# 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.