Silica Analysis

Article written by Jeffrey Stewart, QC Chemist, Analytical Products Group, Inc.

Complete article from Edition 25 APG eNewsletter

Silica is found almost everywhere. Silica ranks second in abundance in the earth's crust behind oxygen. The concentration of silica can range anywhere from one to thirty ppm in natural water to almost 1000ppm in brackish waters and brines.

With so much silica in the environment, how does one test for low levels of silica in a solution? The two main ways that the Power industry uses to test for silica include colorimetric procedures done in a lab and by an online silica analyzer. Within the colorimetric procedure there are two ways of doing the analysis. Both procedures should be done with as little glassware as possible since the glassware can contribute to the silica concentration. Another method for determining silica concentrations is by Ion Chromatography. Another way to measure for Silica is by using a Hach Pocket Colorimeter. With the Hach instrument, only twenty-five milliliters of sample is used. Ten milliliters are used to zero the instrument, and another ten milliliters are color developed for the analysis of silica.

The first method of silica colorimetric analysis is called the Molybdosilicate method. In this method, the analyst dilutes the samples within the range for testing. The range for testing is dependent on what size cell is being used. A one cm cell will test within the range of 200-1300ppb while a five cm cell has a range of 40-250ppb. The sample is diluted within the range. A volume of fifty milliliters is used during the color development. The sample is treated with one milliliter of 1:1 HCl and two milliliters of ammonium molybdate reagent. The sample is then inverted and allowed to set for five minutes. Two milliliters of oxalic acid is added to the sample and allowed to set for two minutes. The oxalic acid is used to remove any interferences from phosphate and decrease interferences from tannin. The sample is then ran photometrically. The down side to this method is the instability of the color produced.

The second method of silica colorimetric analysis is called the Heteropoly Blue method. This method is similar to the Molybdosilicate method except that an additional reagent is added to the sample. A reducing agent made up of 1-amino-2-naphthol-4-sulfonic acid is used. When this reagent is added to the sample, the reaction takes on a blue color. The blue color produced in this reaction is more stable than the yellow color from the molybdosilicate method. Also the range for the heteropoly blue method is much lower. A one-centimeter cell has a range of 20-100ppb at 815nm wavelength while a five-centimeter cell has a range of 4-20ppb. With this method, the samples may need to be diluted to run within the calibration range.

Where can I possibly go wrong in running the test?
There are several places during the analysis of silica that something can go wrong. One is in the preparation of the reagents. All reagents need to be stored in polyethylene bottles. If the reagents are stored in glass containers, silica may leach out of the glass causing high silica blanks.

Another area of concern is the 1-amino-2-naphthol-4-sulfonic acid reagent used in the Heteropoly Blue method. The reagent has a two-month shelf life. As the reagent sits, the solution settles out and can cause inconsistent results.

How can I verify the accuracy of my results?
Whenever a sample of unknown concentration is analyzed, the result is only as accurate as the instrument on which the test is performed. The accuracy and performance of the instrument is directly associated with reliable and accurate standardization or calibration. In order to ensure accurate results, an instrument should be calibrated with analytically prepared calibration standards that are traceable to a NIST standard reference material where possible.