Determination of Polyhexamethylene Biguanide Hydrochloride Using Photometric Colloidal Titration with Crystal Violet as a Color Indicator

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1 ANALYTICAL SCIENCES AUGUST 20, VOL The Japan Society for Analytical Chemistry Determination of Polyhexamethylene Biguanide Hydrochloride Using Photometric Colloidal Titration with Crystal Violet as a Color Indicator Takashi MASADOME,* Takaaki MIYANISHI,* Keita WATANABE,* Hiroshi UEDA,* and Toshiaki HATTORI** * Department of Applied Chemistry, Faculty of Engineering, Shibaura Institute of Technology, Toyosu, Koto, Tokyo , Japan ** Department of Electrical & Electronic Information Engineering, Toyohashi University of Technology, Toyohashi , Japan A solution of polyhexamethylene biguanide hydrochloride (PHMB-HCl) was titrated with a standard solution of potassium poly(vinyl sulfate) (PVSK) using crystal violet (CV) as an photometric indicator cation. The end point was detected by a sharp absorbance change due to an abrupt decrease in the concentration of CV. A linear relationship between the concentration of PHMB-HCl and the end-point volume of the titrant existed in the concentration range from 2 to eq mol L. Back-titration was based on adding an excess amount of PVSK to a sample solution containing CV, which was titrated with a standard solution of poly(diallyldimethylammonium chloride) (PDADMAC). The calibration curve of the PHMB-HCl concentration to the end point volume of the titrant was also linear in the concentration range from 2 to eq mol L. Both photometric titrations were applied to the determination of PHMB-HCl in a few contact-lens detergents. Back-titration showed a clear end point, but direct titration showed an unclear end point. The results of the back-titration of PHMB-HCl were compared with the content registered in its labels. (Received April 26, 20; Accepted July, 20; Published August 0, 20) Introduction Recently, a cationic polyelectrolyte, polyhexamethylene biguanide hydrochloride (PHMB-HCl), is widely used for disinfectants in personal-care products for cosmetics and toiletries, and for sanitizers in swimming pools. Several methods for determining PHMB-HCl have been developed by using spectrophotometry, the voltammetric method, 2 the titrimetric method, 3 capillary electrophoresis, 4 and the HPLC method. 5 A colorimetric determination methods of PHMB-HCl in pool and spa water using complexation between Ni 2+ and PHMB-HCl have been reported. An indirect determination of PHMB-HCl by using adsorptive voltammetry of 2-(5-bromo- 2-pyridyl) azo-5-[n-n-propyl-n-(3-sulfopropyl)amino]phenol on a carbon paste electrode has been reported. 2 Titrimetric methods such as the Kjeldahl method and argentometry, for the determination of PHMB-HCl have been reported. 3 Capillary electrophoresis with contactless conductivity detection for the determination of PHMB-HCl in eye drops has been reported. 4 In these methods, however, the detection limit has been above a few ppm. Lucas et al. 5 have developed a solid-phase extraction method, followed by HPLC analysis using an evaporative light-scattering detector in order to measure PHMB-HCl in contact-lens detergents. This method has been highly sensitive, and has a lower detection limit (0. ppm). The method is very tedious and its apparatus is exaggerated. To whom correspondence should be addressed. masadome@sic.shibaura-it.ac.jp We had developed the potentiometric colloidal titration of PHMB-HCl using hexadecyltrimethylammonium ion as an indicator and a cationic surfactant-selective electrode as an indicator electrode. 6 The titration can simply determine ppm of PHMB-HCl in a contact-lens detergent. Colloidal titration is a familiar method for the determination of natural and synthetic polyelectrolytes. 7 3 The principle is based on the formation of an ion-association complex between cationic polyelectrolyte and anionic polyelectrolyte. Since the ion associate is mainly formed due to an electrostatic interaction, an increase in the concentration of a salt in a sample solution affects the stability constants of the colloidal titration reaction. 4 However, the potentiometric method is inevitably accompanied by a reference electrode, which leaks a salt to a sample solution to some extent. 5 As a result, the leaked salt affects the stability constants of the colloidal titration reaction, which leads to titration errors. 8 0 Therefore, when conducting potentiometric colloidal titration, one should be careful in concerning the leakage of a salt from a reference electrode. In this paper, we report a photometric colloidal titration method of PHMB-HCl using crystal violet (CV) as a color indicator. Photometric titration is an alternative method to potentiometry, and does not require a reference electrode; it is thus free from the salt leakage. Masadome 3 had reported on the determination of a cationic polyelectrolyte, such as poly- (diallyldimethylammonium chloride) (PDADMAC) using CV. The present method was applied to the determination of PHMB-HCl in contact-lens detergents. Interestingly, back-titration with PDADMAC was useful to determine the end-point.

2 88 ANALYTICAL SCIENCES AUGUST 20, VOL. 27 Experimental () (2) (3) (4) (5) Chemicals CV used as an indicator was obtained from Wako Pure Chemicals Co., and used without further purification. A potassium poly(vinyl sulfate) (PVSK) solution for colloidal titration use was obtained from Wako Pure Chemicals Co. and used as a titrant in photometric direct titration. A PDADMAC solution was also obtained from Wako Pure Chemicals Co. and used as a titrant in photometric back-titration. PHMB-HCl was a 20% solution obtained from Avecia. Contact-lens detergents were obtained from local drug stores. Other chemicals of guaranteed grade were used as received. Standard procedure of photometric direct titration A PHMB-HCl solution containing M CV and M HCl (total volume, ml) was titrated with a eq mol L PVSK standard solution. The sample solution was taken into a 0-mm path-length cell for absorbance measurements after the addition of a suitable volume of the titrant. The absorbance of the sample solution was measured at 590 nm with a spectrophotometer (JASCO V-520-SR), and the sample solution in a 0-mm path-length cell was returned to the beaker each time. The point where the absorbance just began to fall was taken as the end point of the present titration. Standard procedure of photometric back-titration A eq mol L PVSK solution (20.0 ml) was added to a PHMB-HCl solution containing M HCl, and stirred for 20 min. After PHMB-HCl reacted with a PVSK sufficiently, a 2.0-mL portion of a M CV solution was added to the sample solution. The excess amount of PVSK was back-titrated with a eq mol L PDADMAC standard solution. Results and Discussion Fig. Effect of the concentration of CV on the sharpness of the end-point detection for the titration of a eq mol L PHMB-HCl solution (50.00 ml) with a eq mol L PVSK standard solution. Concentration of CV: ().2 0 5, (2).0 0 5, (3) , (4) , (5) M. Figure shows the titration curves of a eq mol L PHMB-HCl solution (50.00 ml) with a eq mol L PVSK solution. The absorbance at 590 nm slightly decreased with the addition of PVSK. PVSK preferentially formed an ion-association complex with PHMB, and hence the slight absorbance decrease was due to being diluted with the titrant. After the end point, the absorbance of the sample solution sharply decreased. Then, PVSK reacted with CV, and the concentration of CV decreased abruptly. The concentration of CV affects the sharpness of end-point detection in photometric direct titration. Turning attention to the absorbance after the end points shown in Fig., the greater increasing CV concentration is a sharper absorbance change. However, the concentration of CV at more than M did not improve the sharpness of the end point. Therefore, all of the experimental results of the direct titration described below were obtained by using a M CV solution as an indicator. Effect of inorganic electrolytes in the photometric direct titration In colloidal titration, a great amount of coexisting inorganic electrolytes often lead to titration errors, 8 0 because the increase in the concentration of a salt affects the stability constants of a () (2) Fig. 2 Effect of NaCl and CaCl 2 on the sharpness of the end-point detection of PHMB-HCl ( eq mol L, ml) with a eq mol L PVSK standard solution. () eq mol L PHMB-HCl, (2) eq mol L PHMB-HCl M NaCl, (3) eq mol L PHMB-HCl, (4) eq Mol L PHMB-HCl M CaCl 2. (3) (4)

3 ANALYTICAL SCIENCES AUGUST 20, VOL (c) (d) (e) (f).4.2 (e) (d) 0.3 (c) Fig. 3 Photometric direct titration curves for PHMB-HCl (00.0 ml) containing M HCl and M CV with a eq mol L PVSK standard solution. Concentration of PHMB-HCl: 0, , (c) , (d) , (e) , (f) eq mol L Fig. 4 Photometric back-titration curves for PHMB-HCl with a eq mol L PDADMAC standard solution. Sample solution: PHMB-HCl 77.5 ml eq mol L PVSK ml M CV 2.0 ml + M HCl 2 ml. Concentration of PHMB-HCl: 0, , (c) , (d) , (e) eq mol L. colloidal titration reaction. 4 Therefore, the effect of electrolytes (NaCl and CaCl 2) on the sharpness of the end-point detection of PHMB-HCl was examined. Figures 2 and 2 show titration curves for a coexistent solution of NaCl or CaCl 2. The end point could be detected sharply even in the presence of a 00-fold excess of NaCl and CaCl 2 to the concentration of PHMB-HCl. In the presence of M NaCl, or CaCl 2, the relative titration errors to the result without their salt were.63% for NaCl and 2.50% for CaCl 2. Measurable concentration range of photometric direct titration Most of the biguanide groups in PHMB-HCl dissociate completely at about ph According to the standard procedure, an adequate amount of M HCl was added to the sample solution up to ph 2.5. Figures 3 3(f) show titration curves for a PHMB-HCl solution at different concentrations with a eq mol L PVSK standard solution. Each end point was clear and detected even in the 0 6 eq mol L range, sensitively. The calibration curve of concentration of PHMB-HCl to the end-point volume was linear over the range from 2.0 to eq mol L. The graph equation is Y =.99X Here, Y is the end-point volume (ml) and X the concentration (0 6 eq mol L ) of the PHMB-HCl solution. The factor of a linear regression (γ) of the calibration curve was The relative standard deviation of the end-point volume for three-times titrations of PHMB-HCl was 9.0% for eq mol L,.25% for eq mol L and 4% for eq mol L. Back-titration with PDADMAC When weak anionic polyelectrolytes, such as poly(acrylic acid), are determined by colloidal titration, it is well known that its back-titration using toluidine blue as an indicator is useful. 8 0 The color change of toluidine blue as an indicator is clear only for titration with PVSK. As an alternative way of end-point detection using CV, back-titration with PDADMAC was examined. As to following the standard procedure of the back-titration, titration curves of Figs. 4 4(e) were obtained. Before titration, CV reacted with PVSK, and the absorbance of a sample solution was relatively low because the attachment of CV to the PVSK resulted in a decrease in the absorbance of CV at the maximum absorption wavelength (590 nm). 6 When the titration started, the absorbance of the sample solution increased upon adding the titrant. The free concentration of CV increased by a cation-exchange reaction of PVSK from CV to PDADMAC. After the end point, the absorbance of the sample solution decreased slightly due to dilution with the titrant. The end point was detected as a break point of the titration curve. The sharpness of detection of the end point is important for determining the end point. The angle of the cross point near the end point was about 60 (Fig. 4). On the other hand, the angle for the direct titration was also about 60 (Fig. ) on the same scales of the titrant volume and the absorbance. Thus, the sharpness of the end point was almost equal to that in direct titration. This may be due to the fact that the sharpness of end-point detection depends on the ability to form ion associate between CV and PVSK in direct- and back-titrations. A hump was often observed near to an end point in a photometric titration curve. This hump was due to the coagulation of ion association complexes. 7 The calibration curve of PHMB-HCl was linear over the range from 2.0 to eq mol L. The graph equation is Y =.50X Here, Y is the end-point volume (ml) and X the concentration (0 6 eq mol L ) of PHMB-HCl. The γ of the calibration curve was The relative standard deviation of the end point volume for three-times titrations of PHMB-HCl was.92% for eq mol L,.86% for eq mol L, and 5.93% for eq mol L. Determination of PHMB-HCl in contact-lens detergents At first, PHMB-HCl in a commercially available contact-lens detergent was determined by the direct titration method. Figure 5 shows a photometric direct titration curve for PHMB-HCl in a commercially available contact-lens detergent A with a eq mol L PVSK standard solution. A clear end point was not observed from the direct titration curve. The unclear end point may be because poloxamine (nonionic

4 820 ANALYTICAL SCIENCES AUGUST 20, VOL Fig. 5 A photometric direct titration curve for PHMB-HCl in a eq mol L PVSK standard solution. A commercially available contact-lens detergent A (200.0 ml), M CV (2.5 ml) and M HCl (3.4 ml) was diluted with water to ml. The solution (50.0 ml) was used as a sample solution and titrated with a eq mol L PVSK standard solution. Converted end-point volume / ml Added concentration of PHMB-HCl / μm Fig. 7 Relationship between the concentration of PHMB-HCl added to the diluted commercially available contact-lens detergent and the converted end-point volume of the titrant. Table Determination of PHMB-HCl in commercially available contact-lens detergents Contact-lens detergent Concentration of PHMB-HCl Determination value/μm Nominal value/μm.5 (c) A B 5.5 ± ± Fig. 6 Photometric back-titration curves for PHMB-HCl in a eq mol L PDADMAC standard solution. Sample solution: diluted commercial contact-lens detergent A (2.5 times dilution with water) containing PHMB-HCl of a known concentration (50.0 ml) eq mol L PVSK 20.0 ml M CV 2.0 ml + M HCl ml. Concentration of added PHMB-HCl: 0, , (c) eq mol L. surfactant) contained in the commercially available contact-lens detergent solution may have interfered with the formation of an ion associate between PHMB-HCl and PVSK. Masadome et al. 8 reported that a nonionic surfactant (Triton X-00) interferes with end-point detection in the potentiometric titration of anionic polyelectrolytes. Hattori et al. 7 also reported that the nonionic surfactant interferes with end-point detection in the thermometric titration of anionic surfactants with tetradecyldimethylbenzylammonium chloride. Therefore, back-titration was applied to the determination of PHMB-HCl in a commercially available contact-lens detergent. A commercially available contact-lens detergent A (20.00 ml) was mixed with an appropriate volume of eq mol L PHMB-HCl solution, and the mixed solution was made up to ml with distilled and deionized water (solution a). A -ml portion of M HCl was added to the solution; the ph of solution a was 2.3. Next, a eq mol L PVSK solution (20.0 ml) was added to the solution, and the mixed solution was stirred for 20 min. After adding 2.0 ml of a M CV solution to the solution, the resulting solution was back-titrated with a eq mol L PDADMAC standard solution. Figure 6 shows photometric back-titration curves for PHMB-HCl in a eq mol L PDADMAC standard solution. A linear relationship between the concentration of added PHMB-HCl to solution a and the converted end point volume of the titrant from Fig. 6 was obtained as shown in Fig. 7. Thus, by back-titration, PHMB-HCl completely reacted with PVSK due to having a reaction time of 20 min, and the presence of a cohesive complex of PDADMAC-PVSK. In this case, the converted end-point volume was calculated using the following equation: [(end point volume for 0 eq mol L PHMB-HCl) (end point volume for a sample solution)]. From Fig. 7, the concentration of PHMB-HCl in a commercially available contact-lens detergent A obtained by a standard addition method is.2 ppm ( eq mol L ), and coincides with the nominal value (. ppm, eq mol L ). The concentrations of PHMB-HCl in commercially available contact-lens detergents obtained by the standard-addition method are given in Table. The concentrations of PHMB-HCl obtained by the present method are in accord with the nominal values. This result shows that the present photometric back-titration method can be applied to determine the level of PHMB-HCl in commercially available contact-lens detergents.

5 ANALYTICAL SCIENCES AUGUST 20, VOL Conclusions The proposed photometric direct- and back-titration methods offer a sensitive and simple method for the determination of PHMB-HCl by using CV as a color indicator. The sensitivity and lower detection limit of the present photometric titration method is superior to that of the other methods based on spectrometry, voltammetry, 2 the titrimetric method 3 and capillary electrophoresis. 4 The sensitivity and lower detection limit of the present photometric titration method are inferior to that of the HPLC method. 5 However, the advantage of the present photometric method over the HPLC method is that it requires no expensive instrumentations and no complicated procedures. The lower detection limit of the present photometric titration method is the same as that of our previous potentiometric titration method. 6 In contrast with our previous potentiometric titration, 6 the present method is superior in being free from salt leakage. From the findings, the present photometric titration method is found to be best for the determination of PHMB-HCl. The present method was successfully applied to the determination of trace amounts of PHMB-HCl in a contact-lens detergent. References. T. Rowhani and A. F. Lagalante, Talanta, 2007, 7, T. Hattori, N. Tsurumi, R. Kato, and M. Nakayama, Anal. Sci., 2006, 22, T. Hattori, Y. Nakata, and R. Kato, Anal. Sci., 2003, 9, E. M. Abad-Villar, S. F. Etter, M. A. Thiel, and P. C. Hauser, Anal. Chim. Acta, 2006, 56, A. D. Lucas, E. A. Gordon, and M. E. Stratmeyer, Talanta, 2009, 80, T. Masadome, Y. Yamagishi, M. Takano, and T. Hattori, Anal. Sci., 2008, 24, D. Horn, Prog. Colloid Polym. Sci., 978, 65, R. Senju, Koroido Tekiteihou (Colloidal Titration in Japanese), 960, Nankodo, Tokyo. 9. K. Ueno and K. Kina, J. Chem. Educ., 985, 62, K. Toei and K. Kawada, Bunseki Kagaku, 972, 2, 50.. K. Kina, K. Tamura, and N. Ishibashi, Bunseki Kagaku, 974, 23, K. Kina, K. Tamura, and N. Ishibashi, Bunseki Kiki, 976, 4, T. Masadome, Talanta, 2003, 59, T. Hattori and K. Katai, Anal. Sci., 2008, 24, T. Hattori, Y. Masaki, K. Atsumi, R. Kato, and K. Sawada, Anal. Sci., 200, 26, T. Masadome, Anal. Lett., 200, 34, T. Hattori and H. Yoshida, Anal. Sci., 986, 2, T. Masadome, T. Imato, and Y. Asano, Fresenius J. Anal. Chem., 999, 363, 24.

Shigeya SnTO and SUIIllO UCHIKAWA. Faculty of Education, Kumamoto University, Kurokami, Kumamoto 860

Shigeya SnTO and SUIIllO UCHIKAWA. Faculty of Education, Kumamoto University, Kurokami, Kumamoto 860 ANALYTICAL SCIENCES FEBRUARY 1986, VOL. 2 47 Extraction-Spectrophotometric Determination of Antimony(V) with 2-Hydroxyisocaproic Acid and Citrate, with Application to Differential Determination of Antimony(V)