Ultraviolet-Visible (UV-VIS) spectrophotometry and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize copper naphthenate (Cu-N) complex in treating solution and in Cu-N treated wood. At wavelength 680 ± 2 nm, the absorption was proportional to the amount of Cu-N present in solution. FTIR was used to confirm the presence of the carboxylate group in the Cu-N complex and the carboxylic acid in the non-complexed naphthenic acid (NA). Toluene was used to extract Cu-N from treated wood for UV-vis quantitative analysis. This method can be applicable to the determination of Cu-N in solution and in treated wood with detection limits ranging from 4 to 80 ppm

Kamdem, D. Pascal

Zyskowski, Justin

English

Wood and Fiber Science (E-Journal)

ULTRAVIOLET SPECTROPHOTOMETRY AND FOURIER TRANSFORM[ INFRARED SPECTROSCOPY CHARACTERIZATION OF COPPER NAPHTHENATE Justin Zyskowski Research Assistant and D. Pascal Kamdemt Associate Professor Department of Forestry Michigan State University East Lansing, MI 48824 (Received October 1998) ABSTRACT Ultraviolet-Visible (UV-VIS) spectrophotometry and Fourier Transform Infrared (FTIR) spectroscopy werc used to characterize copper naphthenate (Cu-N) complex in treating solution and in Cu-N treated wood. At wavelength 680 5 2 nm, the absorption was proportional to the amount of Cu-N present in solution. FTIR was used to confirm the presence of the carboxylate group in the Cu-N complex and the carboxylic acid in thc non-complexed naphthenic acid (NA). Toluene was used to extract Cu-N from treated wc~od for UV-vis quantitative analysis. This method can be applicable to the detcrmination of Cu-N in solution and in treated wood with detection limits ranging from 4 to 80 ppm. Kevwords. FTIR, UV-vis, copper naphthenatc, naphthenic acid. INTRODUCTION Today in the United States treated wood is an important material in construction. When exposed outdoors, wood can be destroyed by decay fungi and insects. The protection of wood is achieved by treatment with an appro- priate wood preservative. One of the objec- tives of the American Wood-Preservers' As- sociation (AWPA) is to ensure the quality of the chemical preservatives. The AWPA defines a number of criteria that a chemical formulation must meet in order to be an accepted wood preservative. For Cu-N, the AWPA standard P8-98: 2.3 (AWPA 1998) states, "all of the copper present in the con- centrate (Cu-N) shall be combined as copper naphthenate." It is reported that the presence of water and wood carbohydrates in Cu-N t Member of SWST. treating solution enhances the formation of emulsions and copper oxides (Freeman 1992; Kamdem et al. 1998). It has also been docu- mented that Cu-N-treated utility poles some times fail prematurely, and remedial treat- ments are necessary to extend the useful ser- vice life (Forsyth and Morrell 1993). The early failure has been tentatively attributed to either the quality of Cu-N formulations or the poor treatment with inadequate retention or pene- tration (Kamdem et al. 1998). However, Cu-:N is reported to be an attractive good wood pre- servative because of its low mammalian tax- icity (U.S. Army 1987). Field decay tests have shown Cu-N treated lumber to have a predict- ed service life of 38-43 years (De Groot et a,l. 1988; Gjovik and Gutzmer 1991). Low to:<- icity and successful decay resistance justify the study of the preservative. The quantifica- tion of Cu-N complex in wood and in treating Wood ilr~d Fihrr Sc.fc,n<.~,. 11(4), 1999, pp. 441-446 'C) 1000 hy the Soctrty of Wood Science and Technology 442 WOOD AND FIBER SCIENCE, OCTOBER 1999, V. 31 (4) solution would facilitate the quality control of Cu-N treatment. The objective of this work is to characterize the Cu-N complex qualitatively and quantita- tively in treating solution and treated wood and develop an analytical method to determine Cu-N concentration. If successful, this method will allow direct determination of Cu-N rather than estimate the concentration of elemental copper as suggested in standard A5 in the AWPA Book of Standards (AWPA 1998) or the naphthenic acid (NA) composition. The determination of the amount of Cu in Cu-N solution or treated wood is not sufficient to understand and predict the performance of Cu- N. Each species and form of Cu may have a different degree of solubility and bioavailabil- ity, thus varying wood protection level. It would be beneficial to quantify Cu-N complex rather than individual Cu or NA in treating solution and in treated wood. In order to study Cu-N, it is important to understand its formation. Numerous analytical methods have been used to study the reaction of Cu-N (Craciun and Kamdem 1997). Cu-N is formed when copper, for instance copper hydroxide Cu(OH), or copper oxide, and NA are combined. NA is obtained from crude oil and may contain C5-C35 carboxylic acids (Dzidic et al. 1988). The carboxylic acid groups from NA react with copper to form the Cu-N complex as represented in the following reaction: Nephlhencc Copper Copper Wafer Acid Hydmx~de Naphlhrnale FTIR spectroscopy can be successfully used to follow the carboxylic acid in NA and the car- boxylate of Cu-N (Craciun and Kamdem 1997). Also the chemistry of various metal carboxylates has also been examined using UV-vis (Ingle and Crouch 1988; Mehrota and Bohra 1983). MATERIALS AND METHODS Chemicals Cu-N with the commercial name Perm-E8 from ISK Biosciences, containing 2% copper TABLE 1. Copper-containing compounds, solvent, and corresponding wavelength at maximum absorbtion. Chemical name Solvent A max (nnr) Copper naphthenate Copper napthenate Copper napthenate Copper-8-quinolinate Copper-8-quinolinate Copper ethanolamine Copper hydroxide Copper (11) nitrate Copper (11) acetate Cupric sulfate Copper formate Copper gluconate Copper (11) carbonate Copper (I) oxide Copper (11) oxide Copper oxalate Copper citrate hexane toluene diesel water hexane water watcr w/EDTA water watcr watcr water water W, H, T and D* W, H, T and D* W, H, T and D* W, H, T and D* W, H, T and D* 680 680 680 472 488 642 740 816 775 827 782 804 insoluble insoluble insoluble insoluble insoluble * W: watcr, H. hexanc, T tolusnc, and D dirscl. metal, and Cu-N with a commercial name M GARD S520 from OMG Chemical, containing 8% copper metal, were used after dilution in hexane, toluene, or diesel fuel. Cu-N formu- lations from two sources were used in this study to validate the analytical method since the two formulations are manufactured differ- ently and used commercially. The hypothesis of the method developed in this paper was based on the quantification of copper carbox- ylate interaction taking place in each formu- lation. These solvents and water were used to solubilize Cu-N and other copper containing compounds as listed in Table 1. UV-vis spectrophotometry To scan all Cu containing solutions from 200 nm to 800 nm, a Beckrnan DU 640 B spectrophotometer with a 10-mm light path cuvet from Sigma was used. All UV-vis scans were performed at a speed of 600 nmlmin. FTIR spectroscopy A Nicolet ProtCgC 460 FTIR spectrometer with a horizontal attenuated total reflectance (HATR) accessory from Spectratech was used to scan various solutions from 4000 cm-' to Zv.tko~.v.ski und Kamdem-UV-VIS AND FTIR CHARACTERIZATION OF CU-N 443 650 cm-I. FTIR spectra were collected using 32 scans and a resolution of 4 cm-I. Dilution and analysis of spiked Cu-N solutions A 1 :5 vlv NA to hexane solution, a 1 :2 vlv dilution of Perm E8, and M Gard S520 in hex- ane were prepared. The diluted Perm E8 so- lution was spiked with the NA solution, Cu(OH), and then with both the CU(OH)~ and NA solution. The FTIR spectra of these re- sulting solutions were taken. Dilutions to equalize the original amount of Perm E8 pres- ent in each solution were made, and the ab- sorption of the diluted solutions at the wave- length of peak absorption of Cu-N was taken. Atomic absorption spectrometry The concentration of Cu in the Cu-N solu- tion was determined using a Perkin Elmer 31 10 atomic absorption spectrometer (AAS). The Cu-N was prepared for this analysis by dilution to an approximate concentration of 5 ppm in a 90% ethanol and about 10% water. Solid sample extraction A total of five Cu-N treated southern yellow pine cubes measuring 0.25 by 0.25 by 0.25 in. and an average Cu retention of 0.62 pcf (pound per cubic foot) were soxhlet-extracted with 300 mL aliquots of toluene. The toluene was changed every 18 h until there was no color change as determined by scanning all visible wavelengths by UV-vis. During the soxhlet extraction, six changes of toluene were needed until there was no color change. The extracted toluene solutions were analyzed by AAS for Cu content, and the absorption at the wavelength of peak absorption of Cu-N was read by UV-vis. RESULTS AND DISCUSSION UV-vis spectrophotometry Figure 1 is the UV-vis spectra of hexane, toluene, diesel fuel, and Cu-N diluted in hex- ane. The peak absorption specific to Cu-N di- Wavelength (nrn) FIG. I .  The UV-vis spectra of (a) hexane, (b) toluene, (c) diesel, and (d) 1:s Cu-N in hexane. luted in hexane, toluene, or diesel was ob- tained at 680 ? 2 nm (Fig. Id and Table 1). The same peak was obtained for both Perm.- E8 from ISK Biosciences and M GARD S520 from OMG Chemical. No differences were de- tected between the two formulations of Cu-N, suggesting that indeed the UV-vis method characterizes the carboxylate copper complex. Interference of hexane, toluene, and diesel was negligible on the UV-vis specific peak. Peaks in the UV region due to the organic solvents appear at 230 nm for hexane (Fig. la), 320 nm for toluene (Fig. Ib), and at 410 nm for diesel (Fig. 1c). Several other Cu-based compounds were dissolved and then scanned by UV-vis (Tablle 1). None of the soluble compounds had a peak of absorption at 680 nm. No further attempt was made to scan UV-vis data from the insol- uble copper compounds. FTIR spectroscopy Hexane was used as the solvent for FTIR spectra analysis. No bands due to hexane con- flict with bands at 1708 cm-', 1605 cm-' and 1593 cm-' (Fig. 2a). The FTIR spectrum of the NA solution has a band at 1708 c m '  (Fig. 2b) attributed to the carbonyl stretch from the carboxylic acid. Mixing Cu(OH), and NA re- sults in the decrease of intensity of the band at 1708 cm-I and a new band at 1605 cm-I (Fig 2c). The decrease in intensity of the band at 1708 cm-' is due to the reduction of free 444 WOOD AND FIBER SCIENCE, OCTOBER 1999, V. 31(4) Wavenumber (cm-l) Frc;. 2. The FTlR spectra of (a) hcxane, (b) naphthenic acid, (c) naphthenic acid with copper hydroxide, and (d) copper naphthcnate. carboxylic acid from the NA. The proportion of Cu(OH), and NA favors copper-carboxylate interaction of Cu-N. The new band at 1605 cm is due to the carbonyl stretch in the Cu- N complex. The FTIR spectrum of Cu-N ex- hibits a band at 1593 cm-' from the carbonyl stretch of the Cu-N complex (Fig. 2d). The band due to the carbonyl stretch in the Cu-N complex is different from the corresponding band in Fig. 2c. This shift in wave number from 1605 cm to 1593 cm ' may be due to different ratios of free NA and Cu-N in the solutions. While NA is present in excess with Cu-N, the FTIR band due to the carboxylate in Cu-N appears closer to 1605 cm-' with higher concentrations of NA. Fig. 2 demon- strates how the formation of Cu-N can be monitored by FTIR. The spectra of Cu-N and Cu-N spiked with  lsoocd 1575 em4 Wavenumber (cm-') FIG. 3. The FTIR spectra of (a) copper naphthenatc, (b) coppcr naphenatc with coppcr hydroxide, (c) copper naphthcnate with naphthenic acid, and (d) copper naph- thenatc with naphthenic acid and copper hydroxide. Cu(OH)2 appear in Fig. 3a and 3b, respec- tively. Both spectra have a band at 1593 cm-' due to the carbonyl stretch in Cu-N. The ad- dition of Cu(OH), did not affect the intensity of the band at 1593 cm-' because no free car- boxylic acid from NA was available to form a complex through carboxylation. Figure 3c represents the FTIR spectrum of Cu-N solu- tion spiked with NA. Two bands were ob- served--one at 1708 cm-I from the carboxylic acid and another at 1605 cm-I from the car- boxylate in the Cu-N complex. The band spe- cific to Cu-N appears at 1605 cm ' rather than 1593 c m ' .  As observed in Fig. 2, this may be due to the ratio of NA to Cu-N. Figure 3d represents the FTIR spectrum of the Cu-N spiked with NA and Cu(OH),. The band at Zyskowskr and Kamdem-UV-VIS AND FTIR CHARACTERIZATION OF CU-N 44.5 1708 cm ' (Flg. 3d) is less intense and the band at 1593 cm-' more intense than the cor- responding bands of the sample spiked with only NA (Fig. 3c). The addition of Cu(OH), favors the formation of the carboxylate (1593 cm ') with the carboxylate acid of NA (1708 cm I). Analysis of spiked Cu-N samples Cu-N spiked with NA and/or Cu(OH), were UV-vis scanned. The value of the absorption at 680 nm of the Cu-N solution, Cu(OH), spiked, and NA spiked Cu-N solutions was 0.50. The addition of NA or CU(OH)~ to Cu- N does not increase the absorption at 680 nm. However, when spiked with both NA and Cu(OH)2, the value of absorption increased from 0.50 to 0.67. The increase in absorption is due to the formation of copper-carboxylate interaction with the addition of Cu(OH), and NA. The coinciding increase of the FTIR band intensity at 1593 c m  ' and UV-vis absorption at 680 nm confirm that the absorption at 680 nm is due to the copper carboxylate complex in Cu-N. Quant$cation of Cu-N The UV-vis absorption at 680 nm was used to develop a quantitative method to estimate the amount of copper complex in solution. The original Cu-N solution diluted in ethanol was AAS analyzed and the concentration was de- termined to be 2%. This (lu-N was diluted in all three solvents (hexane, toluene, and diesel) to yield concentrations ranging from 0 ppm to 500 ppm. The absorption of each diluted sam- ple was read at 680 nm. The concentration of Cu-N in each solution wa:j plotted against the corresponding UV-vis absorption at 680 nm. A linear regression with an R2 value of 0.99 was constructed from data obtained with all three solvents. The regression line obtained from dilution of Cu-N in diesel is shown in Fig. 4. This regression line relies on the as- sumption that all Cu present in the initial Cu- N solution was complexed with carboxylic acid as evidenced by the FTIR. FIG. 4. The regression line for copper naphthenate cli- luted in dicsel. The linear portion of the curve is from 4 ppm to 80 ppm. Thc R2- value for the best fit line through these points is 0.9993. Similar regression lines can be made by diluting copper naphthenate in hexane or toluene. Cu-N treated wood was extracted with ta~l- uene, and the extract was analyzed by UV-vis at 680 nm. After the appropriate dilution, the concentration of Cu-N in the extract was dle- termined by fitting the absorption at 680 nm to the calibration curve varying from 4 to 80 ppm (Fig. 4). The value of the concentration of Cu-N extracted from cubes was 4635 pprn, similar to the value of 4610 ppm determined by AAS. This 0.55% difference was attributed to experimental error with extraction, dilution, and absorption reading. This study shows that UV-vis can be used to identify and determine the copper-carbox- ylate complex in solution. The use of AAS to analyze CU for the estimation of Cu-N is still a valid method although it assumes that all copper is present as copper carboxylate corn- plex. CONCLUSIONS UV-vis and FTIR techniques monitored tlhe formation of Cu-N from the mixture of Cu(OH), and NA. Cu-N exhibits a peak spe- cific to the Cu-carboxylate interaction at 680 446 WOOD AND FIBER SCIENCE, OCTOBER 1999, V. 31(4) nm with UV-vis and a band at 1600 cm-I + REFERENCES 10 cm-' using FTIR. AMERICAN WOOD PRESERVERS' ASSOCIATION (AWPA). The UV-vis specific peak at 680 nm was 1998. Book of Standards. Granbury, TX. 162 pp. used to quantify c u - ~  in treatment solution CRACILJN, R., AND D. KAMDEM. 1997. XI'S and FTIR Applied to the study of waterborne copper naphthenate and in the extract from soxhlet-extracted CU- wood preservative, Holzforschung 5 1 :207-213. N treated wood. This method is valid in the DE GROOT, R. c., C. L. LINK, AND J. B. HUFFMAN. 1988. range of 4 to 80 ppm using hexane, toluene, or diesel as solvent. The availability of the three solvents allows different applications of the method. Hexane may be better in FTIR and UV studies of Cu-N treated wood because there are fewer peaks from the solvent. Tolu- ene seems better for use with solid wood soxhlet extraction, and diesel fuel is best suit- ed for treatment solution because it is typically used as a solvent in treatment solution. A similar method could be used to quantify other metal complex-based wood preserva- tives. ACKNOWLEDGMENTS This work was funded by USDAICSREES grant no. 96-34158-2697, under the Michigan State University Hardwood Utilization pro- gram. The authors would like to acknowledge ISK Biosciences and OMG Chemical for sup- plying copper naphthenate. Field trials of copper naphthenatc treated wood. Proc. Am. Wood-Preserv. Assoc. 84: 186-200. Dzlulc, I., A. C. SOMEKVILLE, J.  C. RAIA, AN11 H. V. HART. 1988. Determination of naphthenic acids in California crudes and refinery wastewater by fluoride ion chemical ionization mass spectroscopy. Anal. Chem. 60: 13 18- 1323. FORSYTH, I? G., AND J. J. MORRELL. 1993. Performance of groundline bandage remedial treatments in western U.S. species transmission poles. Annual Meeting-The Inter- national Research Group on Wood Preservation. FREEMAN, M. E 1992. Copper naphthenate: An effective wood pole and cross arm preservative. Proc. First Southeastern Pole Conference. Forest Products Society, Madison, WI, Pp. 68-77. GJOVIK, L. R., AND D. I. GUTZMER. 1991. Comparison of wood preservatives in stake tests. Forest Serv. Res. Note FPL-02. USDA Forest Prod. Lab., Madison, WI. 100 pp. INGLE, J. D., AND S. R. CROUCH. 1988. Spectrochemical analysis. Prentice Hall, Englewood Cliffs, NJ. 325 pp. KAMDEM, D. I?, J. ZHANG, AND M. H. FREEMAN. 1998. The effect of post-steaming on copper naphthenate-treated southern pine. Wood Fiber Sci. 30(2):210-217. MEHROTA, R. C., AND R. BOHRA. 1983. Metal carboxyl- ates. Academic Press, London, UK. 180 pp. U.S. ARMY: ENVIRONMENTAL HYGIENE AGENCY. 1987. Pre- liminary assessment of the relative toxicity of copper naphthenate. Study No. 75-5 1-0497-88. 

Ultraviolet Spectrophotometry and Fourier Transform Infrared Spectroscopy Characterization of Copper Naphthenate

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Ultraviolet Spectrophotometry and Fourier Transform Infrared Spectroscopy Characterization of Copper Naphthenate

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