( 4 ) Goldhaurn, L. R., ANAL. CHEM.24,
1604 (1952). ( 5 ) Gould, T. D., Hine, C. H., J . Lab. Clin. M e d . 34, 1402 (1949). (6) Lloyd, H. A., Fales, H. M., Highet, P. F., VanderHeuvel, W. J. A., Wildman, W. C., J . Am. Chem. Soc. 82, 3481, 3791 (1960). (7) Merck Index, 7th ed. Merck & Co., Inc., U. S. A. (1960).
(8) Royal Canadian Mounted Police Seminar No. 1, Feb. 1, 1954, Headquarters, Ottawa. (9) Smith, Ivor, ed., "Chromatographic and Electrophoretic Techni ues," vol. 1, p. 381, Interscience, J e w York, 1960. (10) Stevenson, G. W., California Associstion of Criminalists, Spring Meeting, 1958.
(11) Stewart, C. P., Stolman, A., 'Toxicology," vol. 1, p. 71, Academic Press, New York, 1960. RECEIVEDfor review May 18, 1961. Accepted July 12, 1901. Work supported by grants from the National Institutes of Health, U. S. Puhlic Health Service (RG-4372 and RG-5802), and from the Research Committee, University of California.
Naphthyl Azoxine S as a Complexometric Indicator JAMES S. FRITZ, JANET E. ABBINK, and MARILEE A. PAYNE Institute for Atomic Research and Departmenf of Chemistry, Iowa State University, Ames, Iowa
b Naphthyl Azoxine SI or NAS, is an excellent indicator for the complexometric titration of metal ions. This indicator can be used for titrations in either acidic or basic solution. Using NASI methods for the titration of 25 elements with EDTA are described. Various masking agents improve the selectivity of EDTA titrations with NAS indicator.
T
HE 7-arylazo derivatives of 8-hy-
droxyquinoline-5-sulfonic acid are excellent metal ion indicators for complexometric titrations (1). Of these, the 7-(l-naphthylazo)-8-hydroxyquinoline-5-sulfonic acid, which has been termed Naphthyl Azoxine, is the best. Unlike Eriochrome Black T, Naphthyl Azoxine can be used in acidic solution and is not blocked by metal ions such as copper(II), cobalt(II), and nickel(I1). 1-(2-Pyridylazo)3-naphthol (PAN) indicator is similar to Naphthyl Azoxine in its application, but in aqueous solution a t room temperature the color change of PAN at the end point of a titration is much slower than with Naphthyl Azosine. Guerrin, Sheldon, and Reilley (8) introduced the indicator, 7-(4-sulfo-lnaphthylazo) - 8 - hydroxyquinoline - 5sulfonic acid, which they called SNAZOXS. This indicator differs from Kaphthyl Azoxine only by a sulfonic acid group in the naphthalene part of the molecule. SNAZOXS retains all of the advantages of Naphthyl Azoxine, but can be used as a metal ion indicator in both acidic and basic solution. Naphthyl Azosine is a satisfactory indicator only in solutions more acidic than about p H 7. When the paper by Guerrin, Sheldon, and Reilley appeared, we had synthesized both the 4sulfo-1-naphthylazo and 6sulfo-2-naphthylazo derivatives of 7-aryl-8-hydroxyquinoline-5-sulfonic acid. However, we had chosen 7 - ( 6 sulfo - 2 naphthylazo) - 8 - hydroxy-
-
quinoline-5-sulfonic acid, which we named Naphthyl Azoxine S or NAS, for development as a metal ion indicator for titrations with EDTA UT other complexing titrants. Actually NAS and SNAZOXS are very similar in properties and application. It is very probable that the two indicators can be used interchangeably for the analyses reported below. One purpose of this paper is to indicate the metal ions that can be titrated with EDTA using NAS indicator. The successful titration of several elements not previously titrated with SNAZOXS or Naphthyl Azoxine is reported. Another purpose is to show how masking agents can be used in conjunction with NAS indicator to increase the selectivity of EDTA titrations. PREPARATION AND PURIFICATION OF INDICATORS
Prepare the indicators by diazotizing either Camino-1-naphthalenesulfonic acid sodium salt or 6-amino-2-naphthalenesulfonic acid sodium salt and coupling with 8-hydroxy-5-quinolinesulfonic acid. Dissolve 10.16 grams (0.021tl) of the amine in 20 ml. of water with gentle heating in a 100-ml. beaker. Add 20 ml. of concentrated HC1 and stir the pasty mixture while cooling to 0" to 5" C. in an ice bath. Dissolve 2.76 grams (0.02M) of sodium nitrite in 8 ml. of water and cool to 0" to 5" C. Slowly add the sodium nitrite to the amine solution through a funnel with its tip extending below the surface of the amine solution. Continue stirring and test for excess nitrous acid with starchiodide paper. Dissolve 9.00 grams (0.02Al) of finely ground 8-hydrosyquinoline-5-sulfonic acid in 40 ml. of water in a 400-ml. beaker. Cool the solution to 0" to 5" C. Slowly add the diazonium salt alternately with a 1.2N solution of NaOH cooled to 0" to 5" C., keeping the p H between 5 and 8. Stir for 10 to 30 minutes. Add NaCl to precipitate the
dye; collect the dye by suction filtration. To purify the dyes, dissolve in a minimum amount of water a t 70" C. Filter to remove any insoluble residue. Add dioxane a t 70" C. equal to four to five times the volume of water and cool. Filter by suction and dry the dye a t 100" C. overnight in an oven. Determine the molecular weight and the per cent of purity of the dyes by a potentiometric titration with 0.02M tetrabutylammonium hydroxide. Use a sleeve calomel with KC1-saturated methanol and glass electrode system. Standardize the tbutylammonium hydroside against benzoic acid. Dissolve approximately 70 mg. of the dye in 7 ml. of water and 25 ml. of acetone. Titrate potentiometrically with 0.02M tetrabutylammonium hydroxide. REAGENTS AND SOLUTIONS
NAS. Prepare a 0.5% aqueous solution of 7-(6-sulfo-2-naphtl1ylazo)-8-hydroxyquinoline-5-sulfonic acid, disodium salt. Metal Solutions. Prepare 0.05M aqueous solutions of the metal nitrates or perchlorates unless otherwise indicated. 0.05M S C + ~ Dissolve . 0.3455 gram of S C Z Oin~ concentrated HC1 and dilute to 100 ml. 0.05M TiC4. Dissolve 0.05111 Tic14 in concentrated HzSOd and evaporate to fumes of HzS04. Dilute back to volume. 0.05M VO+2. Prepare a 0.05M aaueous solution of vanadvl sulfate. 'Citrate. Prepare a io% aqueous solution of ammonium citrate. Tartrate. Prepare a 10% aqueous solution of ammonium tartrate. 2,CPentanedione. Prepare a 10% alcoholic solution. Fluoride. Prepare a 0.1M aqueous solution of sodium fluoride. 0.05M EDTA. Prepare an aqueous solution from the reagent grade disodium salt of (ethylenedinitri1o)tetraacetic acid. Standardize by titrating zinc nitrate (pure zinc metal as primary standard) a t p H 8 to 10 with Friochrome Black T indicator. VOL. 33, NO. 10, SEPTEMBER 1961
1381
(1 to 2 drops of 0.05M) t o return the indicator td a clear yellow. Immediately continue the titration with 0.05M EDTA, taking a sharp change from yellow to orange-pink or pink as the end point. Subtract a blank which is the milliliters of 0.05M EDTA equivalent to the copper nitrate added. When titrations are carried out using masking agents, add the quantity of masking agent indicated in Table I to the diluted sample. Then adjust the pH and carry out the titration as outlined above. Back Titration. Take a sample containing 0.2 t o 0.4 mmole of metal
PROCEDURES
Direct Titration. Take a sample containing 0.2 t o 0.4 mmole of metal ion to be titrated and dilute to approximately 100 ml. Add buffer (several drops of pyridine unless otherwise stated) and adjust the p H (using a p H meter) to approximately the value stated in Tables I or 11. Add 1 or 2 drops of NAS indicator solution and titrate with 0.05M EDTA until the indicator changes to its full pink color. (For some metal ions the color change is sharp; for others it is gradual.) Add enough copper nitrate
Table I.
Titration of Single Metal Ions
(B) Indicates back titration; all others are direct titrations
Comparison Method NAS Method Standard ml. Actual ml. Ion EDTA Conditions EDTA Conditions AIc3 5 . 0 8 f 0.01 (B) Cu, Pyrocat. Violet 5 . 0 9 i 0.01 (B) Cu, pH 8.4 UH 6.0-6.5 Bii3 5 . 0 8 f 0 . 0 1 Xilenol Or., pH 2 5.07 f 0 . 0 2 (B) Zn, PH 6 . 0 Ca+% 5.62 f 0.00 Metal phthalein, pH 10 5 . 6 4 i 0 . 0 1 Tnethanolamine buffer pH 10, 50-75% acetone Cd+* 5.01 f 0 . 0 1 Naph. hoxine, pH 6 5.01 f 0 . 0 1 PH 6 Cot' 5.01 f 0 . 0 1 Naph. Azoxine, p H 6 5.02 f 0 . 0 1 PH 6 Cu+' 5.05 Pure Cuo 5 . 0 6 i0 . 0 4 D Y + ~3.13 f 0.01 Naph, Azoxine, pH 6 3.13 f 0.01 Fe+* 5 . 2 1 f 0.00 (B) Bi, Xylenol Or., 5 . 2 0 f 0.02 (B) Zn, pH 5 . 5 PH 2 Ga+8 4 . 8 7 f 0.00 CU-EDTA, PAN, pH 4.88 f 0.00 (B) Zn, PH 6 2, titrate hot Hg+2 5 . 6 1 Pure HgO 5 . 6 4 f 0 . 0 3 (B) Zn, pH 6, acetate buffer 1n+3 4 . 5 1 0.01 4 . 5 0 f 0.00 (B)Cd, PH 5
;:6"
Mg+z 2.65 f 0.00
2 . 6 4 f 0.00
Mn+2 5 . 5 6 f 0.00
Eriochrome Black T, 5.56 f 0 . 0 1 pH 10, ascorbic acid
M o + ~ 2 . 6 2 f 0.01
2.62 f 0.00
(B) CU H 5-5.5, NH,& buffer, hydrazine sulfate (2 g J 1 tartaric acid (5 g.), 1 : 1 ( 2 ml.), boil 5 min.
g:), tartaric acid ' ( 5 g.), 1:1 sulfuric acid (2 ml.), boil 5 min., 1/3 volume
Ammonia buffer, pH 10, SO% Et. alcohol (25 ml.) pH 8 , 5 , Ascorbic acid
Ni+2
4.97 f 0.01
alcohol Naph. Azoxine, pH 6
4.98 f0.01
Pb+Z
5 . 0 0 f 0.01
Xylenol Or., pH 5 . 5 -
5.00 f0.01
pH 6, titrate slowly near e.p. pH 5.5-6.0
5 09 f 0 . 0 1
PH 6
5.01 f0.01
Acetate buffer, pH 4,
6.0
5.09 f 0 . 0 1
Pr+3
S C + ~ 5.01 f 0.00 Sn"
5 . 0 2 f 0.00
Ti+4
3 . 0 7 f 0.00
Th+4 4 . 8 7 f 0.00 VO+z 3 . 9 2 f 0 . 0 2 Y+a 5.14 f 0.01 Yb+a 5.85 f 0.01 Zn+z 5 . 3 3 f 0 . 0 2 Zr+'
1382
5.13 f0.02
0
Naph Azoxine, pH
5.5-6.0 Xylenol Or., pH 4
(B) Th, Xylenol Or., pH 2, add HC1 NaCl before EDTA (B) CU, PAN, pH 4-5, 3 drops H202
5 02 f 0 . 0 4
Xylenol Or., pH 2 (B) Th, Xylenol Or., pH 2 . 5 , ascorbic acid Naph. Azoxine, pH 6 Naph. Azoxine, pH 6 Xylenol Or., pH 6.0-
4 . 8 5 =k 0 . 0 1 3 . 9 3 f 0.00
+
6.3 (B) Bi, Xylenol Or., pH 1-2
ANALYTICAL CHEMISTRY
3.08 f0.01
5 . 1 4 f 0.01 5 . 8 6 f 0.01 5 . 3 4 f 0.01
fails
titrate slowly near e.p. (B) Cu, pH 3 . 5 , add HCl + NaCl before EDTA (B) Cu, pH 4-5, a c e tate buffer, 3 drops HzOz (B) Cu, PH 3 (B) Zn, pH 6, ascorbic acid PH 6 PH 6 PH 6 (B) cu, PH 3
ion to be titrated and dilute to approximately 100 ml. If the solution is strongly acidic, adjust the p H to approximately 2.0 with ammonium hydroxide. Add 10.00 ml. of 0.05M EDTA. Add a suitable buffer (several drops of pyridine unless otherwise stated) and adjust the pH (using a p H meter) with ammonium hydroxide or nitric acid to approximately the value stated in Table I. Add 1 or 2 drops of NAS indicator solution and back-titrate with 0.05M copper or zinc nitrate. Take the sharp change from pink to yellow or yellow-green as the end point. For titration of tin(IV), follow the above procedure but add approximately 2 grams of sodium chloride before diluting the sample or adjusting the pH. For titration of titanium(IV), follow the above procedure but add 3 drops of 30% hydrogen peroxide before adjusting the pH or adding EDTA. For titration of vanadium, reduce vanadium(V) to vanadium(1V) by addition of solid ascorbic acid. Then titrate vanadium(1V) according to the above procedure. For titration of molybdenum, reduce t o molybdenum(V) by adding 2 grams of hydrazine sulfate, 5 grams of tartaric acid, and 2 ml. of 1:1 sulfuric acid after adding excess EDTA according to the above procedure. Boil for 5 minutes, cool, adjust the pH to 5.0 to 5.5, and back-titrate with copper nitrate. Unlike other metal ions, 2 molecules of molybdenum react with 1 molecule of EDTA. SPECTRAL PROPERTIES
OF
NAS
From nonaqueous acid-base titrations in acetone-water, the molecular weight of purified 7-(6-sulfo-2-naphthylazo)-8hydroxyquinoline-&sulfonic acid, disodium salt (NAS) was 502 f 6 and the molecular weight of the disodium salt of 7-(4-sulfo-l-naphthylazo-8-hydroxyquinoline-5sulfonic acid (SNAZOXS) was 504 f 4. The theoretical molecular weight of the disodium salts of these indicators is 503. Spectral curves of Naphthyl Azoxine, NAS, and SNAZOXS indicators, and of their copper complexes were obtained a t p H 4, 6, 8, and 10. The wave lengths of maximum absorption are given in Table 111. The difference between the wave length of maximum absorption of the free indicator and of the copper complex is an indication of the sharpness of the color change in a titration. Table I11 shows that the three indicators are approximately equivalent in acid solution, but that NAS and SNAZOXS are much superior in alkaline solution. Comparison of the dominant wave length of the free indicator and copper complex is a better measure of the color contrast that can be obtained in a titration end point. The complementary color points for NAS and SNAZOXS, which are given in Table IV, show that these two-indi-
cators are approximately equivalent in their color contrast. In a direct titration with EDTA, NAS changes to a more orange shade of pink than does SNAZOXS, but the copper complex of NAS is a less orange-yellow than SNAZOXS. RESULTS
Results for the titration of individual metal ions with EDTA using NAS indicator are reported in Table I. The “actual” milliliters of EDTA required to titrate the metal ions are the average of 3 to 5 individual titrations; the precision index is the deviation from the mean. Metal ions that react rapidly with EDTA are best determined by a direct titration with EDTA; others are determined by a backtitration with a copper or zinc salt. In general the end point in a direct titration is improved by the addition of a small amount of copper(I1). Some metal ions require special conditions for titration. The titration of titanium(1V) is greatly facilitated by first complexing with hydrogen peroxide. The determination of tin(1V) necessitates the addition of a high concentration of sodium or potassium chloride to avoid precipitation during the formation of the EDTA complex. The titration of magnesium is satisfactory only if carried out in a solution containing a high proportion of alcohol. The titration of calcium is also improved by the addition of some alcohol. NrlS is a valuable indicator because it gives a sharp end point and can be used for the titration of a large number of single metal ions (or total metal ions in mixtures) with EDTA or other complexing titrants. Frequently EDTA titrations using NAS indicator can be made more selective through the use of masking agents. Data for typical titrations are presented in Table 11. T h e actual milliters of EDTA are the average of 3 to 5 individual titrations. Citrate masks iron(III), molybdenum(VI), thorium(IV), and zirconium(IV), and permits the titration of several divalent metal ions. In some cases the titration is good; in others it is fair. Tartrate successfully masks tungsten(V1) As reported in an earlier paper ( I ) , tartrate is a very good masking agent for uranium(V1) or antimony(II1) in EDTA titrations using either Naphthyl Azoxine or NAS indicator. Fluoride masks aluminum I
-_ Table II.
Use of Masking Agents in EDTA Titrations with NAS Indicator
Theory, Ion Ion MI. Actuala M1. Titrated Added EDTA EDTA Cd+a Al+a 5 . 2 7 5 . 2 6 f 0.00 C U + ~Al+* 5 . 1 9 5.18 f 0 . 0 1
Actualb M1. EDTA 5 . 2 5 f 0.00
...
fails
Pb+* A1+8 VO+2 Al+3
Y+3
5.05 5 . 0 5 f 0.00 3.97 fails 5 . 1 0 5 . 0 9 f 0.01
Yb+3 Al+3 Co+2 Cr+3 Cuf2 Cr+J Ni+2 C r + 3 Cd+z Fe+3
4.60 5.16 5.19 4.97 5.38
4 . 6 0 i 0.00 5.17 f 0.00 5.20 f 0.01 4 . 9 8 f 0.00 5 . 3 9 f 0.00
4 . 9 8 %l’O.OO 5.39 f 0 . 0 1
Cu+S Fe+’
5 . 2 6 5 . 2 5 f 0.00
5 . 2 7 It 0 . 0 1
Mn+2 Fe+3
5.65
Pb+2
4 . 8 0 4 . 8 0 f 0.00
Fe+3
Cd+* ZrC4 C U + ~ Zr+4 C O + ~Zr+‘
5 . 4 1 5 . 3 6 f 0.00 5 . 0 5 5 . 0 0 f 0.00 5.17 5 . 1 6 f 0.01
a
5 . 1 7 %‘O.OO
3 . 7 7 f 0.00 5.20 f 0.01 5119 Ooi 4 . 9 7 f 0.00 4 . 9 5 f 0.00 5.02 f O . 0 1 4 . 9 9 f 0.00 5.20 f 0.01 5.46 f 0.02
4 . 8 2 f 0.01
fails ...
... *..
... fails
... ... ...
- -
10.m1. citrate, 50% acetone, pH 8 . 5 10 ml. citrate, 50% acetone, pH 8.5 (B) CU, pH 3-5 10 ml. citrate. 9 ~ _. . ~ .DH ~ 2 g. NaC1, 25’rhl. NaF, pH 4 2 g. NaCI, 25 ml. NaF, pH 6 2 g. NaC1, 25 ml. NaF,. pH _ 6 10-ml. citrate, pH 8 10 ml. citrate. DH 6 . 5 10 ml. tartraie; pH 5-6 (B) Cu, 6 drops H20t, 50 ml. tartrate, pH 4 . 5 10 ml. citrate, pH 6 . 5 (B) 10 ml. Zn,citrate, 10 ml. pH citrate, 6 . 5 pH 9 ~~
4.99 2‘0.01
...
10 ml. 2,4pentsnedione, pH 8 Sod. acetate, pH 5 . 5 - 6 . 5 Sod. acetate, pH 6 . 8 Sod. acetate, pH 6 . 5 10 ml. citrate. 50% acetone, a---
...
3.75 5.19 5.79 4.97 4.93 5.03 5.00 5.19 5.48
~-~
5.10 2 ‘ 0 . 0 1
fails
Al+a Mg+2 C U + ~ MOO.-* Cu+z sn+iNi+2 Sn+* Zn+2 Sn+‘ Ni+2 Th+‘ Zn+l Th+‘ CU+’ Wad-' Ti+‘ WO4r2
Conditions
1 : 1 mole ratio of ion added to ion titrated. 5 : 1 mole ratio of ion added t o ion titrated.
Table 111.
PH 4 6 8 10
Comparison of Wave Lengths of Maximum Absorption
Naphthyl Azoxine DifferInd, Cu:Ind, ence, mp rnp mp 516 511 484 484
442 442 445 449
74 69 39 35
NAS Ind, mp
498 497 494 489
(111) and tin(1V) and permits the accurate titration of copper(II), nickel(II), zinc(II), etc. Fluoride also masks beryllium(II), niobium(V), and titanium(1V) (1). 2,4-Pentanedione makes it possible to titrate copper(II), lead(II), or yttrium(II1) without interference from aluminum(II1). The titration of several metal ions in the presence of chromium(II1) is possible in the presence of acetate. LITERATURE CITED
( 1 ) Fritz, J. S., Lane, U’. J., Bystroff, A. S.,ANAL.CHEM.29,821 (1957).
DifferCu:Ind, ence, mp mp 421 423 427 422
77 74 67 67
SNAZOXS DifferInd, Cu:Ind, ence, mp mp mp 444 445 449 452
512 504 501 496
68 59 52 44
Table IV. Complementary Color Points
NAS NAS :Cu SNAZOXS SNAZOXS:Cu
5
Y
0.139 0.144 0.163 0,142
0.224 0.086 0.277 0.127
(2) Guerrin, G., Sheldon, M. V., Reilley, C. N., Chemist Analyst 49,36 (1960).
RECEIVEDfor review May 1, 1961. Accepted July 10, 1961. Contribution No. 1014. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
VOL. 33,
NO. 10, SEPTEMBER
1961
1383
