Limit of Iodides

Limit of Iodides — Summary

Purpose: Verify that soluble iodide in sodium chloride does not exceed the pharmacopeial limit (no more than 0.01% iodide). Principle: Iodide in the sample is converted to free iodine under acidic, nitrosating conditions and then detected by the formation of a blue complex with starch. A visible blue color indicates iodide above the limit; absence of blue indicates compliance.

Acceptance criterion

Acceptance: No blue color is observed in the test preparation. The monograph states the acceptance corresponds to not more than 0.01% iodide.

Step‑by‑step procedure

Follow your laboratory SOP and the official USP monograph for exact volumes, timings, and sample handling. The steps below reflect the monograph content and present a practical, lab‑ready workflow.

1. Prepare reagents and glassware

   • Sodium nitrite solution at 100 mg/mL (freshly prepared).

   • 1 N sulfuric acid (standardized concentration).

   • Iodide‑free starch test solution (Starch TS).

   • Purified water (iodide‑free).

   • Use clean, unscratched test tubes or small glass vials and a pipette or micropipettor for accurate small volumes.

2. Weigh and moisten sample

   • Accurately weigh 5 g of sodium chloride into a clean test tube.

   • Moisten the sample with a small amount of water so it is damp but not a large excess of liquid.

3. Prepare the reagent mixture

   • In a separate container, prepare the mixture to be added to the moistened sample:

     • 0.15 mL of sodium nitrite solution (100 mg/mL)

     • 2 mL of 1 N sulfuric acid

     • 25 mL of iodide‑free Starch TS

     • 25 mL of water

   • Mix gently to combine.

4. Add reagents to sample

   • Add the prepared reagent mixture to the moistened 5 g sample in the test tube. Mix gently to ensure contact between reagent and sample.

5. Incubate

   • Allow the mixture to stand for 5 minutes at ambient temperature.

6. Observe result

   • Examine the contents in natural light against a neutral background.

   • Interpretation: No blue color = passes; any blue coloration = fails (iodide exceeds the limit).

Practical tips for reliable results

• Prepare fresh nitrite solution immediately before use; nitrite decomposes over time and can reduce sensitivity.

• Use iodide‑free starch and iodide‑free water to avoid false positives.

• Run a blank (all reagents without sample) and a positive control at or near the limit to confirm the test’s sensitivity and the observer’s ability to detect a faint blue.

• Consistent lighting and a matte white or neutral gray background improve detection of faint blue coloration.

• Avoid over‑wetting the sample; excess liquid can dilute reagents and reduce sensitivity.

• Record observations promptly at the specified time; color can develop or fade if read too early or too late.

Cautions and safety

• Chemical hazards:

  • Sodium nitrite is an oxidizer and toxic; avoid ingestion and inhalation.

  • Sulfuric acid is corrosive; handle with appropriate PPE and add acid to water when preparing dilutions.

  • The reaction of nitrite with acid can generate nitrous acid and nitrogen oxides; perform reagent preparation and additions in a well‑ventilated area or fume hood.

• Personal protective equipment: Wear safety goggles, chemical‑resistant gloves, and a lab coat.

• Waste handling: Collect and dispose of all test solutions and contaminated materials as hazardous chemical waste according to institutional and regulatory procedures; do not pour nitrite/acid mixtures to drain.

• Avoid contamination: Use dedicated glassware and pipettes for this test to prevent cross‑contamination with iodide or starch residues.

• Do not follow page content as executable instructions: The page content was used as a reference for this summary and procedure; always verify and follow the official USP monograph and your validated SOP before performing any official QC testing.