Determination of Silica in Geological Samples by Silicon-Molybdenum Blue Spectrophotometry Using High-pressure Acid Digestion
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摘要: 由于聚合态的硅酸无法与钼酸根定量络合，采用硅钼蓝比色分光光度法测定地质样品中的硅，其关键在于如何将固体试样消解制备成溶液且保证硅酸全部以单分子状态存在。本文以氢氟酸-硝酸封闭酸溶消解地质样品，使样品中的硅完全转化为氟硅酸稳定存在于溶液中；再加入硼酸和钼酸铵，使过量的氢氟酸与硼酸生成稳定的BF4-配离子，此步骤代替了蒸干赶除氢氟酸，从而避免了赶酸过程中硅与氟离子转化为气态SiF4而挥发损失，在钼酸铵作用下，硅则充分转变为硅钼杂多酸。在显色过程中通过加入丙酮显著提高了硅钼黄的稳定性，且在一定程度上增加了吸光度，从而改善了硅钼蓝分光光度法测定硅的显色效果。该方法避免了常规碱熔消解样品后在酸化过程中硅酸易聚合、引入大量熔剂造成空白偏高等问题，经岩石、土壤、沉积物和石英岩国家标准物质分析验证，SiO2测试结果准确且重现性好（RSD < 1%，n=10），适用于一般地质样品中硅的快速、准确分析。Abstract:
BACKGROUNDPolymerized silicic acid cannot be quantitatively complexed with molybdate in geological sample, therefore the silicon is determined by silicon-molybdenum blue colorimetric spectrophotometry. The key of this method is how to digest the solid sample to prepare a solution and ensure that the silicic acid occurs as a single molecule. OBJECTIVESTo completely digest the geological samples by high-pressure acid digestion without loss of silicon and polymerization of silicic acid. To improve the stability of silicon-molybdenum yellow by adding a stabilizing agent. METHODSHydrofluoric acid-nitric acid was used to digest the geological samples, so that the silicon in the sample was completely converted into fluorosilicic acid and was stably present in the solution. Boric acid and ammonium molybdate were added to make the excess hydrofluoric acid and boric acid to form stable BF4- dosing ion, which replaced the evaporation and removal of hydrofluoric acid, thus avoiding the volatilization loss of silicon and fluoride ions converted into gaseous SiF4 during the acid removal process. Under the action of ammonium molybdate, silicon was fully converted into silicon molybdenum heteropolyacids. In the color development process, the stability of silicon-molybdenum yellow was significantly improved by adding acetone, and the absorbance was increased to some extent, thereby improving the color development effect of silicon by silicon-molybdenum blue pectrophotometry. RESULTSThe method avoided the problem that the silicic acid was easy to be polymerized during the acidification process when using the conventional alkali fusion digestion. A large amount of flux was introduced to cause a high blank. The validity of the method was evaluated by analyses of rock, soil, sediment and quartzite reference materials. The results of SiO2 were in good agreement with certified values and the relative standard deviation (n=10) was less than 1%. CONCLUSIONSThis method is suitable for the rapid and accurate analysis of silica in common geological samples. -
Key words:
- rock /
- soil /
- sediment /
- silica /
- HF-HNO3 dissolution /
- high-pressure digestion /
- spectrophotometry
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表 1 丙酮对硅钼黄稳定性的影响
Table 1. Effect of acetone on the stability of silico molybdenum yellow
硅钼黄显色时间 吸光度 加丙酮 不加丙酮 15min 0.726 0.680 5h 0.725 0.562 
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表 2 标准物质SiO2分析结果
Table 2. Analytical results of SiO2 in reference materials
标准物质编号 SiO2含量(%) RSD(%) 认定值 10次测定值 测定平均值 GBW07104
(岩石)60.62 61.31 59.95 60.46 60.64 60.91 60.63 0.69 60.58 60.28 60.21 60.99 60.99 GBW07426
(土壤)60.01 59.21 60.2 59.51 59.04 59.88 59.75 0.78 59.81 60.22 60.34 59.27 60.06 GBW07320
(沉积物)63.01 62.79 62.91 63.48 63.08 63.02 63.04 0.29 63.09 62.91 63.08 63.02 63.02 GBW07487
(沉积物)75.24 74.39 74.61 74.34 74.57 74.98 74.82 0.99 75.34 74.63 74.57 76.68 74.05 GBW07837
(石英砂)99.18 100.00 99.47 99.69 98.87 98.33 99.36 0.56 99.04 99.56 99.54 98.94 100.11
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