Precise Silica Analysis: The Role of Oxalic Acid in Spectrophotometric Determination

Introduction to Silica Analysis

Silica analysis in water samples is crucial for determining the amount of silicon dioxide (SiO2) present in natural or treated water. Spectrophotometric methods are widely used for this purpose due to their accuracy, precision, and ease of implementation. This article delves into the specific role of oxalic acid in enhancing the accuracy of silica analysis through a detailed examination of the reagents and procedures involved.

Chemicals Required for Spectrophotometric Analysis of Silica in Water Samples:

Ammonium Molybdate as Ligand

Ammonium molybdate is a key reagent in silica analysis due to its ability to form a yellow silicomolybdic acid complex. This complex has an absorption maximum at 410nm with a molar extinction coefficient of 1500 l/mol/cm and a limit of detection down to 0.0001 mol/lit. This high sensitivity makes it ideal for precise silica quantification. However, phosphate compounds present in water can also react with ammonium molybdate, forming a yellow phosphomolybdic polyacid that absorbs at the same wavelength as silica. This interference must be minimized.

The Role of Oxalic Acid

Oxalic acid plays a critical role in silica analysis by selectively breaking down the yellow phosphomolybdic complex. Unlike acetic acid, which I previously used, oxalic acid is more effective in the presence of interfering metal ions due to its stronger reducing property. When added to the silica solution, oxalic acid helps to isolate the silicomolybdic acid complex, thereby minimizing interference from phosphorylated compounds.

Titration Reaction and Optical Density Measurement

The titration reaction involves adding equal amounts of molybdate and oxalic acid (along with sulfuric acid) to the silica solution. This process forms a complex that is then measured for optical density. This optical density is used to determine the original silica content in the sample. The resulting absorbance at 410nm is particularly important for accurate quantitative analysis.

Reduction Agent ANSA

Additionally, a reducing agent such as ANSA (1-amino-2-naphthol-4-sulphonate) is added to reduce the silicomolybdic acid in both silica and phosphate analysis to molybdenum blue complex, which exhibits an absorption maximum at 810nm with a molar extinction coefficient of 40000 l/mol/cm. This complex allows for accurate measurement of even trace amounts of silica, down to concentrations as low as 20 ppb.

Conclusion:

Oxalic acid is indispensable in silica analysis for its ability to selectively disrupt the interfering phosphomolybdic complex, ensuring more accurate and reliable results. Together with reducing agents and sulfuric acid, it forms a critical part of the effective silica analysis procedure.