IAC SERIES 1400 SILICA TECHNICAL INFORMATION

Why Measure Silica? | Simple and Reliable Operation

Simple and Reliable Operation

The microprocessor controlled transmitter produces accurate and reproducible data, which are displayed in a bar/numeric and graphic format. 
The self diagnostic feature monitors the operating conditions and whenever the measuring results are unreliable the system error alarm is triggered and the possible cause is displayed, like no sample, no standard, photometer error, no reagent or dirty cell.

The analyzer has a microprocessor based display/control unit and an analytical part.

The heart of the analytical part has a measuring cell, in which a magnetic stirrer to mix the reagents with the sample and, a 860 nm LED light source with a light detector to measure the intensity of the color formation color (by absorbance).

The sample enters the analyzer through a needle inlet valve and an electromagnetic inlet valve to the measuring cell from where it overflows into the drain. When the measuring cell is rinse out by the sample, the electromagnetic inlet valve closes and about 25 ml of sample is trapped inside the cell.
Before the first reagent is added, the light intensity is measured and set to zero absorbance. In this way sample turbidity and color and light source aging is automatically compensated.

Reagent 1:	Ammonium-molybdate reacts with reactive silica and phosphate to form a weak yellow color. The reaction time is 240/300 seconds.
Reagent 2:	Oxalic acid is added to decomplex the phosphate-molybdate compound and to stop further reaction of the molybdate with the silica. For measurements in de-ionized water, boiler feedwater without phosphate, condensate and low levels of silica this reagent is not required.
Reagent 3:	Reducing agent converts the weak yellow color in a more intense blue color, which                                is very suitable for low ppb measurements. The reaction time is 300/360 seconds.

In the final phase, the absorbance is measured and converted to concentration value.

The sample cell is flushed at the end of the analysis with fresh sample and the analyzer is ready for a new measurement.

The calibration uses another electromagnetic valve located after the needle inlet valve that is activated to rinse and fill the measuring cell with a standard solution (recommended 100 ppb as Si). The absorbance measured with the standard solution is then equal to its concentration value. The calculated conversion factor is shown on the display and used to calculate the concentrations in the sample.

Detection Method

The concentration measurement of reactive silica is based on the well documented color reaction with ammonium molybdate.

To improve the lower limit of detection, a reducing reagent is added to convert the yellow molybdosilic acid to molybdenum blue.

In the presence of phosphate, an additional reagent is added prior to the reducing agent to remove the phosphate interference.

The analysis is batch-wise, which gives the most stable results compared with other continuous flowing systems