A sensitive spectrophotometric method for the determination of methenamine has been developed without any separation steps. Bromocresol green is adsorbed on Sephadex LH-20 gel but the sorption decreases in the presence of methenamine due to ion-pair formation between bromocresol green and methenamine in solution. This attenuation was used to the microdetermination of methenamine by measurement of absorbance of the solid phase (Sephadex LH-20 gel) in a 1.0 mm cell at 625 nm. Methenamine could be determined in the concentration range of 0.42–1.68
Methenamine (MT), (CH2)6N4, also known as 1,3,5,7-tetraazatricyclo[3.3.1.13,7]decane, urotropine, hexamine, hexamethylenetetramine, formin, and aminoform (Figure
The molecular structure of methenamine.
Many methods have been developed for the determination of milligram amounts of MT; these methods have been cited by Madsen et al. [
Determination of MT in the presence of formaldehyde and ammonia is difficult because most of methods are indirect determinations based on the determination of the amount of formaldehyde released after hydrolysis of MT in acidic condition [
HPLC and GC methods have been described for the determination of MT with expensive apparatus that use organic solvents [
This study presents a sensitive Solid Phase Spectrophotometry (SPS) method for the direct determination of microgram amounts of MT in pure aqueous solutions in the presence of formaldehyde. The determination of MT was carried out with inhibition of the sorption of bromocresol green (BCG) on a solid phase from an aqueous solution, and the subsequent measurement of absorption, directly in the solid phase. SPS in visible region has been less used for determination of organic compounds [
Methenamine forms an ion-pair with BCG in solution and therefore, the sorption of BCG on the solid phase decreases. We used the Sephadex LH-20 as solid phase. Sephadex LH-20 medium is based on hydroxypropylated dextran that has been cross-linked to yield a polysaccharide network [
Citric acid, sodium hydroxide, potassium hydroxide, sodium acetate, acetic acid, and bromocresol green (all from Merck) were of the highest purity available and used as received. The methenamine powder (99.5%) was from Sina Chemical Industries Co. (Iran). It was recrystallized from ethanol and dried at 60°C to constant weight and used for preparation of standard solutions of MT.
The Sephadex LH-20 gel (mesh 25–100 mm) (Aldrich) in its original dry state without pretreatment was used as solid support.
The stock solution was prepared by dissolving an appropriate amount of MT in water to obtain a concentration of 3.0 × 10−4 M. Citrate buffer, pH 4.0, was prepared by titrating aqueous 0.1 M disodium citrate (19.2 g citric acid +200 mL of 1 M NaOH/1000 mL H2O) with 0.1 M HCl. Bromocresol green was dissolved in 4.0 × 10−4 M NaOH and was diluted to 100.0 mL with the pH 4.0 citrate buffer to produce a 1.0 × 10−4 M solution. Buffer solutions and bromocresol green stock solutions were adjusted to an ionic strength of 0.2 with KCl. Wastewater sample was obtained from Sina Chemical Industries Co.
Weighing of materials was performed by using an analytical balance model Sartorius MCBA 100 with precision of ±0.0001 g. pH measurements were carried out with a Metrohm 691 pH-meter. A GBC spectrophotometer model Cintra 6 was used for spectrophotometric measurements.
A 10-mL sample containing 0.42–1.68
Wastewater sample containing MT was appropriately diluted with water to get the required concentration, and then the general procedure was followed. The amount of MT was calculated from a calibration graph.
The BCG color reagent (pKa = 4.66) occurs in two acid-base forms in weakly acidic aqueous solutions with the absorption maximum at 430 nm (BCGH− form) and 615 nm (BCG2−) [
The decreases of absorbance of BCG in the Sephadex LH-20 phase in the presence of of MT (1-mm cell, similarly packed with Sephadex LH-20 equilibrated with water as reference). [BCG] = 10−4 M; 70 mg Sephadex LH-20; from up to down [MT] = 0.0 and 1.26
Optimum pH for the formation of ion-pair and fixation of BCG on Sephadex LH-20 falls below 4.0 (Figure
Influence of pH on the absorbance of BCG in solid phase. Conditions: 1.26
The optimum stirring time before and after adding of Sephadex LH-20 was 4 min and 5 min, respectively (Figure
Influence of stirring time on the absorbance of BCG in solid phase. Conditions: 1.26
Stability of absorbance of BCG in solid phase on duration time. Conditions: 1.26
The calibration graph is reasonably linear for the concentration ranges 0.42–1.68
Analytical parameters.
Intercept | 0.001 |
Slope (mL |
0.164 |
Linear dynamic range ( |
0.42–1.68 |
Correlation coefficient | 0.9984 |
RSD (%) | 0.03 |
Detection limit ( |
0.05 |
ΔA for sample |
|
aAverage of three determinations.
Reproducibility was measured for a series of four independent determinations containing 1.26
The sensitivity, expressed as molar absorptivity, of the proposed method is compared in Table
Comparison of sensitivity of some methods for the determination of methenamine.
Method | Molar absorptivity |
Reference |
---|---|---|
Hantzsch esters formation |
|
[ |
Complex of MT with iodine |
|
[ |
FerroZine method |
|
[ |
Proposed method (SPS) |
|
This work |
Methenamine wastewater that is formed in the production of MT usually contains residual amounts of MT and formaldehyde. It has been reported that formaldehyde is the most serious interference in the determination of MT [
Effect of formaldehyde on the determination of 1.26
Content ( |
Absorbance of solid phasea |
---|---|
0 | 1.680 |
3.2 | 1.680 |
6.4 | 1.680 |
aAir as reference.
The method was applied to the determination of MT in wastewater sample by calibration curve method (ΔA versus concentration of MT). To check the accuracy of the proposed method, conductometric method [
Determination of methenamine in industrial wastewater sample.
Obtained content of MT by SPS after dilution (10000 times) ( |
Obtained content of MT by conductometric method after dilution (1000 times) ( |
---|---|
|
|
aReference [
Methenamine at the microgram level can be determined efficiently with BCG using the solid phase spectrophotometry technique without expensive apparatus. The proposed method has been applied to the determination of MT in industrial wastewater sample with good result. The method is simple and more sensitive as compared to others commonly used at the microgram level. The method does not use organic solvents, extra organic compounds as reactants, and pretreatment of the sample. Therefore, the method is environmentally friendly and can be considered as a green analytical method. It has been demonstrated that formaldehyde does not interfere in the determination of MT by proposed method.
The authors express their appreciation to Sina Chemical Industries Company and Payame Noor University for supporting this study.