Analyst, January, 1968, Vol. 93, &5. 13-19 13 Heterocyclic Azo Dyestuffs in Analytical Chemistry Part I. The Ligand Properties of 2-(2-Pyridylazo)-l-naphthol and its Sulphonated Analogues BY R. G. ANDERSON* AND G. NICKLESS (Department of Inorganic Chemistry, School of Chemistry, The University, Bristol 8) The preparation, from 2-hydrazinopyridine and the respective 1 , 2-naph- thaquinones, of 2-( 2-pyridylazo)-l-naphthol (a-PAN) and its derivatives con- taining sulphonic acid groups in positions 4 to 8 of the naphthalene ring, respectively, is described. The metal-complexing properties of the dyes are discussed in relationship to the position of the sulphonic acid group, and compared with those of 1-(2-pyridylaz0)-2-naphthol @-PAN). The dye with the sulphonic acid group in position 8 appears to be the most promising colorimetric reagent.All the new dyes are shown to be superior indicators to /3-PAN for the complexometric titration of copper. ~-(%PYRIDYLAZO)-~-NAPHTHOL (p-PAN) was first proposed as an analytical reagent in 1955 by Cheng and l3ray.l Since then it has been used in the complexometric and colorimetric determination of many metals, particularly in conjunction with solvent extraction pro- cedures.293 The dye is noted for its high sensitivity, the stability of its complexes, and the characteristic colour changes produced on chelation. It , therefore, seemed worthwhile to us to study other pyridylazonaphthol derivatives, and, in particular, those containing sulphonic acid groups, which can be used in purely aqueous media. A variety of other pyridylazonaphthol dyes has been studied with a view to use in analytical chemistry.In particular, sodium 2-pyridyldiazotate has been coupled with a-naphth~l,~ 2,7-dihydro~ynaphthalene,~ chromotropic acid,6 H-acid,6 2,3-dihydroxynaphtha- lene-6-sulphonic acid (6,7-dihydroxynaphthalene-2-sulphonic acid)' and 8-hydroxyquinoline.8 The resultant dyes have found limited applications in analytical chemistry. However, the reactions of sodium 2-pyridyldiazotate with phenols and naphthols are, in general, slow, and the diazotate fails to react completely with many phenolic substances. We have shown9 that 2-hydrazinopyridine reacts smoothly with 0- and 9-benzoquinone to give two pyridineazo dyestuffs not available by conventional synthetic procedures.This reaction has been used to prepare 2-(2-pyridylazo)-l-naphthol (a-PAN), an isomer of p-PAN, and its derivatives containing sulphonic acid groups in positions 4 to 8 of the naphthalene ring, respectively, and to study their possible analytical usefulness. / HO I HO a-PAN @-PAN EXPERIMENTAL Spectroscopic and potentiometric measwements-The techniques used to measure the stability constants and visible spectra of the complexes have been described previou~ly.~ All measurements were made in 60 per cent. aqueous methanolic solutions at an over-all ionic strength of 0-1 M and a temperature of 25" C for potentiometric or 20 & 2" C for spectro- scopic methods. Solutions of all dyes containing sulphonic acid groups were standardised by the technique of spectrophotometric titration before use.1° 2-(2-PyridyZazo)-l-na~hthoZ (a-PA N)-2-Hydrazinopyridine was prepared by the method of Fargher and Furness,ll and 1,Znaphthaquinone by the method of Fieser.l2 A 3.2-g sample of 1,2-naphthaquinone was dissolved in 200ml of 60 per cent.aqueous methanol and a * Present address: Department of Chemistry, The University, Leicester. 0 SAC and the authors.14 ANDERSON AND NICKLESS : HETEROCYCLIC A20 [ANalyst, Vol. 93 solution of 2-2g of 2-hydrazinopyridine in 40ml of G M hydrochloric acid added. When the resulting solution was neutralised with ammonia solution an 84 per cent. yield of a-PAN was precipitated. The product, a red - brown powder melting at 102" to 104' C, was found by thin-layer chr~matographyl~ to be homogeneous. (Found: carbon, 72-1 per cent. ; hydrogen, 4.6 per cent.; nitrogen, 16.7 per cent.Calculated for Cl,HllN30: carbon, 72.3 per cent.; hydrogen, 4-4 per cent.; nitrogen, 16.9 per cent.) A sample of the copper complex was prepared by heating together equimolar amounts of copper chloride and a-PAN in ethanol. The complex was precipitated from the solution as a dark, copper-coloured powder. (Found: carbon, 52-4 per cent.; hydrogen, 3.2 per cent.; nitrogen, 11.8 per cent. Calculated for CU.C~,H,~N,O.C~: carbon, 51-9 per cent.; hydrogen, 2.9 per cent; nitrogen, 12-1 per cent.) 1-(2-Pyridylazo)-2-naphthoZ (P-PAN)-A commercial sample was obtained from Hopkin and Williams Ltd., and was standardised by spectrophotometric titration before use.lO 2-(2-Pyridylazo)-l-na~hthol-4-sul~honic acid (a-PA N-4.S) , [4-hydroxy-3-(2-pyridyZazo)- naphthalene-1-sulphonic acid]-A solution containing 1.1 g of 2-hydrazinopyridine in 20 ml of water was added to a solution containing 2.6 g of sodium 1,2-naphthaquinone-4-sulphonate and 12 ml of 72 per cent.perchlolic acid in 100 ml of water. The resulting orange precipitate was dissolved in sodium hydroxide solution, filtered and re-precipitated with hydrochloric acid. a-PAN-4S was thus found by thin-layer chromatography to be an orange - red homogeneous powder.13 (Found: carbon, 52.3 per cent. ; hydrogen, 4.1 per cent. ; nitrogen, 12.6 per cent. Calculated for Cl,HllN,0,S.H20: carbon, 51.9 per cent.; hydrogen, 3.8 per cent. ; nitrogen, 12-1 per cent.) 2- (2-Pyridylazo) -1-naphthol-5-sulPhonic acid (a-PA N-5S), [5-hydroxy-6- (2-pyridy1azo)- nap ht hulene- 1 -sztl$honic acid] -2-Amino- 1 -naphthol-5-sulphonic acid (6-amino-5-hydroxy- naphthalene-1-sulphonic acid) was prepared as follows : benzene diazonium chloride was coupled with sodium 1-naphthol-5-sulphonate (5-hydroxynaphthalene-1-sulphonate) in weakly alkaline conditions.The resulting dye was dissolved in sodium hydroxide solution and reductively cleaved with sodium dithionite. The aniline produced was removed and the solution neutralised with hydrochloric acid, when a 72 per cent. yield of the amine was precipitated. A 0.98-g sample of potassium dichromate was dissolved in 2 M hydrochloric acid and 2.4 g of 2-amino-1-naphthol-5-sulphonic acid (6-amino-5-hydroxynaphthalene-1-sulphonic acid) added , which dissolved to give a solution of 1,2-naphthaquinone-5-sulphonic acid (5,6-naphthaquinone-l-sulphonic acid).2-Hydrazinopyridine (1.1 g) in water (20 ml) was then added to the quinone solution, when an immediate orange precipitate of dye was formed. This was purified via its sodium salt and was found by thin-layer chromatography to be homogeneous.n (Found: carbon, 52.7 per cent.; hydrogen, 3.7 per cent.; nitrogen, 11.8 per cent. Calculated for C,,H,,N,O,S.H,O: carbon, 51.9 per cent. ; hydrogen, 3.8 per cent. ; nitrogen, 12-1 per cent.) Although we tried to take melting-points the sulphonic acid reagents decomposed, presumably by losing the azo-nitrogens. Thus in this work we have found it impossible to take melting-points of any of the sulphonic reagents. The other sulphonated dyestuffs were prepared in exactly analogous ways, starting from the appropriate sodium naphthol snlphonate.All were orange-to-yellow dyes that were found to be homogeneous by thin-layer chromatography on cellulose.15 2-(2-Pyridylazo)-l-naphthol-6-sulphonic acid (a-PA N-6s) , [5-hydroxy-6-(2-$yridylazo)- naphthalene-2-sulphonic acid]-This was prepared from sodium 2-naphthol-6-sulphonate (6-hydroxynaphthalene-2-sulphonate) . (Found : carbon, 50.6 per cent. ; hydrogen, 3.7 per cent. ; nitrogen, 11.1 per cent. Calculated for Cl,HllN30,S.l&H20: carbon, 60.6 per cent.; hydrogen, 3.9 per cent.; nitrogen, 1143 per cent.) 2-(2-PyridyZazo)-l-na~hthol-7-sul~honic acid (a-PA N-7s) , [8-hydroxy-7-(2-pyridylazo)- naphttCtalene-2-suZphonic acid]-This was prepared from sodium 2-naphthol-7-sulphonate (7-hydroxynaphthalene-2-sulphonate) .(Found: carbon, 49-7 per cent. ; hydrogen, 4.0 per cent. ; nitrogen, 11-1 per cent. Calculated for C,,H,,N,0,S.2H20: carbon, 49.3 per cent. ; hydrogen, 4.1 per cent.; nitrogen, 11.5 per cent.) 2-(8-Py~idylaxo)-l-na~hthol-8-sul~honic acid (a-PAN-8S), [8-hydroxy-7-(2-pyridylazo)- naphthalene-1-sul~lzonic acid]-This was prepared from sodium 2-naphthol-8-sulphonate (7-hydroxynaphthalene-l-sulphonate) . (Found : carbon, 48.3 per cent. ; hydrogen, 4.2 perJanuary, 19681 DYESTUFFS IN ANALYTICAL CHEMISTRY. PART I 15 cent. ; nitrogen, 10.5 per cent.. Calculated for Cl5H,,N,O4S.2~H,O : carbon, 48.1 per cent. ; hydrogen, 4.3 per cent.; nitrogen, 11-2 per cent.) For the sulphonic acid derivatives, water is always present when the dyestuffs are formed, as described here, from aqueous solutions.We have tried therrnogravimetric studies to elucidate the actual number of water molecules, but the whole thermogravimetric analysis pattern is complicated not only because of the loss of water but also because of the loss of nitrogen through decomposition. These two losses occur in the same temperature region. The solvents used in the thin-layer chromatography were- (1) Fifty millilitres of 40" to 60" C light petroleum, 50 ml of diethyl ether and 5 ml of absolute ethanol. (2) Sixty millilitres of butanol, 20 ml of absolute ethanol and 20 ml of 2 M ammonia solution. (3) Forty millilitres of isopropyl alcohol, 80 ml of ethyl methyl ketone and 30 ml of 0.88 ammonia solution.RESULTS AND DISCUSSION 1,2-Naphthaquinone has been shown to give azo dyes of a-naphthol exclusively on reaction with aromatic hydrazine deri~atives.l*~~~ At various values of pH the visible spectra of a-PAN and of its complexes show a close similarity to those of its sulphonated derivatives, but are distinct from those of B-PAN. The amine produced from the reduction of the sodium mono-phenylhydrazone of 1,2-naphthaquinone-4-sdphonate was found by infrared spectro- photometry to be identical with that produced from the reduction of 2-phenylazo-l-naphthol- 4-sulphonic acid (4-hydroxy-3-phenylazonaphthalene-l-sulphonic acid) but to be different from l-amino-2-naphthol-4-sulphonic acid (4-amino-3-hydroxynaphthalene-1-sulphonic acid). a-PAN and P-PAN were found by thin-layer chromatography to be different compounds.Furthermore, we have found that 1-(2-benzothiazolylazo)-2-naphthol-6-sulphonic acid [5-(2-benzothiazolylazo)-6-hydroxynaphthalene-2-sulphonic acid] and 1,2-naphthaquinone mono-( 2-benzothiazolylhydrazone)-6-sulphonic acid are different compounds. On the basis of the evidence given below it is seen that azo dyes of l-naphthol only were obtained. We have previously described the use of both /3-PANlg and a-PAN and its sulphonated derivatives17 as chromatographic spray reagents. /%PAN was noted for its high sensitivity, giving pink, violet and green colours against a yellow background with many metal ions. a-PAN was found to be similar to /3-PAN in sensitivity, but it was noticed that the back- ground colour was orange - red, and the metal complexes had violet, blue and green colours.a-PAN-6S and a-PAN-7S were slightly less sensitive than a-PAN, whereas a-PAN-4S, a - P A N 6 and a-PAN-8S were more sensitive. It was noted that the spots contrasted particularly well when a-PAN-4S was used as a spray reagent. The visible spectra and acid-dissociation constants of the dyes are contained in Tables I and 11. In these tables, and in the ensuing discussion, reference to a cation, neutral molecule or anion of a sulphonated dye is made without regard to the sulphonic acid group, as this group remains ionised under nearly all normal experiment a1 conditions. All the dyes show hypsochromic and bathochromic shifts on protonation and ionisation, respectively. Table I and Fig. 1 show that the spectra of a-PAN and its sulphonated deriva- tives are essentially similar but distinct from those of /?-PAN.In general, /3-PAN is seen to TABLE I VISIBLE SPECTRA OF THE DYES Cation Neutral molecule (PH - 0) (PH - 6 ) Amax., mp E x 10-8 Amax., m p Q x r 1 8-PAN .. 425 16.2 470 17.2 a-PAN . . 460 14.8 482 16-8 a-PAN-4S .. 465 16-8 477 19.1 a-PAN-5S . . 463 16.6 485 19.1 a-PAN-6S .. 465 16-3 492 19-2 a-PAN-7S .. 465 16.6 487 19.1 a-PAN-8S .. 468 14-7 488 17.3 Anion r 495 13.2 514 21-6 498 20.9 610 22-3 513 22-5 61 2 22-4 632 22.3 (PH - 13) Amax., m p E x16 ANDERSON AND NICKLESS : HETEROCYCLIC AZO [Analyst, Vol. 93 Wavelength, mu Wavelength, mu Fig. 1. Absorption spectra of (a) u-PAN and (b) p-PAN, concentration lo-%, at A, pH 0; B, pH 6; C, pH 13; and D, the nickel (11) complex [1:1] at pH 9 absorb at somewhat shorter wavelengths than a-PAN.Further, although pK,, is about the same for both dyes, pKoH is considerably lower for a-PAN than it is for /3-PAN. This fact has a considerable effect on the behaviour of the two dyes in analytical chemistry. However, there is also a considerable variation in the bathochromic shift produced on ionisation among the sulphonated derivatives of a-PAN. This shift is smallest for CC-PAN-~S, about the same for a-PAN-5S, a-PAN-6S and a-PAN-7S, larger again for a-PAN and very large for a-PAN-8s. This trend is best considered by comparing the bathochromic shifts observed on passing from the cation to the anion. If the magnitude of this total batho- chromic shift is compared with the magnitude of pKNH or pKoH or better, log PZH (the sum TABLE I1 ACID-DISSOCIATION CONSTANTS OF THE DYES P-PAN .... .. U-PAN .. .. .. u-PAN-4S .. .. .. a-PAN-5S .. .. .. a-PAN-6S . . .. .. u-PAN-~S .. .. .. a-PAN-8S . , .. .. * Determined spectrophotometrically. ~ K N E PKOH 2.32* 12.00* 2*29* 10-00* 243* 8-63t 248* 9-13? 2*86* 10.447 2.39* 9-11? 2.46* 9-09? t Determined potentiometrically. 5 Fig. 2. Bathochromic shift (Av) plotted against log fig for the equilibrium, RH,+ + R- + 2H+, for various PAN derivativesJanuary, 19681 DYESTUFFS IN ANALYTICAL CHEMISTRY. PART I 17 of pKN, and pKoH), exactly the same relationship is noticed, the only exception being the pKNH of a-PAN, which is low. If log /3f is plotted against the shift in wavelength or fre- quency involved in removing two protons from the molecule (see Fig.2), a definite, if em- pirical, relationship between the two can be seen for /I-PAN, or-PAN and its sulphonated derivatives. Thus, the magnitude of the frequency shift is directly related to the ease of protonation, or the basicity, of the Ligand. Tables I11 and IV summarise results relating to the spectra and stability constants, respectively, of the metal complexes of the dyes. The copper complex of a-PAN is seen to absorb at longer wavelengths than does that of p-PAN. With the nickel and zinccomplexes there are shoulders to the main peaks that lie at longer wavelengths for the a-PAN complexes and at shorter wavelengths for the p-PAN complexes. These two facts explain why the complexes of a-PAN appear to be more blue in colour than those of /%PAN.As a result of the greater acidity of the hydroxyl group in a-PAN, it was found that the copper complex of a-PAN is thermodynamically considerably less stable than that of /3-PAN. It is observed that for a-PAN and its sulphonated derivatives the peak wavelengths for the metal-complex spectra are lowest for or-PAN-4S, about the same for a-PAN-SS, cc-PAN-GS, or-PAN-7S and a-PAN, and highest for a-PAN-8s. It is also observed that the log K, values for the metal complexes are qualitatively directly related to the bathochromic shift involved in passing from the cation to the 1 : 1 complex. Thus exactly the same behaviour was observed with the ligands on chelation as on ionisation. This is not unexpected, as the more basic a ligand is, normally the more stable are its chelates, assuming that steric and other effects are constant.I t would be unwise, however, to draw any hard and fast conclusions concerning TABLE I11 SPECTRA OF THE METAL COMPLEXES Zinc( 11) * Amax., mP P-P-4N . . 545 514 a-PAN .. 545 CX-P~~N-~S . . 528 - a-PAN-5S . . 543 - a-PA4N-6S . . 545 - a-PAN-7S . . 543 - K-PAN-~S . . 589 555 x 10-3 22.6 20.8 25.2 26-0 23.2 25-1 26.0 27.1 28-2 - - - - Copper( 11) - mp e x ~ O - ~ 650 20.8 566 21.2 555 22.6 567 22.5 570 22.3 569 22.0 582 22.4 Zinc( 11) Copper( 11) Nickel( 11) Cobalt (111) Palladium(II1 Amax., - - I - - - - - - - - - Nickel (11) Palladium (I I) * -7 mp E x1O-3 mp E xlOp3 552 18.6 - - 519 18.6 - 548 22.0 - 563 41.8 650 11.5 532 45.8 612 12.9 580 41.2 - - 547 46.1 631 9.1 582 42.6 675 8.9 549 45.3 630 10.7 579 41.8 670 9.5 547 45.5 628 10.6 592 43.3 703 12.5 556 43-8 652 13.1 Amax., Amax., - - - 1:1 a t p H 9 - 1: 1 at pH 5 - 1:2 at pH 9 - l : l a t p H 5 - 1:1 a t p H 5 Nickel(I1) - a-PAN and /?-PAN - 1 : 1 at hH 9 TABLE IV STABILITY CONSTANTS OF THE METAL COMPLEXES Copper (11) * log Kl p-PAN... . 17.0 IX-PAN.. . . 14.6 a-PAN-4S . . 13.8 a-PAN-6S . . 14-3 a-PAN-7S . . 14-5 a-PAN-8S . . 15.7 wPAN-~S . . 14.6 - - - 8-6 8.5 17.1 8.8 8.7 17.5 9.2 8.1 17.3 8.9 7.9 16.8 9.9 7.6 17-4 * Determined spectrophotometrically. t Determined potentiometrically. Cobalt (111) * Amax., mp E x ~ O - ~ - - - - - - 615 11.5 583 12.8 635 9-1 598 10.6 635 10.5 603 11.9 630 10.8 598 12.2 653 10.2 612 11.6 - - - 5.9 6.1 11.9 6.0 6.5 12.5 6.2 6-5 12-7 6.2 5-8 12.0 8.3 6.2 14.518 ANDERSON AND NICKLESS : HETEROCYCLIC A20 [Analyst, Vol.93 log K1 and the bathochromic shift discussed above, because the former refers to the reaction of the anion with the metal cation, for which the wavelength change involved is more or less the same for each dye. Nevertheless, as far as analytical applications are concerned, it is the shift from the cation or neutral molecule that is of greater interest. The relationship between complex stability and position of the sulphonic acid group discussed above cannot be applied to the log K, values, as these are abnormally small for a-PAN-7s and a-PAN-8S, because of the steric effects encountered in trying to pack two ligand molecules around one metal atom. It has thus been shown that the order of basicity of a-PAN and its sulphonated deriva- tives, with respect to proton dissociation and metal-complex formation, is as follows- a-PAN-4S < a-PAN-5S = a-PAN-7s < a-PAN-6S < a-PAN < a-PAN-8S (The results show that a-PAN-6S is slightly more basic than a-PAN-5s or a-PAN-7s.) The charge distribution in the 1-naphthol molecule is such that electron-withdrawing substituents will have a maximum acid-strengthening effect on the hydroxyl group if they occupy positions 2 or 4 or, to a lesser extent, 5 or 7 of the naphthalene ring.The sulphonic acid group is just such a group, and this explains why a-PAN-4S is the most acidic ligand, a-PAN-6S is slightly more basic than a-PAN-5s or a-PAN-7S, and all four ligands are more acidic than a-PAN itself.To explain the high basicity of a-PAN-8S, one must consider the sulphonic acid group itself. In position 8 of the naphthalene ring, it is close enough to the hydroxyl group for the sulphonic oxygen atoms to shield the hydroxyl proton, possibly with the formation of a weak hydrogen bond, and thus raise its acid-dissociation constant. For the same reason, a high electron density around the phenolic oxygen atom causes it to form stronger bonds with metals. Through internal hydrogen-bonding, this trend of basicities is also transmitted to the heterocyclic nitrogen atom, thus enhancing further the difference in chelating power of these ligands. All the complexes of the sulphonated dyes have molar extinction coefficients that are close to those of a-PAN and slightly higher than those of /3-PAN.As a colorimetric reagent, therefore, a-PAN-8S seems to be the most promising of the new dyes, because of its high pKoH value, the high stability of its complexes and the large bathochromic shift produced on chelation. Perhaps the most familiar use of /3-PAN is as an indicator for the direct complexometric titration of copper with EDTA.2 Further, excess of EDTA may be back-titrated with standard copper solutions by using P-PAN as indicator, and the copper complex of /I-PAN has also been used as an indicator for the titration of metals that form only weak complexes with /I-PAN. The end-point of the complexometric titration of copper with /&PAN is slow because of the small difference in stability between the copper - P-PAN complex and the copper - EDTA complex18 (log K, = 18*3), and also because of the insolubility of the copper - /%PAN complex.Heating of the solution and the addition of organic solvents have been recom- mended to produce a faster colour change at the end-point.19 With a-PAN and its sulphonated derivatives, however, not only is the gap in stability larger, but there is also the possibility of forming water-soluble copper - indicator complexes. It was to be expected, therefore, that these dyes would be better indicators for the titration of copper. Further, although the copper complexes of these dyes are less stable than that of /I-PAN, because of the greater acidity of the dyes, chelation was found to take place at about 0.2 pH units lower than Copper solutions (0-01 M) were titrated directly against EDTA at a pH of 4.5 to 6.0 first with /3-PAN and then a-PAN and its sulphonated derivatives as indicators.All the dyes of 1-naphthol gave faster colour changes of from blue - violet to green at the end-point than /3-PAN, and it was, therefore, concluded that the new dyes were an improvement on /I-PAN for the titration of copper. The difference between the stability of the zinc - /3-PAN complex20 (log K, = 11.2) and the zinc - EDTA complexl8 (log K1 = 16.1) is large, and the titration of zinc with P-PAN as indicator is satisfactory. With a-PAN and its sulphonated derivatives the gap in stability is even larger still, and, in this case, displacement of the metal from the indicator complex begins to occur before the end-point is reached.As a result, the end-point becomes less easy to see if the solutions are more dilute than about 0.1 M, and consequently /%PAN is considered to be the best indicator for the titration of zinc. with p-PAN.January, 19681 DYESTUFFS IN ANALYTICAL CHEMISTRY. PART I 19 The dye produced from the reaction of sodium 2-pyridyldiazotate with l-naphthol has been studied as an analytical reagent for metal ions.* The dye gives sensitive colour changes with a wide range of metals and forms extractable complexes of high stability with copper and zinc. This dye has been assumed to be 4-(2-pyridylazo)-l-naphthol. A slightly impure sample was prepared by condensing 2-hydrazinopyridine with 1,4-naphthaquinone. The metal-complexing reactions of this dye were found to be most undistinguished and quite unlike those reported for the dye prepared from l-naphthol.As a chromatographic spray reagent, the compound gave colours only with metals in groups VIII and Ib of the Periodic Table, and these were of a low sensitivity. It was a yellow dye with a pKNK value of 4.0 in 50 per cent. aqueous methanol, as opposed to the value of 2.54 in 50 per cent. aqueous dioxan found for the dye prepared from the diazotate. Further, 4-(2-pyridylazo)-l-naphthol failed to give the distinctive green complex with cobalt, produced by all the other pyridylazo- naphthol dyes. D. Betteridge (in a private communication) has subsequently shown by infra- red spectroscopy that the dye obtained by Betteridge, Todd, Fernando and Freisefl was, in fact, identical with the 2-(2-pyridylazo)-l-naphthol described in this paper.The dye has recently been suggested as an extractive indicator in the titration of EDTA with copper.21 1. 3. 4. 6. 8. 9. 10. 11. 12. 13. 14. 15. 16. > Y. 3. r- 4 . 17. 18. 19. 20. 21. REFERENCES Cheng, K. L., and Bray, R. H., Analyt. Chem., 1955, 27, 782. Anderson, R. G., and Nickless, G., Analyst, 1967, 92, 207. Busev, A4. I., and Ivanov, V, M., J . Analyt. Chem., USSR, 1964, 19, 1150. Betteridge, D.. Todd, P. K., Fernando, Q., and Freiser, H., Analyt. Chem., 1963, 35, 729. Sommer, L., 2. analyt. Chem., 1960, 171, 410. Sommer, L., and HniliCkovA, M., Naturwissenschaften, 1958, 45, 544. HniliEkovP, M., and Sommer, L., 2. aizalyt. Chem., 1960, 177, 425. Busev, A. I., Ivanov, V. M., and Talipova, L. L., Zh. Analit. Khim., 1963, 18, 33. Anderson, R. G., and Nickless, G., Analytica Chim. Acta, 1967, 39, 469. Pease, B. F., and Williams, M. B., Analyt. Chem., 1959, 31, 1044. Fargher, R. G., and Furness, R., J . Chem. Soc., 1915, 107, 691. Fieser, L. F., Org. Synth., 1937, 17, 68. Pollard, F. H., Nickless, G., Samuelson, T. J., and Anderson, R. G., J . Chromat., 1964, 16, 231. Zincke, Th., and Bindewald, H., Bey. dt. chem. Ges., 1884, 3026. Kamel, M., and Amin, S. A., Indian J . Chem., 1964, 2, 232. Pollard, F. H., Nickless, G., and Jenkins, H., in West, P. W., Macdonald, A. M. G., and West, T. S. , Editors, “Analytical Chemistry 1962 : The Proceedings of the International Symposium, Birmingham, in Honour of Fritz Feigl,” Elsevier Publishing Company, Amsterdam, London and New York, 1963, p. 160. Pollard, F. H., Nickless, G., and Anderson, R. G., Talanta, 1966, 13, 725. Schwarzenbach, G., and Freitag, E., Helv. Chim. Acta, 1951, 34, 1503. Cheng, K. L., Analyt. Chem., 1958, 30, 243. Corsini, A., Yih, I. Mai-Ling, Fernando, Q., and Freiser, H., Ibid., 1962, 34, 1090. Retteridge, D., Talanta, 1966, 13, 1497. Received June 14th, 1967