Arsenic (As) is not found in any nutrient solutions. It occurs in the following forms:  arsenite (As³⁺) and arsenate (As⁵⁺) . It is highly toxic.

The following methods are available for determination:

• Atomic absorption spectrometry (AAS) with hydride generator (HG-AAS): High sensitivity.
• Inductively coupled plasma mass spectrometry (ICP-MS): Very precise.
• Spectrophotometry with silver diethylthiocarbamate: color development by complex formation.
• Electrochemical methods (e.g. ASV): High sensitivity.
• Iodometric titration: Suitable for As³⁺.

Titration of arsenic with iodine solution (I₂)

1. Principle of the method

Arsenic(III) ions (As³⁺) are oxidized to arsenic(V) by iodine (I₂) in acidic solution:

The endpoint is detected using starch solution as an indicator ( blue → colorless ).

2. Chemicals

• 0.01 mol/L iodine solution (I₂)
• 1 mol/L hydrochloric acid (HCl)
• 0.1 mol/L sodium thiosulfate solution (Na₂S₂O₃)
• Starch solution (indicator)

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Magnetic stirrer
• Graduated pipettes (10 mL, 50 mL)

4. Implementation

1. Add 10 mL of 1 mol/L HCl to 10 mL of nutrient solution.
2. Carefully heat the solution to 40°C.
3. Slowly add 0.01 mol/L iodine solution while stirring.
4. After the yellow color disappears, add starch solution.
5. Continue titrating until the blue color disappears.

5. Calculation of the arsenic concentration

The concentration of As³⁺ is calculated as follows

:

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