Standards Development

Scope

This standard specifies the method for detection of silica in water and wastewater.

This colorimetric method determines molybdatereactive silica. It is applicable to most waters, though some may require filtration and dilution to remove interferences from colour and turbidity. This method is useful for concentrations as low as 20 g/l.

Purpose

The objective is to develop an effective and precise method for measuring silica in water, addressing the concerns raised by all stakeholders.

Justification for an additional standard method for silica measurement:

ISO 16264 Standard is based on Flow Injection Analysis (FIA) and Continuous Flow Analysis (CFA), employing spectrophotometry to control silica levels in industrial water, particularly boiler feed water, due to the risks of silica deposition on turbine blades. The method's lower detection limit, according to device calibration, is approximately 0.2 ppm as SiO. However, there is no evidence of calibration data for concentrations around 0.02 ppm as SiO in the standard's comparative results.

ISO 15923-1 Standard measures silica using photometry, with a calibration limit near 0.1 ppm as SiO. Similar to ISO 16264, there is no record of calibration outcomes for concentrations of 0.02 ppm as SiO in inter-laboratory comparisons.

ISO 11885 Standard utilizes Inductively Coupled Plasma (ICP) instrumentation, which can measure silica concentrations in low range. However, the high cost of ICP equipment limits its accessibility to many laboratories.

Need for a New Method:

Monitoring silica at concentrations around 0.02 ppm as SiO is critical in boiler feed water, groundwater, surface water, wastewater, and particularly industrial water, due to the danger of silica scale formation on turbine blades. To address this, a modified method based on ASTM D-859, which employs spectrophotometry, is proposed. This method is capable of accurately measuring silica concentrations around 0.02 ppm as SiO.

The absence of silica in boiler feed water is critically important in the industry. Therefore, determining silica at low levels (as low as 0.02 ppm) is essential.

In the standard ASTM-D859 method, preparing one of the reagents requires 1-amino-2-naphthol-4- sulfonic acid, sodium hydrogen sulfite, and sodium sulfite. The new method replaces these harmful substances with ascorbic acid, which is safe, environmentally friendly, and free from sulfur compounds. This makes the solution preparation safer, easier, and more economical.

Another method, ISO-16264, is used in online systems. In this method, the volumes of reagents added to the sample are unknown. In the new method, reagent volumes are known. This new method eliminates the need for fume hoods, enhancing the health and safety of laboratory staff, saving time in solution preparation, and increasing instrument lifespan by using non-scaling reagents.

Finally, New method is based on the reaction of soluble silica with molybdate ion to form a greenish yellow complex, which is then converted to a blue complex by reduction with ascorbic acid.

This method covers the photometric determination of molybdatereactive silica in water. Given the complexity of silica chemistry, the form of silica measured is defined by this method as molybdatereactive silica, which includes dissolved simple silicates, monomeric silica, silicic acid, and an undetermined fraction of polymeric silica.

The effective range of this method spans 20 to 1000 g/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L at the lower wavelength (640 nm). It's particularly useful for treated industrial waters and can be applied to natural waters and wastewaters after filtration or dilution. For seawater or brines, this method is only applicable if the matrix is matched.

Analysts should be aware that precision and accuracy statements for reagent water solutions might not be applicable to waters with different matrices.