New Catalysts for the Borohydride Dyeing Process - American

Nov 8, 2010 - 5000 Monastir, Tunisia, Higher Institute of Fashion of Monastir, Stah ... and Société Industrielle des Textiles (SITEX), 5070 Ksar Hel...
4 downloads 0 Views 669KB Size
Download PDF
Ind. Eng. Chem. Res. 2010, 49, 12333–12338

12333

New Catalysts for the Borohydride Dyeing Process N. Meksi,*,†,‡ M. Ben Ticha,† M. Kechida,§ and M. F. Mhenni† Research Unit of Applied Chemistry and EnVironment, Faculty of Sciences of Monastir, 5000 Monastir, Tunisia, Higher Institute of Fashion of Monastir, Stah JabeursRoute Korniche, 5019 Monastir, Tunisia, and Socie´te´ Industrielle des Textiles (SITEX), 5070 Ksar Hellal, Tunisia

In the borohydride dyeing process, indigo cannot be reduced by sodium borohydride to its reduced form without the addition of catalyst. This catalyst, which is a metallic salt, is used to activate the reduction procedure of the reducing agent. So, the reduction reaction of indigo depends significantly on the nature of this catalyst. In this paper, the effect of 12 different metallic salts on the performances of the indigo reduction reaction has been discussed. These performances were evaluated by measuring the indigo reduction yield as well as the color yield (K/S) of the dyed samples of cotton. In these studies, it was found that the copper-based catalysts were the best and offered maximum performance. 1. Introduction Currently, indigo (C.I. Vat Blue 1) is a dye which is mainly used to color cotton yarns for denim fabrics. This dye is insoluble in water. Dyeing textile with indigo involves usually a dissolving step of the dye. This step consists of the reduction of indigo by a reducing agent in the presence of an alkali such as sodium hydroxide.1-4 Thus, a water-soluble leuco form of indigo (leuco-indigo) is obtained which can be used to dye textile fibers. After soaking it in the reduced dye solution (dyeing bath), it is important to expose the textile out in the air to oxidize the dye back to its insoluble form. These two steps (immersion in the dyeing bath/exposure to air) is repeated many times to achieve a dark blue color. Classical processes for dyeing by indigo use generally sodium dithionite as reducing agent (Figure 1). These processes present several disadvantages:5-8 (a) ecological problems resulting from the quality of the wastewaters (some of the byproducts formed in the decomposition of sodium dithionite are sulfur compounds which can heavily contaminate the environment, and the pH of wastewaters generated from these dyeing processes is very high (pH ) 12-14); so, this requires great quantities of acids for the neutralization); (b)technical problems such as the problem of the storage of the reagents, especially sodium dithionite, the difficulty of control of the dyeing bath, and the color variation of the dyed fabrics, etc. In a previous work,9 we developed a novel process to reduce indigo by sodium borohydride in the presence of potassium nickel cyanide K2Ni(CN)4 as catalyst. The present process gives the opportunity to reduce indigo and all kinds of vat dyes in total absence of alkali. The reduction of indigo and the cotton dyeing with the leuco-indigo form prepared in these conditions (pH range ) 9-10.5 and temperature ) 55 °C) could offer several economic and ecological advantages with a good preservation of cotton fiber qualities. In the borohydride dyeing process, it is necessary to use catalyst. This catalyst is a metallic salt. Without it, sodium borohydride fails to reduce indigo. This is attributable to the strong stability of the indigo carbonyl groups. As explained in the literature,10-12 as vat dye, indigo has a conjugated molecular * To whom correspondence should be addressed. E-mail: [email protected]. † Faculty of Sciences of Monastir. ‡ Higher Institute of Fashion of Monastir. § SITEX.

structure. The indigo atoms have sp2 hybridization. These properties as well as the presence of intra- and intermolecular hydrogen bonds in its structure confer to the indigo molecule a great stability and consequently a low chemical reactivity of their carbonyl groups. So, the reduction of indigo carbonyl groups with hydride ion H- generated by sodium borohydride becomes very difficult in this case. However, it is very probable that the addition of metallic salt in the medium leads to the creation of chemical interactions between the metal atom of the catalyst and the oxygen atom of the indigo carbonyl group. These interactions provoke perturbation on the electronic cloud of the carbon atom of the indigo carbonyl group. Thus, the nucleophilic center existing in this carbon atom could be activated. So, this can facilitate considerably the attack of H-. Consequently, the reduction of the indigo carbonyl group becomes very possible. It appears here that the role of the catalyst consists probably of activating the reduction procedure of sodium borohydride. So, the performances of the indigo reduction reaction depend not only on the catalyst amount in the medium9 but also on the nature of this metallic salt used, i.e., its cation and its anion. In this work, we try to study the effect of the catalyst nature on the reduction yield and the cotton dyeing quality. Then, we determine the best catalysts for the borohydride dyeing process which offer the most excellent performances. 2. Experimental Section 2.1. Chemicals and Materials Used. Indigo (C16H10N2O2, BEZEMA AG) was commercial grade. Sodium borohydride (NaBH4, Acros Organics) and sodium hydroxide (NaOH, Kaustik JSC) were laboratory grade. All of these chemicals were used for the reduction without further purification. Calcium chloride hydrate (CaCl2 · 2H2O, PS Park), manganese(II) chloride hydrate (MnCl2 · 4H2O, Riede-deHaen), iron(III) chloride (FeCl3, Riede-deHaen), cobalt chloride hydrate (CoCl2 · 6H2O, Fluka), nickel chloride hydrate (NiCl2 · 6H2O, Fluka), zinc chloride (ZnCl2, Aldrich), copper chloride hydrate (CuCl2 · 2H2O, Acros Organics), copper sulfate hydrate (CuSO4 · 5H2O, Fluka), copper nitrate hydrate (Cu(NO3)2 · 3H2O, Panreac Quimica SA), copper bromide (CuBr2, Fluka), and copper acetate hydrate (Cu(CH3COO)2 · H2O, Riede-deHaen) were used as catalysts for the indigo reduction reaction. Potassium nickel cyanide (K2Ni(CN)4) was synthesized using potassium cyanide and nickel sulfate as described in the

10.1021/ie100974d  2010 American Chemical Society Published on Web 11/08/2010

12334

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

Figure 1. Dyeing process by indigo.

literature.13 The potassium nickel cyanide obtained was purified as follow: It was washed in chloroform and filtered to remove residual potassium cyanide. Then, the obtained product was recrystallized in methanol. Setamol WS (BASF AG) and potassium hexacyanoferrate (K3Fe(CN)6, Riede-deHaen) were used for the titration of leucoindigo. Commercially bleached but unfinished cotton fabric with the following specifications was supplied from SITEX, Tunisia: plain weave; ends per inch, 33.02; picks per inch, 38.1; warp count, 10.5 open end; weft count, 15 open end; weight, 204 g/m2. 2.2. Reduction of Indigo by Sodium Borohydride. A solution containing 0.34 g of indigo and 1.41 × 10-5 mol of catalyst was prepared by adding them to 130 mL of distilled water. This solution was stirred and heated to 55 °C. Then, 0.34 g of sodium borohydride (previously dissolved in 20 mL of distilled water) was added. After 5 min, 20 mL of distilled water was introduced. This indigo reduction was carried out at 55 °C and was performed under nitrogen atmosphere to prevent oxidation. The redox potential and pH in the reduction medium were measured every 3, 5, or 10 min using respectively a Pt electrode versus (Ag/AgCl, 3 mol/L KCl) as a reference electrode with a potentiometer (Metrohm pH meter 744) and a pH meter (Knick pH meter 765 Calimatic). Generally, we supposed that the end of the reduction reaction was reached when the redox potential in the solution rose rapidly and then remained quasi stable. 2.3. Evaluation of the Lost Water by the Hydrolysis Reaction. In the reduction of indigo by sodium borohydride, the volume of water in the dyeing bath decreased. So, when the reduction of indigo was achieved, the compensation for this loss was accomplished through the addition of measured distilled water at the same temperature of the reduction to the initial volume of 170 mL (this weight of the amount of added water was measured). 2.4. Determination of the Indigo Reduction Yield. The molar concentration of the reduced indigo form in the dyeing bath was determined by potentiometric titration with potassium hexacyanoferrate (K3Fe(CN)6) analogous to the method described in ref 9. This was utilized to calculate the indigo reduction yield using the following equation: R/% )

Cli × 100 Cin.

where Cli is the molar concentration of leuco-indigo and Cin. is the molar concentration of indigo initially introduced into the medium. 2.5. Dyeing Process. The reaction medium obtained after the reduction procedure was used as a dyeing bath. The fabrics were dyed in the way principally similar to that reported in ref 9.

2.6. Dyeing Quality Evaluation. The dyeing quality was evaluated using a color yield parameter (K/S). So, the reflectance of the dyed samples was measured at 660 nm on SpectroFlash SF300 spectrophotometer with dataMaster 2.3 software (Datacolor International). Then, the K/S value was determined according to the Kubelka-Munk equation:14-16 K/S )

(1 - R0)2 (1 - R)2 2R 2R0

where R is the decimal fraction of the reflectance of dyed fabric, R0 is the decimal fraction of the reflectance of undyed fabric, K is the absorption coefficient, and S is the scattering coefficient. 3. Results and Discussion The aim of the present work is to determine the best catalysts for the borohydride dyeing process. The work consists of two steps. First, the anions of several metallic salts were kept as chloride Cl-, we investigated the effect of the nature of their cation on different parameters controlling the dyeing bath performances (the oxidoreduction potential of the dyeing bath and the indigo reduction yield) as well as the dyeing quality of the colored fabrics (K/S), and we compared the obtained results with those of K2Ni(CN)4, the catalyst initially used in the previous study. In this part, the following metallic salts were studied as catalysts for the reduction reaction of indigo by sodium borohydride: CaCl2, MnCl2, FeCl3, CoCl2, NiCl2, CuCl2, and ZnCl2. From the metallic salts above, the best catalyst and especially its cation were determined. Then, in the second part of this work, this catalyst cation was used and kept constant. Then, we investigated the effect of the nature of its anion on the dyeing bath performances and the dyeing quality of the colored fabrics. The obtained results were compared, as was done previously, with those of K2Ni(CN)4. 3.1. Effect of the Nature of the Catalyst Cation. 3.1.1. Effect of the Nature of the Catalyst Cation on the Evolution of the Oxidoreduction Potential of the Dyeing Bath. For the chloride-based catalysts cited previously, after the addition of the reducing agent (NaBH4), the evolution of the redox potential of the medium with time was studied. The purpose of this study is to determine the reduction reaction duration. This parameter represents the time necessary to consume all of the amount of sodium borohydride present in the medium. This time is generally indicated by the following: (a) disappearance of the soap from the solution area which is formed essentially by the generation of hydrogen gas (this hydrogen gas is produced by the hydrolysis reaction of sodium borohydride); (b) the quasi stability of the redox potential of the medium which occurs after a rapid jump step. The experimental results of this study are shown in Figure 2. In this figure, it can be seen that both the rapid jump and the

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

Figure 2. Effect of the nature of cation catalyst on the evolution of redox potential of the dyeing bath.

quasi stability steps of the redox potential are very clear in the case of K2Ni(CN)4, whereas they are slightly visible in the case of NiCl2 and CoCl2. So, for these later catalysts, we suppose that the quasi stability of the redox potential indicates the end of the reaction of reduction, and the time of the beginning of this quasi stability step is noted as the reduction reaction duration. However, in the case of CuCl2, the rapid jump step of the redox potential is visible but its final step of quasi stability is not observable. This can be probably explained by the appearance of secondary reactions in the medium which occur between copper and several forms of borates after exhausting all of the amount of sodium borohydride. These forms of borates can be produced by both the indigo reduction and the hydrolysis reaction.17-19 So, for CuCl2, we suppose that the rapid jump step of the redox potential indicates the end of the reaction of reduction, and the time of the beginning of this rapid jump step is taken as the reduction reaction duration. Besides, it was observed that this time corresponds really to the visual indicator of the end of the reduction reaction which is stopping of hydrogen gas generation in the medium (exhausting of the sodium borohydride amount in the medium). Contrary to the situation of the previous catalysts, the redox potential of the reduction medium in the case of the following metallic salts CaCl2, MnCl2, FeCl3, and ZnCl2 exhibits a different behavior. In Figure 2, both the rapid jump step and the final step of quasi stability of the redox potential for these metallic salts are not observable. We judged it is good to stop the reaction reduction after duration of 180 min. The absence of the rapid jump step in this case can be probably attributed to the presence of secondary reduction reactions in the medium between metals and sodium borohydride. In Figure 2, it can be also seen that, with the exception of NiCl2, all of the obtained curves are different from the curve of K2Ni(CN)4. It appears that the nature of the catalyst affects significantly the evolution of the redox potential of the reduction medium. At the end of the reduction reaction, the volume of the medium was adjusted by addition of measured distilled water to the initial volume of 170 mL as described in the experimental part. Then, the potentiometric titration with K3Fe(CN)6 was effectuated in order to determine the molar concentration of the reduced indigo form in the medium as well as the indigo reduction yield. It appears after that only the reduction reactions effectuated with CoCl2, NiCl2, and CuCl2 as catalysts gave concentration of the reduced indigo form non-null.

12335

Figure 3. Effect of the nature of cation catalyst on the reaction duration.

Figure 4. Effect of the nature of cation catalyst on the appearance of the reduced form of indigo (the beginning of indigo reduction).

For the case of MnCl2 and FeCl3, the obtained amounts of the reduced indigo form were so small to be detected by potentiometer. For the case of catalysts allowing an appreciable reduction of indigo, their reaction durations have been determined, and the corresponding results are shown in Figure 3. Each value presented in this figure is an arithmetic mean calculated by starting from the results of two tests. This figure shows that the reductions effectuated with the two catalysts containing nickel were achieved before those catalyzed by CoCl2 and CuCl2. On the other hand, the appearance of greenish color (the color of the reduced form of indigo) during reaction is a good indicator attesting that the reduction of indigo by sodium borohydride took place. For each studied catalyst, we noted the moment when this green color appeared. The obtained results are reported in Figure 4. Each value presented in this figure is the mean calculated from the result of two experiments. Figure 4 reveals that the reduction of indigo by sodium borohydride starts more quickly (2 min only) in the presence of CuCl2 than in the presence of the other catalysts studied, including K2Ni(CN)4. However, this reduction reaction is achieved last after those catalyzed by metallic salts containing nickel (see Figure 3). This can be attributed to the difference in kinetics between the reduction reaction of indigo and the hydrolysis reaction for each kind of studied catalyst. In addition, in the case of MnCl2 and FeCl3, it was observed that a small greenish nuance appeared after respectively 70 and 38 min. But, in the case of CaCl2 and ZnCl2, the greenish color was not observed. This confirms that the reduction of indigo in the presence of these two metallic salts did not happen. 3.1.2. Effect of the Nature of the Catalyst Cation on the Reduction Yield. The main reaction of the indigo reduction

12336

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

Figure 6. Effect of the nature of cation catalyst on the color yield.

Figure 5. Effects of the nature of cation catalyst on reduction yield and the hydrolysis of sodium borohydride. Table 1. Ionization Potential and Ionic Radius of the Studied Cations22 cation 2+

Ca Mn2+ Fe3+/Fe2+ Co2+ Ni2+ Cu2+ Zn2+

ionization potential (V)

ionic radius (Å)

11.868 15.636 30.643/16.18 17.05 18.15 20.29 17.96

99 80 64/74 72 69 72 74

is accompanied by a competitive reaction which is the hydrolysis of sodium borohydride. These two reactions were estimated respectively by calculating the reduction yield of the indigo and evaluating the loss of water from the dyeing bath, the same techniques as described in ref 9. For each chloride-based catalyst, the indigo reduction yield and the loss of water were calculated. The obtained experimental results are reported in Figure 5. Each point represented in this figure is the mean value of two experiments. The experimental deviation of the evaluation of the weight of lost water and the indigo reduction yield are also given. Figure 5 reveals that the copper-based catalyst is the catalyst which gives the maximum reduction yield. This can be probably attributed to the strong stability of the complex formed through the interactions created between copper and the oxygen atom of the indigo carbonyl groups. The origin of this strong stability was explained in the rule of the complexes stability suggested by Irving and Williams.20,21 Indeed, with the exception of iron(III) that has problems related to the stability of its ion in aqueous medium, Irving and Williams showed that, for the divalent series of metals Ca, Mn, Fe, Co, Ni, Cu, and Zn, when the ionic radius decreases, the ionization potential increases (Table 1).22 Consequently, the stability of the complexes increases gradually to reach a maximum at the copper complexes and then decreases with zinc: Ca < Mn < Fe < Co < Ni < Cu > Zn Thus, the complex (indigo-copper) seems probably to be the most stable complex in comparison with the complexes formed between indigo and the other metals. As a result, the reduction of the indigo becomes more effective and the reduction yield increases. Figure 5 reports also the effect of the nature of the catalyst cation on the loss of water. In this figure, it can be observed that the consumption of water is more important in the case of catalysts CaCl2 and MnCl2. Generally, it can be noticed that

Figure 7. Evolution of the mean value of the reduction medium pH versus the nature of the cation catalyst.

this loss depends not only on the nature of the cation used but also on the duration of the reduction reaction. 3.1.3. Effect of the Nature of the Catalyst Cation on the Dyeing Quality. The obtained mediums after the indigo reduction reactions effectuated in the presence of chloride-based catalysts were used as dyeing baths for cotton fabrics. Then, we evaluated the dyeing quality of the colored samples by measuring K/S at 660 nm. The experimental results of the influence of the catalyst cation on the color yield are reported in Figure 6. The values represented in this figure are the mean of two experiments. In this figure, it can be seen that copper offers the maximum color yield. This can be attributed to the maximum reduction yield obtained when CuCl2 was used as catalyst for the reduction of indigo by sodium borohydride (Figure 5). In addition, the small difference in the color yield values between the two nickel-based catalysts K2Ni(CN)4 and NiCl2 can be explained by the difference between the pH of their medium (see Figure 7). This difference in pH can probably affect slightly the absorption of the dye by cotton. 3.2. Effect of the Nature of the Catalyst Anion. From the previous study of the influence of the nature of cation catalyst on the performances of the indigo reduction by sodium borohydride and the quality of the dyeing, it was found that, among the chloride-based catalysts studied, the salt containing copper CuCl2 gives the maximum yields. Thus, we keep in this part the catalyst cation to copper, and we study the effect of its anion on the evolution of the potential redox, the indigo reduction yield, and the color yield. The studied anions are as follows: the chlorides (Cl-), bromides (Br-), sulfates (SO42-), nitrates (NO3-), and acetates (CH3COO-). The obtained results were compared as was done previously with those of K2Ni(CN)4.

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

12337

Figure 8. Effect of the nature of the anion catalyst on the evolution of redox potential of the dyeing bath.

Figure 10. Effects of the nature of the anion catalyst on the indigo reduction yield and the hydrolysis of sodium borohydride.

Figure 9. Effect of the nature of the anion catalyst on the reaction duration.

3.2.1. Effect of the Nature of the Catalyst Anion on the Evolution of the Oxidoreduction Potential of the Dyeing Bath. The nature of the anion of the copper salt was varied in the reaction medium. The effect of this variation on the evolution of the redox potential of the medium was investigated. The experimental results are shown in Figure 8. In this figure, it can be observed that all of the curves representing the evolution of the redox potential with time have the same shape whatever is the anion of the cupric catalyst used. All of these curves constituted three parts: In the first part, the potential redox of the medium increases rather quickly. The second part represents a slow increasing of redox potential when the time increases. The second part is a rapid jump of redox potential. Finally, in the third part, the potential redox of the medium tends to take again a rather fast increase. Thus, it appears that, contrary to K2NiCN)4, the final step of quasi stability is not observable for all catalysts containing copper. This can be attributed to the presence of the secondary reactions of reduction of the ions present in the medium. For these catalysts containing copper, we supposed that the moment when the redox potential of the medium begins the third part of its evolution is taken as the reduction reaction duration. That moment also corresponds to the visual sign of the end of the reduction reaction indicated by the end of the generation of hydrogen gas H2, which reveals the exhaustion of the sodium borohydride amount. The reduction reaction duration of the copper-based catalysts is reported in Figure 9. In this figure, it can be observed that all of the catalysts containing copper have reduction reaction duration practically similar. These reduction reaction durations are equal to 140 min. However, these durations remain much higher than that obtained with K2Ni(CN)4 (76 min). 3.2.2. Effect of the Nature of the Catalyst Anion on the Reduction Yield. The effect of the nature of the cupric catalyst

anion on the indigo reduction yield and the extent of the hydrolysis reaction of sodium borohydride were also investigated. The evaluation of the indigo reduction yield and the estimation of the sodium borohydride hydrolysis were carried out as described in ref 9. The experimental results of these two studies are reported in Figure 10. Each point of the represented curves is the mean value of two experiments. The experimental deviation of the evaluation of the weight of water lost and the indigo reduction yield are also reported in this figure. Figure 10 shows that the yields of the reduction reactions catalyzed by cupric salts can be classified according to the following order: Cu(CH3COO)2 > CuSO4 > CuCl2 > Cu(NO3)2 > CuBr2 (a) Taking into account the experimental deviations calculated for these evaluations, we can consider that the reduction yields obtained with both copper sulfate and copper chloride are similar. The same is found for the reduction yields obtained with both copper nitrate and copper bromide. The indicated order a corresponds to the classification of the anions of various copper salts according to the concept of acid-bases/hardness-softness of Pearson:23-25 CH3COO- > SO42- > Cl- > NO3- > Br- (classification of decreasing hardness) Copper is mentioned by Pearson23-25 as a species of middle hardness. So, its affinity toward the various studied anions can be regarded as similar. On the other hand, the various anions are solvated differently. This solvatation decreases when hardness decreases. Therefore, the order of solvatation is the same one as that of decreasing hardness. An important solvatation of the anion induces a decreasing of interactions between the anion and the cation of the catalyst. Thus, the availability of the cation rises. This availability can explain a great catalytic activity. This catalytic activity can even exceed that of the catalyst initially used K2Ni(CN)4. It is the case of copper diacetate Cu(CH3COO)2 which gives a reduction yield equal to 47.17%, whereas, in the presence of K2Ni(CN)4, the reduction yield obtained is 43.03%. Figure 10 shows also that taking into account the experimental deviations of evaluation, the loss of the hydrolyzed borohydride is practically similar whatever is the copper salt used. The lost

12338

Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010

Acknowledgment We are particularly grateful to Mr. Rhaiem Salah, General Manager of SITEX (Socie´te´ Industrielle des Textiles), and all the technical and administrative staff of this company for their assistance and valuable contribution. Literature Cited

Figure 11. Effect of the nature of the anion catalyst on the color yield.

water which is consumed specially in the hydrolysis reaction of sodium borohydride is about 71 g. This loss is, in all the cases, higher than that obtained when K2Ni(CN)4 was used as catalyst. 3.2.3. Effect of the Nature of the Catalyst Anion on the Dyeing Quality. The obtained medium after the indigo reduction reaction in the presence of copper-based catalysts was used as dyeing baths for cotton fabrics. The evaluation of the dyeing quality of the colored samples was carried out using the color yield parameter. The experimental results representing the effect of the nature of the cupric catalyst anion on the color yield are reported in Figure 11. The values represented in this figure are the mean of two experiments. Figure 11 reveals that K/S depends considerably on the indigo reduction yield. The higher the reduction yield is, the higher the color yield is. The maximum value of the color yield was obtained in the case of Cu(CH3COO)2 as catalyst. Moreover, it can be observed in Figure 11 that the values of K/S obtained with the other cupric salts are higher than that of K2Ni(CN)4, although some of these copper salts gave reduction yield lower than that of K2Ni(CN)4 (Figure 10). This indicates that the reduction reaction of indigo in the presence of copper-based catalyst offers usually very interesting dyeing results. Conclusion This paper describes an unconventional process of indigo reduction. This process consists of reducing indigo by sodium borohydride in the presence of a metallic salt as catalyst. In this work, the influence of the nature of the catalyst (its cation and its anion) on the performances of this process was investigated. The study of the effects of several metal salts (12 metal salts) showed that the cation which gives the best yield of the reduction reaction of indigo by sodium borohydride is copper (Cu2+), whereas the anion which gives the best yield of this reduction reaction is acetate (CH3COO-). This study showed also that the reduction of indigo by sodium borohydride in the presence of catalyst containing copper always offers very interesting yields for the reduction and the dyeing. However, this kind of catalyst needs more time to achieve the reduction of indigo than the initially used catalyst potassium nickel cyanide. On the other hand, use of the copper salts can result in better sanitary conditions and less health hazards than K2Ni(CN)4. Moreover, they present a better commercial availability and lower cost.

(1) Hughey, C. S. Indigo dyeing: an ancient art. Text. Chem. Color. 1983, 15, 103. (2) Fernelius, W. C.; Renfrew, E. E. Indigo. J. Chem. Educ. 1983, 60, 633. (3) Parmar, M. S.; Satsangi, S. S.; Prakash, J. Indigo dyed warp yarn: An essential requirement for manufacturing denim. Colorage 1996, 43, 45. (4) Blackburn, R. S.; Bechtold, T.; John, P. The development of indigo reduction methods and pre-reduced indigo products. Color. Technol. 2009, 125, 193. (5) Roessler, A.; Jin, X. State of the art technologies and new electrochemical methods for the reduction of vat dyes. Dyes Pigm. 2003, 59, 223. (6) Bozˇicˇ, M.; Kokol, V. Ecological alternatives to the reduction and oxidation processes in dyeing with vat and sulphur dyes. Dyes Pigm. 2008, 76, 299. (7) Xin, J. H.; Chong, C. L.; Tu, T. Colour variation in the dyeing of denim yarn with indigo. Color. Technol. 2000, 116, 260. (8) Westbroken, P.; De Clerck, K.; Kiekens, P.; Gasana, E.; Temmerman, E. Improving quality and reproducibility of the indigo dye process by measuring and controlling indigo and sodium dithionite concentrations. Tex. Res. J. 2003, 73, 1079. (9) Meksi, N.; Kechida, M.; Mhenni, F. Cotton dyeing by indigo with the borohydride process: Effect of some experimental conditions on indigo reduction and dyeing quality. Chem. Eng. J. 2007, 131, 187. (10) Nair, G. P.; Shah, R. C. Sodium borohydride in vat dyeing. Text. Res. J. 1970, 40, 303. (11) Langland, J. T.; Kreevoy, M. M.; Wade, R. C. Acid-catalyzed borohydride reduction of vat dyes. Text. Res. J. 1975, 45, 532. (12) Kreevoy, M. M.; Taft, R. W. The evaluation of inductive and resonance effects in reactivity. III. Extra resonance energies of conjugation. J. Am. Chem. Soc. 1957, 79, 4016. (13) Fermeliusn, W. C. Inorganic Synthesis; Robert E. Krieger: New York, 1978; Vol. II, pp 227-228. (14) Kubelka, P.; Munck, F. Ein beitrag zur optik der farbanstriche. Z. Tech. Phys. 1931, 12, 593. (15) Etters, J. N. Quality control of cotton denim in indigo dyeing. Am. Dyest. Rep. 1992, 81, 17. (16) Etters, J. N. Reproducibility in indigo dyeing. Am. Dyest. Rep. 1994, 83, 26. (17) Hua, D.; Hanxi, Y.; Xinping, A.; Chuansin, C. Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. Int. J. Hydrogen Energy 2003, 28, 1095. (18) Davis, R. E.; Bromels, E.; Kibby, C. L. Boron hydrides. III. Hydrolysis of sodium borohydride in aqueous solution. J. Am. Chem. Soc. 1962, 84, 885. (19) Wee, J.-H.; Lee, K.-Y.; Kim, S. H. Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems. Fuel Process. Technol. 2006, 87, 811. (20) Irving, H.; Williams, R. J. P. The stability of transition-metal complexes. J. Chem. Soc. 1953, 3192. (21) Ringbom, A. Complexation in Analytical Chemistry; Robert E. Krieger: New York, 1979; pp 6-9. (22) Weast, R. C., Ed. Handbook of Chemistry and Physics; CRC Press: Cleveland, OH, 1977-1978; pp F-213, F-214, and E-68. (23) Pearson, R. G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85, 3533. (24) Pearson, R. G. Hard and soft acids and bases, HSAB, Part 1 Fundamental principles. J. Chem. Educ. 1968, 45, 581. (25) Ringbom, A. Chemical Hardness: Applications from Molecules to Solids; Wiley-VCH: New York, 1997.

ReceiVed for reView April 27, 2010 ReVised manuscript receiVed August 19, 2010 Accepted October 20, 2010 IE100974D