Silica

Silica, also known as silicon dioxide (SiO₂) or mineral quartz, is a derivative compound of the chemical element silicon (Si) and makes up approximately 59% of the Earth's crust and a large part of common sand. Silicon links up with oxygen to form silicates, which are the most common suite of minerals.

Silica is found as dissolved silica or suspended silicate particles in most natural waters. Silica is used in many industrial applications, including food and pharmaceutical production, as an additive and for water filtration.

Analytical Procedures and Methods for Silica

Analytical procedures for silica include the Silicomolybdate Method for high range measurement and the Heteropoly Blue Method for low range measurement. The Heteropoly Blue method is an extension of the Silicomolybdate method to increase sensitivity.

Silica methods from Hach^® determine molybdate-reactive silica. The terms 'molybdate-reactive' or 'molybdate-unreactive' do not imply reactivity, or lack of it, toward other reagents or processes. Occasionally, a sample contains silica which either reacts very slowly with molybdate or does not react at all. In at least one of its forms, silica does not react with molybdate even though it is capable of passing through filter paper being present in dissolved or colloidal form. It is not known to what extent such “unreactive” silica occurs in waters.

“Molybdate-unreactive” silica can be converted to the “molybdate-reactive” form by heating or fusing with alkali, such as in a sodium bicarbonate digestion. See Standard Methods 4500-SiO2 for details.

In both Silicomolybdate and Heteropoly Blue Methods the color formed is directly proportional to the amount of molybdate-reactive silica present in the original sample. Total silica can be determined by performing analyses using the Electrothermal Atomic Absorption Method (Standard Methods 3113B), the Inductively Coupled Plasma Mass Spectrometric Method (Standard Methods 3125) or the Inductively Coupled Plasma Method (Standard Methods 3120).

Terms such as “colloidal,” “crystalloidal,” and “ionic” have been used to distinguish among various forms of silica but such terminology cannot be substantiated. Hach does not provide solutions or support for any of the methods for total silica.

At Hach, find the testing equipment, resources, training and software you need to successfully monitor and manage silica levels in your specific process application.

The Method 8282 has specific recommendations for maintenance of the labware and cleaning the Pour-Thru cell.

Labware cleaning:

Fully clean all containers that are used in this test to remove any traces of silica. Use plastic containers for all analysis and storage because glass can contaminate the sample with silica. Small bottles or flasks with screw-type closures work well.

• Clean containers (do not use phosphate detergents), then rinse with high quality deionized water of low-level silica concentration.
• Soak for 10 minutes with a 1:50 dilution of Molybdate 3 Reagent in low-level silica water.
• Rinse repeatedly with either low-level silica water or the sample before use. Keep containers tightly closed when not in use.
• Fill the Pour-Thru Cell with this same mixture of Molybdate 3 and water. Soak for 10 minutes.
• Rinse with low-level silica water.

Keep containers tightly closed when not in use and rinse the labware repeatedly with either low-level silica water or the sample before use.

Pour-Thru cell cleaning:

The Pour-Thru Cell can collect a buildup of products with color, especially if the reacted solutions stay in the cell for long periods of time after measurement.

• Rinse the Pour-Thru Cell with a 1:5 dilution of ammonium hydroxide solution to remove the color.
• Fully rinse with deionized water.
• Put a cover on the Pour-Thru Cell funnel when it is not in use.

The following can also be done to clean the Pour-Thru cell:

• Clean Pour-Thru cell assembly with phosphate free detergent, then rinse with high quality DI water.
• Soak for 10 minutes in 1:50 dilution of molybdate 3 reagent in low-level silica water. This should rid the sample of any adhering silica.

The EZ Series EZ1034 can analyze from 1 µg/L up to 100 µg/L of Si and the EZ1035 can measure up to 1 mg/L Si with an internal dilution.

The reagent 1 (R1) blank value can be found on the reagent bottle. If the sample has very low silica concentrations and has issues with high readings (one to two ppb high), which are not caused by blueing of the sample cell or instrument issues, then a field blank determination can be performed to adjust the blank value. Clean the sample cell by following the procedure listed in the user manual.

The method used to determine the blank on the 5500sc Silica Analyzer is the standard additions method. This method requires a stable sample concentration feed to the analyzer for the duration of the testing, which will be a couple hours. Begin by performing a grab sample test with the sample water, and record the analyzer reading. Collect 500 ml of sample, and to this add 2 ml of 500 ppm standard, then run this spiked sample as a grab sample and record the reading. Collect 500 ml of sample, and to this add 4 ml of 500 ppm standard, then run this spiked sample as a grab sample and record the reading. Repeat running grab samples spiked with 6, 8, and 10 ml, recording each reading.

When the spiked grab sample readings have been collected, then chart these on graph paper (or in Excel). Spike value - 0, 2, 4 etc., ppb is the x-axis; sample reading in the y-axis. Draw a best-fit straight line through the charted points. Where the line crosses the x-axis is the value of the reagent blank. (The intercept will be a negative value, but it is entered as a positive blank value.)

Determination of reagent blank is performed at concentrations near the detection limit of the analyzer, and the spiked sample readings noted above are subject to noise and interference. The labware and mixing vessels must be scrupulously clean for good results. It is best to repeat each standard addition at least three times to assure results are repeatable. Final accuracy of the blank determination will probably be around +/- 1 ppb, and very likely in this range of agreement with the value on the Molybdate reagent bottle.

A disagreement of 1-2 ppb between two analyzers running sample that is itself only 1-2 ppb (or even less) can be very difficult to isolate. If the process system is stable and runs a known baseline response following regeneration, and no other fault can be found in the analyzer, the most practical solution may be to adjust the blank value on this analyzer so that readings match the known baseline.

The reagents and standards for the 5500sc have the following shelf life:

5500sc silica reagent set product # 6783600 includes:

• 5500sc reagent 2 silica 2-liter product # 6774902 has a shelf life of 720 days from the date of manufacture unopened.
  
• 5500sc reagent 3 silica 1.8-liter product # 6775202 has a shelf life of 720 days from the date of manufacture unopened.
  
• 5500sc reagent 1 silica 2-liter product # 6774802 has a shelf life of 720 days from the date of manufacture unopened.

The exact date and month of expiration will be printed on the label for each bottle.

The reagents for the 5500 silica/phosphate analyzer are a different concentration versus the older Series 5000 silica/phosphate analyzers reagents and therefore, are not compatible. Due to the difference in concentrations, the reagent volumes are different and can cause accuracy issues on a different model of analyzer.

Please note, the calibration standards for the 5500sc silica/phosphate analyzers are the same with the Series 5000 model.

If a standard bottle for the Series 5000 is unopened and has not expired, the contents may be used to calibrate for the same parameter on a 5500sc analyzer.

Before use transfer the content to the correct bottle style for the 5500sc systems. An incorrect bottle type may result in an air or standard leak that could damage the system.