1,2-Hexanediol UV spectrophotometry

1,2-Hexanediol UV spectrophotometry is an analytical method based on the absorption characteristics of substances towards ultraviolet light. 1,2-hexanediol, as a six carbon diol containing two adjacent hydroxyl groups, has a conjugated system in its molecular structure with characteristic absorption peaks in the wavelength range of 200-400 nm. In ethanol medium, when the concentration of 1,2-hexanediol is between 0.01-1.00 mmol/L, the absorbance of the solution at 267 nm shows a good linear relationship with the concentration, and the correlation coefficient can reach 0.9993.
The experiment requires the use of a quartz colorimetric dish to eliminate the interference of glass material in absorbing ultraviolet light. The preheating time of the instrument shall not be less than 30 minutes, and the baseline drift shall be controlled within ± 0.002 Abs. The standard curve drawing should include 7 concentration gradients, with each concentration measured in parallel 3 times and the mean taken. Sample pretreatment requires filtration through a 0.45 μ m microporous membrane to eliminate the Rayleigh scattering effect caused by suspended solids.
The pH value of the solution has a significant impact on the measurement results. When the pH of the medium deviates from neutrality, the hydroxyl dissociation of 1,2-hexanediol will alter the molecular conjugation system, resulting in a red or blue shift of the absorption peak. The experiment found that within the pH range of 6.8-7.2, the absorbance value at 267 nm remained stable, and for every 0.5 unit change beyond this range, the absorbance deviation could reach 5% -8%.
The temperature control should be maintained at 25 ± 0.5 ℃. An increase in temperature can cause a change in the refractive index of the solution, resulting in a decrease in the apparent molar absorptivity. The measured data shows that for every 1 ℃ increase in temperature, the absorbance at 267 nm decreases by about 0.8%. For samples containing trace metal ions, it is recommended to add 0.1 mmol/L disodium EDTA as a masking agent, which can effectively eliminate the coordination interference of transition metal ions such as Fe ^ 3+and Cu ^ 2+.
When using the standard addition method for quantitative analysis, the addition amount should be 50% -150% of the estimated concentration of the sample. For complex matrix samples, it is recommended to use second-order derivative spectroscopy to eliminate background interference. The detection limit of the method was calculated to be 0.0072 mmol/L, and the quantification limit was 0.024 mmol/L. The intra day precision RSD is 0.45%, and the inter day precision RSD is 1.2%.
In terms of industrial applications, this method has been successfully used for online monitoring of 1,2-hexanediol impurities in the production process of ethylene glycol. Compared with gas chromatography, the relative error of the results obtained by the two methods is less than 3%. The study also found that when there is more than 0.1% 1,3-propanediol in the solution, overlapping absorption peaks will occur at 254 nm, and partial least squares method needs to be used for multivariate correction.
Experimental precautions include: avoiding the use of plastic containers to prevent the dissolution of plasticizers; The outer wall of the colorimetric dish needs to be thoroughly cleaned with mirror paper; Wash the colorimetric dish three times with the test solution before each injection; If the accumulated usage time of deuterium lamps exceeds 1000 hours, the attenuation of light intensity should be detected. The criteria for determining abnormal data follow the 3 σ criterion, and measurements that exceed three times the standard deviation of the mean should be retested.
This method has the characteristics of simple operation and fast analysis speed compared to traditional chemical titration, with a single sample detection time of no more than 10 minutes. However, it should be noted that high concentration samples may produce negative deviations due to exceeding the linear range of Beer's law. In this case, a 1 cm optical path colorimetric dish should be selected for dilution and measurement. With the popularization of diode array detectors, the application prospects of this method in process analytical chemistry will be even broader.