Limit of Iodides
Step-by-StepLimit Instructionsof 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.
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MoistenPrepare 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.
Weigh and moisten sample
Accurately weigh 5 g of Sodiumsodium Chloridechloride ininto a clean test tube.
AddMoisten the sample with a freshlysmall preparedamount mixtureof of:water so it is damp but not a large excess of liquid.
Prepare the reagent mixture
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In a separate container, prepare the mixture to be added to the moistened sample:
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0.15 mL of
SodiumsodiumNitritenitrite solution (100 mg/mL). -
2 mL of 1 N
SulfuricsulfuricAcid.acid -
25 mL of
Iodide-iodide‑free StarchTS.TS -
25 mL of
water.water
WaitMix gently to combine.
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.
Incubate
Allow the mixture to stand for 5 minutes.minutes at ambient temperature.
Observe result
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Examine the
substancecontents in naturallight.light against a neutral background.
Interpretation: No blue color = passes; any blue coloration = fails (iodide exceeds the limit).
AcceptancePractical Criteriatips for reliable results
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NoPrepare 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 observedan (Acceptance:oxidizer NMTand 0.01%)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.