Principle of Measurement

1. UV Phosphomolybdate Method (Most modern analyzers)

How it works:

1. Inorganic phosphate reacts with ammonium molybdate in an acidic medium.

2. Forms a phosphomolybdate complex.

3. The complex is measured photometrically in the UV range (usually 340 nm).

4. The absorbance is directly proportional to phosphate concentration.

Advantages:

• Fast and highly specific

• Suitable for fully automated analyzers

• Minimal interference

2. Reduced Phosphomolybdate Method (Colorimetric)

How it works:

1. Phosphomolybdate complex is formed.

2. The complex is reduced to a blue-colored compound (“molybdenum blue”).

3. Measured at 600–700 nm, depending on dye.

Advantages:

• Strong color intensity

• Stable reaction for manual or semi-automatic systems

Reagent Components

Phosphorus reagent kits typically include:

R1 (Acid/Molybdate reagent)

• Ammonium molybdate

• Sulfuric acid or perchloric acid

• Stabilizers

• Surfactants

• Preservatives

R2 (Reducing reagent) (if applicable)

• Reducing agents (e.g., ascorbic acid, stannous chloride)

• Dye stabilizers

Some kits are single-reagent formulations.

Packaging Formats

• Liquid-stable ready-to-use reagents

• Two-reagent systems (R1 + R2)

• Analyzer-specific cartridges

• Optional calibrators and controls

Common volumes: 25 ml, 50 ml, 100 ml, 250 ml.