420
ANALYTICAL CHEMISTRY
calcium n = 2.33 a t p H 11.70. By increasing the pH to 12.4
it was possible to obtain a solution that was composed primarily of the 3 to 1 calcium complex. The values given were all obtained from curves that were drawn a t 540 mp. Curves drawn a t 520, 530, 550, and 560 mfi are very similar and indicate the same values of n. A modified method of continuous variations that was used by Watters and Aaron (IO)in their study of copper pyrophosphate complexes was utilized in the present work in order to provide additional evidence for the presence of the 3 to 1 complex for magnesium. This is shown in Figure 5. The value of n, the number of ligands per cation, is obtained from the relation n = (2 - Xmax,)/(l- Xmx) where Xmx, represents the fraction of solution B present a t the point where the difference curve Y is a maximum. Since X,,, = 0.54, n = 3.18. This is reasonable since a maximum in the difference curve Y a t 0.50 would indicate that a 1 to 1 complex is formed between the 2 to 1 complex and the dye. This is the same as a 3 to 1 complex of dye with metal. A wave length of 640 m r was used because data a t 640 m r showed greater deviation from the straight line of no reartion than did data that was obtained when using wave lengths between 520 and 560 mu. Data obtained a t 630 and 650 mu resulted in curves that were very similar to those in Figure 5. This modified method of continuous variations was also used with calcium a t p H 11.7 and pH 12.4. The refiults were not so well defined as those for magnesium. Nevertheless, they indicate reaction between the 2 to l complex and the dye. CONCLUSIONS
T h e dye Eriochrome Black T forms 1 to 1, 2 to 1, and 3 to 1 complexes with magnesium and also with calcium.
Schwarzenbach and Biedermann (9) suggested that calcium and magnesium have a coordination number of 6 when they form 1 to 1 complexes with the dye. Their proposed formula for these complexes showed one bond with each of the phenolic oxygens, one with each of the azo nitrogens, and one with each of two water molecules. A possible explanation of the 3 to 1 complexes that are indicated in the present work is that the dye anion acts as a bidentate through the two phenolic oxygens or through one azo nitrogen and an adjacent phenolic oxygen, and that the alkaline earth metal ion has a coordination number of 6, thus forming an octahedral complex in whirh the spadl orbitals of the metal are involved in bond formation. LITERATURE CITED
(1) Beta, J. D., and Noll, C. A . , J . Am. Water W o r k s Assoc., 42, 49 (1950).
(2) Biedermann, W., and Schwarzenbach. G., Chimia (Switz.), 2, 56 (1948). (3) Cheng, K . L., Kurtz, T.. and Bray, R. H., ANAL.CHEY., 24, 1640 (1952). (4) . , Debnev. E. W.. Nature. 169. 1104 (1952). ( 5 ) Diehl, H., Goetz, C. -4., Hach, C. C.:J. Water W o r k s Assoc., 42. 40 - - (iwn.
Ah.
- 7
* - - - - I
(6) Harvey, A. E., Jr., Komarmy, J. AI., and Wyatt, G . M., ANAL. CHEM.,25, 498 (1953). (7) Hol, P. J., and Leedertse, G. C. H., Chem. Weekblad, 48, 181 (1952). (8) Kinnunen, J., and hIerikauto, B.. Chemist Analyst, 41, 76-9 (1952). (9) Schwarzenbach, G., and Biedermann, W., H e h . Chim. Acta, 31, 678 (1948). (10) Watters, J. I., andAaron, A, J . Am. C h e m Soc., 75, 611 (1953) REcmvan for review June 15, 1954. Accepted October 4, 1954. From a thesis presented to the Graduate School of The Ohio State University by Allen Young in partial fulfillment of the requirements for the degree of master of science.
New Organic Reagent for Silver and Copper BERNARD GEHAUF and JEROME GOLDENSON Chemical Corps Chemical and Radiological Laboratories, Army Chemical Center,
A red dye prepared from l-phenyl-3-methyl-5-pyrazolone, pyridine, sodium cyanide, and chloramine-T was found to give deep blue compounds with silver and cuprous ions. The sensitivity of a test for metals based on the use of this dye is 1 part in 600,000 for silver and 1 part in 250,000 for copper. As a reagent for silver, the dye can be used as an outside indicator for the titration of chlorides or silver.
I
N T H E course of a search for a satisfactory method of detecting microamounts of cyanides, a blue-green dye was obtained when a mixture of sodium cyanide, pyridine, and l-phenyl3-methyl-5-pyrazolone was treated with chloramine-T. While investigating the stability of this coloring matter under various conditions it was found that a red compound was formed when the blue dye was boiled for a short time in aqueous alkaline solution. Investigation of this new red compound, to which the name Zolon Red has been applied, proved that it is a true dyestuff with interesting properties as such but of even more interest because of its properties of forming deep blue insoluble compounds with silver and cuprous ions. This latter property was of sufficient interest to warrant an investigation of its possible uses as an organic analytical reagent. PREPARATION O F Z O W N RED
Four grams of pyridine and 18 g r a m of l-phenyl-3-methyl-5pyrazolone were stirred to a uniform paste and diluted with
Md.
250 ml. of water. Two and one half grams of sodium cyanide were then added, and while stirring, 250 ml. of an aqueous solution containing 14 grams of chloramine-?' were added over a period of 10 minutes. A red color formed immediately, rapidly changing through purple to blue. When all of the chloramine-?' had been added, the stirring was continued until tests made by placing a drop of the reaction mixture on filter paper gave a pure blue spot with no trace of a red ring. The mixture was alloTved to stand until a thick paste of dye separated. This was then filtered with suction and the filtrate, which retained a considerable amount of dissolved dye, was treated with 20 grams of sodium chloride, and the salted out dye was added to the original filter cake. After being pressed down well on the filter, the cake was washed once by displacement with an equal volume of water. The blue-black dye paste was then transferred to the original reaction vessel and broken up into a thin paste with a small amount of water. TKOhundred and fifty milliliters of water and 20 grams of sodium carbonate were added, and the niivture mas brought to boiling. This was continued until the conversion to a red dye was complete. The conversion was followed by placing drops of the reaction mixture on filter paper and observing the colors displayed by the spot. When a clear red spot with no blue or purple center was obtained, the heating was terminated and the mixture was allowed to cool to room temperature. The thick deposit of red fibrous crystals n-hich separated was filtered off with suction and washed twice by displacement with a volume of water equal to that of the filter cake. The wet cake, which consisted of the sodium salt of the dye, was broken up in 500 ml. of water and reprecipitated as the free acid of the dvestuff by adding a slight excess of 10% hydrochloric acid. Thebrickred precipitate was filtered with suction, washed thoroughly with water, and dried. The product was very slightly soluble in water, freely soluble in
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 a variety of organic solvents to an orange color, and soluble in aqueous alkaline solutions to a pure magenta color (absorption maxima, 490 to 530 mp). The alkaline salts of the dye, in addition to being water-soluble, were also soluble in various organic solvents such as acetone, alcohol, and pyridine. REACTIONS OF ZOLON RED WITH METALS 4ND SENSITIVITY
42 1
solutions when papers dyed with Zolon Red are used m an outside indicator and in the manner indicated. STRUCTURE OF ZOLON RED
The structure of Zolon Red has not been determined. However, it appears to be closely related to the blue dye from which it is prepared. This latter substance is a polymethine dye corresponding to the following general formula:
The reactions of Zolon Red Tvith metals were studied by adding
a saturated alcoholic solution of the free dye acid to aqueous solutions of the metals at approximately 0.1N dilution buffered with sodium acetate. Only silver and cuprous ions gave deep blue insoluble compounds. Gold gave a red purple precipitate after standing. Mercuric salts reacted after long standing to form a red flocculent precipitate. No reaction under the above conditions was obtained with the following metals: lead, zinc, cadmium, iron, nickel, cobalt, platinum, and palladium. The blue compounds of silver and copper when first formed appeared as highly dispersed colloids which, after standing for 8ome time, separated out as a blue-black precipitate. The colloidal suspensions showed a maximum absorption band in the visible spectrum a t 590 to 620 mp. When separated and dried, the solid wab almost black, with a strong metallic bronze reflection. The silver compound was found to be stable under ordinary conditions. The cuprous compound was unstable, owing to oxidation to the cupric state. Both the silver and cuprous compounds were destroyed by acids or salts that formed insoluble or nonionized complexes with the metals, the red dye being regenerated in the process. Iodides and thiocyanates will destroy the cuprous-Zolon Red compound. The sensitivity of Zolon Red to silver and cuprous ions was investigated. Ten milliliters of aqueous solutions of known metal content were buffered with sodium acetate and then teated by adding 6 drops of a saturated solution of Zolon Red in alcohol. Color changes were observed in a 0.75-inch test tube viewed vertically. The limit of sensitivity was found to be: silver, 1 part in 600,000; copper, 1 part in 250,000. The same sensitivity could be obtained on paper dyed with Zolon Red. In this case the buffered solutions of the metals were applied from a capillary tube to the paper; by this means the insoluble blue compounds were formed and concentrated in a small area. ANALYTICAL POSSIBILITIES O F ZOLON RED
While the analytical possibilities of Zolon Red have not been thoroughly explored, it has been shown that it can be used as an indicator in the volumetric titration of chloride with silver or vice versa. Although the blue silver complex is destroyed by chlorides mith regeneration of the red dye, a sharp end point cannot be obtained when it is used as an internal indicator because of adsorption of the dye and blue complex by silver chloride. However, the end point can be determined sharply with 0.1N
hi N,A c=o CH,-C-~=Crl(-CH=CH),-CI
A
””-?
;
-cH,
Three dyes of this type are known. The first, n = 0, is a yellow substance prepared by Knorr ( 2 ) . The second, n = 1, is magenta, and the third, n = 2, the dye under discussion, is blue green. The series is of interest because it illustrates in a striking manner the effect of the increased length of conjugated carbon chain in shifting the maximum absorption band in the spectrum from the short to the longer wave lengths. In this case, the shift is remarkably uniform (I). The same sequence of colors is also noted in the preparation of the blue-green dye. As prepared in this investigation, three distinct stages in the reaction can be observed. The first, in which the pyridine ring is ruptured by the action of cyanogen chloride, results in the formation of a yellow substance, enolized glutaconic aldehyde, which has the same auxochromophoric system as the yellow dye prepared by Knorr ( 2 ) . In the second stage of the reaction the aldehyde is condensed with one molecule of the pyrazolone, resulting in an unstable product that has an auxochromophoric system of seven conjugated carbon atoms. This is magenta in color, has weak dyeing properties, and forms blue compounds with silver ions. The resemblance of this compound, both in regard to its color and reactions with silver, to Zolon Red would seem to indicate that the latter also has an auxochromophore of seven conjugated carbons. A further effect of interest as possibly throwing some light on the composition of Zolon Red is the color that the blue-green dye displays when dissolved in concentrated sulfuric acid. This is a brilliant magenta, which reverts to the original blue-green when the solution is diluted with water and the free acid is neutralized with alkali. LITERATURE CITED
(1) Rurk, R. E., and Grummitt, 0.. “Advances in Nuclear and Theoretical Organic Chemistry,” p. 99, Chap. I V by Brooker L. G. S., Interscience, New York, 1945. (2) Knorr, L., Ann., 238, 184 (1887). RECEIVED for review July 14, 1954. Accepted December 3, 1954.
