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Determination of Silicate in Mineral Water by Ion Chromatographic Column Derivatization

Item No.: YQ004
Abstract: A method for the determination of silicate in mineral water by anion exchange separation and post-column derivatization was established. The results show that the separation limit of phosphate in the mineral water matrix can be used to eliminate
Abstract: A method for the determination of silicate in mineral water by anion exchange separation and post-column derivatization was established. The results show that the separation limit of phosphate in the mineral water matrix can be used to eliminate the interference of the phosphate in the mineral water. The limit of the method is 0.8mg • L-1, the recovery is 97.7 ~ 105.9%, the reproducibility (RSD%, n = 3) 0.26% to 0.88%. Compared with the national standard method, this method has the advantages of high sensitivity and no phosphate matrix interference.
Key words: silicate, ion chromatography, post - column derivatization, mineral water
Silicon is a trace element necessary for the human body and is generally present in the form of silicic acid. Polycyclic acid can effectively maintain the body's electrolyte balance and physiological function, with the promotion of bone and tooth development, conducive to bone calcification, prevention of osteoporosis and other effects of drinking water in the level of silicon and cardiovascular disease incidence was negatively correlated. Polycyclic acid can effectively relieve arteriosclerosis, cardiovascular and heart disease, increase skin elasticity.
At present, the determination of silicate in mineral water or drinking water and groundwater is silicon molybdenum yellow spectroscopy [1], silicon-molybdenum blue spectroscopy [1], ion chromatography non-inhibitory conductance method [2], ion chromatography- Method [3], ICP-AES [4] and so on. Ion chromatography non-inhibitory conductivity method is less sensitive, ICP-AES method although the sensitivity is high but the instrument is expensive. Silicon molybdenum blue spectroscopy and silicon molybdenum yellow spectroscopy, although highly sensitive, are susceptible to phosphate interference in the sample. Ion Chromatography - Post-Column Derivatization The use of anion exchange separates the silicate from other anions and reacts with the developer, with a dual selectivity. In this paper, an anion exchange separation - post - column derivatization method for the determination of silicates in mineral water is proposed. The method has high accuracy and good reproducibility, and eliminates the interference of phosphate in the mineral water to the silicate color reaction.
1 experimental part
1.1 Instruments and reagents
Instrument: PIC-10 type ion chromatograph (Qingdao Pu Ren Instrument Co., Ltd., with UV - visible detector)
Analysis of balance, precision 0.1mg (Germany Sartorius company)
     0.22μm filter: Qingdao Pu Ren Instrument Co., Ltd. production
Reagents: Na2CO3, NaHCO3, KH2PO4, Na2SiO3 • 9H2O, Na2MoO4, HNO3, C12H25OSO3Na (Analytical Pure, Shanghai Erbi Chemical Reagent Co., Ltd.)
Ultra pure water (resistivity greater than 18.2MΩ / cm)
1.2 chromatographic conditions
Column: NJ-SA-4A (250 mm x 4.6 mm), eluent: 2.50 mM Na2CO3 + 1.8 mM NaHCO3, flow rate: 1.50 mL min, derivatization: 20 mM Na2MoO4 + 200 mM HNO3 + 6mMC12H25OSO3Na at a flow rate of 0.5 mL / min. Detector: UV - Visible detector, detection wavelength: 410nm. To retain the time qualitative, peak area quantitative. Injection volume of 25mL.
1.3 Sample pretreatment
Filtration with 0.22 mm filter was analyzed.
2 Results and discussion
2.1 Selection of Derivatives
Silicate itself does not have obvious UV-visible absorption, but in acidic conditions can react with molybdate to produce silicon molybdenum yellow, which in a certain reducing agent conditions can produce silicon molybdenum blue. Silicon molybdenum yellow and silicon molybdenum blue have obvious visible light absorption, can be used as post-column derivatization method for the detection of silicate. [5] shows that silicon molybdenum yellow and silicon molybdenum blue compared to the sensitivity, accuracy and reproducibility of the two are better, but the silicon molybdenum blue higher sensitivity, narrow linear range, suitable for the determination of silicon Salt concentration is low. Silicon molybdenum blue method applied to the ion chromatographic column after the derivative system, the need for derivatives and reductants were prepared and stored [6], an increase of the preparation of the instrument. Determination of mg / L grade silicate solution, we use silicon molybdenum yellow method.
2.2 Silicate linearity and quantitation limits
Under the determined chromatographic conditions, the standard values ​​were sequentially entered into the ion chromatograph according to the order of concentration from low to high, and the limit of quantification was calculated at 10 times the signal to noise ratio (S / N = 10). The linear range, linear equation, correlation coefficient and quantitation limit of SiO32- are shown in Table 1, where Y is the peak area and X is the mass concentration of the component (mg • L-1). The sample obtained by processing 1.4 is input to an ion chromatograph. The silicate ion chromatogram in mineral water is shown in Fig.

Figure 1 Determination of silicate in mineral water by ion chromatography (1: SiO32-)
Table 1 Linear range of SiO32-, linear equation, correlation coefficient and detection limit
  Linear range /mg·L-1 Linear equation Correlation coefficient/ R limit of quantitation /mg·L-1 Determination of silicate in GB8538-2016 /mg·L-1
SiO32- 0.5~50.0 Y=3734X+8.88 0.9998 0.8 1.0
Under the known chromatographic conditions, the silicate external standard curve has a good linearity, the sensitivity is better than that of GB8538-2016 in the silicon molybdenum yellow-spectrophotometry, and this method uses only 25mL injection volume, and GB8538-2016 Using a 50 mL water sample. The reason for this method is that the separation of the silicate in the column is also a process of enrichment, and thus has a more sensitive response under the same developer conditions.
2.3 Spike recovery and method reproducibility
    In the commercial mineral water samples were added silicate for spiked recovery experiments, parallel determination of 3 times. The recoveries and reproducibility results are shown in Table 2.
Table 2 in the mineral water silicate spiked recovery and reproducibility
sample Backgroundmg·L-1 Add tomg·L-1 measured  valuemg·L-1 Recovery rate /% RSD/%(n=3)
A 20.5 2 22.53 22.39 22.65 101.2 0.58
5 25.40 25.44 25.31 97.7 0.26
B 18.0 2 20.11 20.19 20.05 105.8 0.35
5 23.31 23.08 23.49 105.9 0.88
It can be seen from Table 2 that the method has high accuracy in the determination of silicate in mineral water, the recovery rate is close to 100% and the reproducibility is less than 1.00%, which is a reliable method for the determination of silicate in mineral water.
2.4 Phosphate on the silicate determination of interference
    GB8538-2016 silicon molybdenum yellow method used in the determination of silicate, it should be noted that mineral water phosphate in the matrix also has a similar color reaction [1,5], phosphate will result in high silicate determination results, this effect Can be eliminated by adding oxalic acid solution. In this method, the anion is first separated by a column to react with the developer, while the phosphate and the silicate retain a large difference in the column. First, the silicate in the mineral water samples was directly measured, and then 20mg / L H2PO4- was added to the mineral water samples. The retention time, peak area and peak height of the silicate were found under the same chromatographic conditions. Basically consistent, repeated determination of silicate has not yet found the retention time, peak area, peak height has changed significantly. Thus, there is no interference of the phosphate with the silicate assay.
3 Conclusion
    The silicate in mineral water was tested using anion exchange separation - post - column derivatization. The method uses anion exchange separation to enrich the silicate, improve the sensitivity, but also can eliminate the phosphate on the silicon molybdenum yellow color reaction interference for the mineral water products in the silicate detection provides a reliable method.