Jawaharlal Nehru Technological University Hyderabad B.Tech IV ANALYTICAL INSTRUMENTATION Previous Question Paper (February/March - 2022 - R18)

Silica analysis in water, particularly in high-pressure boiler feed water, is primarily performed using the Molybdosilicate method based on colorimetric analysis. The process involves the following steps:

1. The water sample is treated with an acidic solution of ammonium molybdate.
2. Silica reacts with the molybdate ions to form a yellow-colored complex known as silicomolybdic acid.
3. To increase sensitivity and eliminate interference from phosphates, a reducing agent such as amino-naphthol-sulfonic acid or stannous chloride is added, which converts the yellow complex into a more intense molybdenum blue complex.
4. The intensity of the blue color is directly proportional to the concentration of silica in the sample.
5. A spectrophotometer or colorimeter measures the absorbance of the solution at a specific wavelength (typically around 810 nm), and the concentration is determined by comparing it against a calibration curve generated from standard silica solutions.

A conductivity meter measures the ability of an aqueous solution to carry an electrical current, which is directly related to the concentration of dissolved ions. The principle of operation is based on Ohm's Law and the behavior of electrolytes.

The measurement system typically consists of a conductivity cell with two inert metallic electrodes (usually platinum) immersed in the sample. An alternating current (AC) is applied to the electrodes to prevent polarization and electrolysis of the solution. The meter measures the conductance (G), which is the reciprocal of resistance (R):

Where:

• κ (Kappa) is the specific conductivity (S/cm).
• A is the surface area of the electrodes.
• L is the distance between the electrodes.

The ratio L/A is known as the cell constant. Since the cell constant is fixed for a given probe, the instrument calculates the specific conductivity by multiplying the measured conductance by the cell constant. Because conductivity is highly temperature-dependent, modern meters incorporate an internal temperature sensor (RTD or thermistor) to apply automatic temperature compensation (ATC), typically normalizing the reading to 25°C.