Spectrophotometric Determination of Trace Magnesium in Water

Magnesium is one of the elements closely related to human survival. Currently, the main methods for magnesium determination are EDTA-2Na titration and atomic absorption spectrometry [1, 2]. The former requires heating to release magnesium after calcium determination, making the operation more complicated and the endpoint difficult to observe. The latter has high accuracy and is easy to operate, but the instruments are expensive. This paper utilizes the fact that adding magnesium to Congo red alkali solution causes the absorption spectrum to shift towards the red cluster by Δ60 nm, accompanied by a coloring effect; under the reaction conditions, the yellow staining of the reactant changes to red staining [3]. The presence of salicylic acid can improve the selectivity and sensitivity of magnesium determination. Experimental results show that the method has high precision and good recovery rate. The application of this method to detect magnesium in drinking water is simple, rapid, and yields satisfactory results.

Materials and Methods

1. Instruments and Reagents: UV-1206 spectrophotometer (Shimadzu, Japan), constant temperature water bath (Zhangjiagang Medical Instrument Factory), 10 g/L salicylic acid ethanol solution, 0.3 g/L Congo red solution, 2 mol/L sodium hydroxide solution. Magnesium Standard Stock Solution: Accurately weigh 10.140 g of magnesium sulfate (MgSO4·7H2O), dissolve in water, and dilute to 1000 ml. Store in a polyethylene bottle [4]. The magnesium content of this solution is 1.00 mg/ml. Dilute to 10.0 μg/ml immediately before use. All reagents are analytical grade, and the water is deionized water.

2. Test Method: Take 25 ml of water sample (to ensure the magnesium content is within 50 μg) into a 25 ml colorimetric tube. Separately, take 0, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 ml of magnesium standard solution into 25 ml colorimetric tubes and add pure water to the mark. Add 1.0 ml of salicylic acid ethanol solution to the sample tube and standard tube, mix well; then add 1.0 ml of Congo red solution and mix well again. Incubate in a 37℃ water bath for 15 min. Remove and add 1.0 ml of sodium hydroxide solution. Place at room temperature for 30 min. Using a 1 cm cuvette, zero the reagent blank and measure the absorbance A at a wavelength of 550 nm.

Results and Discussion

1. Conditional Experiments: (1) Absorption Spectrum and Absorption Coefficient. The absorbance A of the reagent blank and the magnesium complex at different wavelengths was determined according to the experimental method, and the absorption spectra were plotted. The maximum absorption peaks of the reagent blank and the magnesium complex were at 490 nm and 515 nm, respectively. Due to the high absorption of the reagent blank, the difference in absorption was greatest at 550 nm after subtraction using the reagent blank as a reference tube. 550 nm was selected as the measurement wavelength in this method. The absorbance coefficient of the magnesium complex at 550 nm was 3.97 × 10³. (2) Amount of Complexing Agent. The experimental results showed that the concentration of Congo red affected the linear range, blank value, and sensitivity of the method. Increasing the concentration of Congo red widened the linear range, but the blank value increased accordingly, and the sensitivity decreased, resulting in reduced reproducibility. 1.0 ml of 0.3 g/L Congo red solution was selected in this method. (3) Effect of sodium hydroxide dosage. Experiments showed that absorbance increased with increasing sodium hydroxide concentration, reaching its maximum in a 0.04 mol/L solution. Absorbance changes were minimal in higher concentrations of sodium hydroxide solution (figure omitted). The method used 1.0 ml of sodium hydroxide. (4) Salicylic acid dosage control. Salicylic acid concentration directly affects the sensitivity and selectivity of the method, and also masks trace amounts of Ca2+, Al3+, and Fe3+. The optimal dosage is 0.7–1.5 ml, and the method used 1.0 ml. (5) Reaction temperature, time, and color stability. Experiments showed that magnesium reacts slowly with salicylic acid and Congo red at room temperature, exhibiting poor stability. Complete reaction is only achieved under heating conditions. Color stability was maintained for 24 hours with virtually no change in absorbance (table omitted). The experiment selected a reaction temperature of 37℃ and a reaction time of 15 min.

2. Linear range and detection limit: Absorbance was determined for different Mg2+ contents according to the experimental method. Experiments showed that the Mg2+ content had a good linear relationship with absorbance A in the range of 0–2.0 μg/ml, and the regression equation was:

A = 0.00643C (μg/25ml) + 0.00324 Correlation coefficient r = 0.999. After 22 blank reagent determinations, the limit of detection of this method was calculated to be 2.0 μg.

3. Composition of the complex: The complex was determined by the molar ratio method and the continuous molar variation method. Its composition was: magnesium: Congo red = 4:1.

4. Effect of coexisting ions: The determination was carried out under the condition that the magnesium content was 1.00 μg/ml and the relative error was < ±5%. The following coexisting substances (in μg/ml): K+, Na+, Cl- (500), SO42- (50), NO3- (30), Ca2+ (2) did not interfere with the determination. But 1 μg... Positive interferences were generated by Mn²⁺, Fe³⁺, Sr²⁺, and Al³⁺ at 1 ml⁻¹; negative interferences were generated by Zn²⁺, Cu²⁺, Ni²⁺, and Li⁺ at 1 μg·ml⁻¹. Interferences can be eliminated using potassium cyanide and sulfosalicylic acid. This method is applicable to the analysis of drinking water quality.

5. Sample Analysis: (1) Synthetic Sample Analysis. Twelve determinations were performed on a synthetic water sample with a magnesium content of 1.6 μg/ml, yielding a result of 1.62 ± 0.06 μg/ml, with a relative standard deviation of 3.7%. (2) Water Sample Analysis. The method was used to analyze drinking water samples, and the results are shown in Table 1. The standard recovery rate was 96%–106%.

Table 1: Water Sample Analysis Results

References:

[1] National Standard of the People's Republic of China GB/T8535-1995. Test Methods for Drinking Natural Mineral Water

[2] Compilation Group of "Complete Guide to Water Quality Analysis". Complete Guide to Water Quality Analysis. Chongqing Branch of Science and Technology Literature Press, 1989: 284

[3] Putilina. O. N; Makareskaya. V. V. ∥жанал. хим—1990, 45(6)—1173—1177

[4] Translated by Song Enlie et al. Practical Inorganic Photometric Analysis. Liaoning People's Publishing House, 1983: 74