Y+3 Zn+2 (2:3) Dithiocarbamate 8 84 Dy +3 A I +3 Sulfosalicylate .41 + 3 Sulfosalicylate 4 95 Er +3 i11A3(2:5) Sulfosalicylate 22 22 La + 3 -41+ 3 Sulfosalicylate 14 66 Pr +3 .4l+3 Sulfosalicylate Sulfosalicplate 16 06 Sm +3 A1+3 (2: 1) Sulfosalicylate 18 75 Y+3 AlA3(2: 1) Sulfosalicylate Y +3 14 10 Cu (1:3) Thiourea Ratio of ion added to rare earth is 1 : 1 except where noted. Y+3 Nd +3
(I
1 1 12
ANALYTICAL CHEMISTRY
Actual 4.61 4.62 4.62 4.63 4.65 4.63 3.49 3.48 3.51 3.48 3.50 3.51 16.10 18.72 2.99 14.16 14.09 3.22 3.21 3.21 3.20 4.96 4.98 22.46 22.38 4.97 4.97 4.97 4.98 9.38 14.07 22.44 4.97
4.96 14.06 8.83 8.81 4.95 22.23 14,67 14.62 16.13 18.i 8
14.10
Difference Azo. 00
-0.02 -0.02 -0.01 +0.01 -0.01 -0.01
-0.02
+o. 01
-0.02 f 0 . 00 +0.01 $0.04
-0.03 10.01 +0.06 -0.01
+o. 02 +0.01 +o. 01
Azo.00
-0.01 +0.01
+0.04
-0.04
0.05M E D T A , taking the sharp color change from violet t o orange or pink as t h e end point. I r uranium(V1) is present, t h e color change at t h e end point is from violet t o clear blue. If t h e sample contains a high percentage of uranium, dilute until t h e uranium(V1) concentration is less t h a n 0.002M and titrate t h e thorium with 0.01M or 0.005M E D T A using a microburet. The end point in the thorium titration is very sharp in the p H range 1.7 to 3.0, if the thorium is diluted enough. It is not necessary to titrate slowly in the vicinity of the end point, as with some other indicator procedures. LTnlike most indicators, arsenazo permits the sharp titration of thorium in the presence of moderate amounts of weakly complexing substances such as sulfate, sulfosalicylste, catechol, and carboxylic acids. The tolerance for other cations that complex with EDTA is poor; in general, metals with a formation constant of or greater interfere. Calcium and magnesium do not complex a t the p H employed and therefore do not interfere. Uranium(V1) does not interfere. This is important, because uranium is frequently present in solutions to be analyzed for thorium. When uranium(VI) is present, the color change is from violet to blue instead of the usual change from violet to orange, because uranium(V1) forms a blue arsenazo complex as soon as the thorium-arsenazo complex has been broken by reaction with EDTA a t the end point. The thorium end point is sharp, if the concentration of uranium(V1) is not excessive (not much above 0.002M). However, samples where the ratio of uranium to thorium is high can still be analyzed for thorium merely by diluting to reduce the concentration of uranium(V1) in the solution to be titrated. The thorium content is determined by taking a suitable aliquot and titrating with 0.01M or 0.005M EDTA. Data for individual titrations of thorium are given in Table 11.
10.00
fO.00 10.00
+o. 01 -0.01
-0.03
+o. 02
f0.00 -0.01 -0.04
-0.01 -0,03 10.00 +0.01
f0.01 -0.04
+o .07
+O. 03 10.00
TITRATION OF RARE EARTHS A N D YTTRIUM
Procedures. GENERAL PROCEDURE. Transfer a sample containing 0.25 t o 1.0 mmole of rare earth to a 250-ml. beaker and dissolve in water or concentrated acid (perchloric or hydrochloric). Evaporate the bulk of acid (if acid is used) and dilute t o approximatply 100 ml. Add 4 or 5 drops of pyridine, then adjust the pH t o 5.5 t o 6.5 n-ith ammonium hydroxide or dilute acid (perchloric, nitric, or hydrochloric). Add 2 drops of 0.5% arsenazo indicator solution and titrate with 0.05M E D T A , taking t h e s h a m violet to orange-red c o l o r change as: the end point.
CALCIUMOR ~ I A G K E S IPRESEST. U~I Follow the general procedure but keep the p H below 6.0 and preferably at 5.5. Titrate rather S ~ O W ~ J near the end point. .kLUhlIKUhf PRESENT.F O h r T the general procedure but add 50 ml. of 0.1M sulfosalicylic acid before adding the pyridine or adjusting the pH. URAKIUBI,IRON,COBALT,NICKEL, COPPER,ZINC.CADMIUM, SILVER,MERCURY, AND/OR LEADPRESENT. Weigh or measure into a 250-ml. beaker a sample containing not more than 1 mmole of rare earth or 2 mmoles of combined foreign metals. Dissolve in water or concentrated perchloric or hydrochloric acid. Evaporate most of the acid used and dilute to approximately 50 ml. Add 8 nil. of 0.lM sulfosalicylic acid per 0.1 mmole of rare earth present. Adjust the p H t o approximately 1.6 with dilute ammonia or perchloric acid. Add slo~vly, with stirring, 2 ml. of freshly preparcd 0 . 5 M diethyldithiocarbamate per 0.1 mmole of foreign metal ion. The p H a t this point should be in the range 5.5 to 7.0. If the precipitate formed is not too bulky or highly colored, titrate the rare earth according to the generaleprocedure nithout removing the dithiocarbamate precipitate. If extraction is necessary. transfer the sample t o a separatory funnel; nash the beaker with small portions of chloroform and water, and transfer the washings to the separatory funnel. Add about 35 ml. of chloroform. w i r l to extract the dithiocarbamate, allow the layers to settle, and draw off the chloroform layer into the original beaker. (Shake vigorously only if swirling fails to remove the precipitate.) Repeat the extraction 1 to 2 more times, using 20-ml. portions of chloroform. Wash the combined chloroform extracts with water containing a little dithiocarbamate and combine with the main aqueous solution. Rinse the aqueous layer into a beaker, adjust the p H to 6 with pyridine and perchloric acid, add 3 to 5 drops of arsenazo indicator, and titrate within 15 minutes with 0.05Ji EDTA. The rare earths and yttrium are similar to their chemical beharior and can be conveniently considered as a group for the following discussion. Actually there is a significant difference in the strength of the various rare earth E D T A complexes; the formation constants vary from lanthanum ( 1 0 1 6 5 ) to lutecium (1019*) in fairly regular intervals. Yttrium behaves like one of the higher rare earths. This difference in strength of the complexes is reflected in the p H for the E D T A titration using arsenazo indicator. The higher rare earths give sharp end points at p H 5.0 or a b o w ; the lower rare earths are best titrated a t a pH of 5.5 or above. The optimum p H range for direct titration with EDT-4 is 5.0 or 5.5 to approximately G.5. At higher p H values a masking agent such as sulfosalicylate must be
prcwnt to prevent rare earth hydroxides from precipitating. I n alkaline sulfosalicylate solution, however, the reaction rate of rare earths with E D T A is slow, and numerous false end points are obtained when direct titration is attempted. Satisfactory results can be obtained u p to p H 9 or 10 by adding a slight excess of E D T A and backtitrating after a minute or two with a standard rare earth solution. The precision and accuracy of the arsenazo indicator method ITere checked by titrating standard solutions of representative rare earths. The rare earth solutions were standardized by E D T A titration using naphthyl azosine indicator (8) or gravimetrically by precipitation with oxalate. The data. s h o m in Table 111, indicate excellent precision and accuracy for the arsenazo method. The problem of determining rare earths in the presence of other metal ions naq given careful attention. By
Table V
proper adjustment of p H , interference from calcium and magnesium can be avoided. I n moderate amounts, neither calcium nor magnesium interferes if the p H is lower than 6.0. Interference from metals that form stronger EDTA complexes than calcium ( K = 1Olo9, cannot be avoided hy p H control. The most useful method for improving the selectivity of the rare earth titration is the use of masking agents. Titration of rare earths in the presence of aluminum is a n important example. illthough aluminum normally interferes by forming a strong E D T A complex ( K = 10l6I), the presence of a moderate amount of sulfosalicylate will completely mask aluminum. The rare cnrth cnd point is sharp, arid the violet color does not recur after the true end point has been reached. Cyanide can be used to inask some of the divalent metal ions. By using cyanide, very good results were obtained for titration of rare earths in
Titration of Rare Earths after Dithiocarbamate Extraction
EDTA, Ml. Ion Titrated Dy + 3
a
Ion
iiddeda
Actual Fe + 3 3b50 3.50 Dy T3 uoz +Z 3 52 3 51 3 52 3 50 Pr + 3 Cd + 2 5 24 I 5 22 c o +2 5 22 c u +a 5 22 y i +2 5 22 P b +2 5 22 Zn +2 5 21 Fe + 3 4 64 4 62 4 62 uoz +* 4 65 4 62 4 64 4 63 Tb+3 Fe +3 5 68 5 69 5.70 coz +* 5.66 5.63 5.67 5.66 5-+ 3 Cd +2 4.94 4.92 4.93 c o +2 4.93 4.92 c u +2 4.93 4.93 Hg +2 4.94 4.93 h-i '2 4.91 4.92 Pb +2 4.93 4.94 Zn +2 4.93 4.92 uoz+2(2:1) 4.37 4.37 4.38 4.36 4.35 Y+3 Fe + 3 4.37 4.36 4.37 4.36 Y+3 Fe+S (2:l) 4.37 4.37 Ratio of ion added to ion titrated is 1: 1 unless otherwise noted. Theory 3,50
Difference 1 0 .oo
10.00 -0 01 1 0 00 -0 02 -0 02 -0 02 -0 02 -0 02 -0 02 -0 03 -0 02 -0 02 -0 03 -0 01 -0 02 1 0 01 +0.02 -0.03 10.01
10.00 -0.02 -0.01 -0.01 -0.02 -0.01 -0.01
+o.oo
-0.01 -0.03 -0.02 -0.01 fO.00 -0.01 -0.02 10.00 f0.01 -0.01 -0.02 -0.01 ZkO.00
-0.01
+o.oo
VOL. 30, NO. 6, JUNE 1958
1 1 13
the presence of copper, cobalt, and nickel. It is not possible t o mask zinc and cadmium adequately with cyanide, however, and if these metals are present the rare earth titration fails. Several other masking agents were tried. Copper is efficiently masked by thiourea, mercury(I1) interferes a t the pH used, and other divalent metals are not masked. Iodide is a useful and selective masking agent for mercury(I1). Triethylenetetramine, 2,3-dimercaptopropanol, and sodium sulfide were tried without success as masking agents for zinc. Some limited success was obtained in masking lead with 2,3-dimercaptopropanol, but only small amounts of lead can apparently be tolerated (Table IV). Diethyldithiocarbamate is very useful for avoiding interference by metals that react with hydrogen sulfide. I n many cases rare earths can be titrated with EDTA directly in the presence of the metal dithiocarbamates. If the metal dithiocarbamates are highly colored or a large amount of precipitate is present, the interfering metal dithiocarbamates are removed by eutraction (occasionally a double extraction is required) into chloroform. Sulfosalicylate is also added to prevent precipitation of rare earth hydrokides during the extraction. The method is particularly important, in that it permits the simple and accurate determination of yttrium and rare earths in samples containing uranium. Interferences by iron, cobalt,
nickel, copper, zinc, silver, cadmium, mercury, and lead are also avoided (Table V). Metal ions studied for which interferences could not be avoided were thorium, zirconium, and chromium. However, methods are in the literature for separating these metal ions from the rare earths, and it is probable that in at least some cases the E D T A titration with arsenazo could be used following the separation. DISCUSSION
Because of the similarity in chemical structure of arsenazo and p-sulfophenylazochromotropic acid (SPADNS), their indicator properties were compared. The absorption spectra of the thorium complexes are very similar, but thorium forms a strong complex with arsenazo over a much longer p H range than with SPADNS. The thorium and rare earth complexes with arsenazo appear to be definitely stronger than the corresponding complexes with SPADSS. One evidence of this is that thorium can be titrated with E D T A a t a lower p H using arsenazo. This is not conclusive in itself, but suggests greater complex stability. Stronger evidence is that many metal ions interfere in the titration of thorium using arsenazo, but both thorium and rare earths can be titrated in the presence of conipleving anions such as sulfosalicylate and sulfate. On the other hand, by use of SPADSS, thorium can be titrated in
the presence of metal ions such as zinc, aluminum, cerium, and lanthanum (6), but sulfosalicylate and other complexing anions interfere. This indicates that the thorium SPADKS complex breaks up a t a higher thorium concentration (lower pTh value) than the thoriumarsenazo complex. Although virtually all of the titrations reported in this paper were carried out with 0.05M EDTA, good end points are obtained even when much more dilute EDTA is employed. The possibility of using arsenazo as a n indicator for microtitrations and as a reagent for the quantitative spectrophotometric determination of rare earths and uranium is now being investigated. LITERATURE CITED
(1) ilnderegg, G., Flaschka, H., Sallmann, R., Schwarzenbach, G., Helti. Chini. Acta 37. 113 11953). Banerjee, ' G . , A4nal. Chin[. A c t a 16, 56, 62 (1957). Baneriee, G., Z. anal. Cheni. 146, 417
(1955). Ihid., .147, 105 (1955). Ibid., 148, 349 (1955). Datta, S. K., Ihid., 149, 328, 333 (1956); 150, 347 (1956). Fritz, J. S., Ford, J. J., A s a ~ CHEAI. . 2 5 , 1610 (1953). Fritz, J. S., Lane, W. J., Bystroff, A. S., Ihid., 29, 821 (1957). Kuznetsov, V. I., Zhur. .Inal. Khzni. 7, 226 (1952).
RECEIVED for review August 12, 1957. Accepted January 24, 1958. Contribution 631. n'ork performed in Ames Laboratory, L-. S. Atomic Energy Commission.
Determination of Acids and Basic Nitrogen Compounds in Petroleum Products IRA KUKIN' Gulf Research & Developmenf Co., Piffsburgh, Pa.
b Nonaqueous titrations with visual indicators were used to determine acids and weakly basic nitrogen compounds, including oils which were too dark to b e readily titrated by existing color-indicator methods.
C
oLoa-indicator titrations, where applicable, are rapid and simple. When naphthenic acids were titrated b y a potentiometric method (ASTlI D 644) ( I ) , sharp inflection points could not be obtained when the acid number was less than 0.1; the indicator end point had to be approximated from a prePresent Address, L. Sonneborn Sons, Inc., 300 Fourth Ave., New York 10, X. Y. 1
1 1 14
ANALYTICAL CHEMISTRY
determined e.m.f. value. In such cases, it was more convenient to titrate to a n indicator end point, using a titration flask (6) which facilitated the detection of sharp visual end points. As no A S T l I method exists for determining weakly basic nitrogen conipounds in distillate fuels, a nonaqueous titration with methyl violet as the indicator ivas developed. ACID NEUTRALIZATION METHOD
Apparatus, Reagents, and Method.
BURETASSEMBLY.Two drying tubes containing Ascarite were sealed t o a 5-ml. microburet of t h e Koch type, graduated in 0.01-ml. subdivisions (Figure 1).
ALCOHOLIC SODIUM HYDROXIDE. Diesolve about 16 grams of sodium hydroxide (analytical grade) in 100 ml. of methanol-ethanol (1 to l), and filter off the small amount of insoluble carbonate. As required, prepare approximately 0.03.V titrant solution by diluting with absolute ethanol. KO discoloration or floc formed when the alcoholic solutions of sodium hydroxide were stored for about a year. hIETHOD. Weigh 20 grams of the Oil into the titration flask. Add benzeneisopropyl alcohol (1to 1) to theetchmark (120 ml.) on the flask. Add 5 drops of p-naphtholbenzein indicator (Q), 0.1% in methanol. Stir the solution with a magnetic stirrer, and titrate with 0.035 alcoholic sodium hydroxide. The rate of addition of sodium hydroxide can be
