Potassium Iodide: Properties, Uses & Lab Guidelines

Potassium iodide is a versatile inorganic salt widely employed in research laboratories as a source of iodide ions, a component in biochemical assays, a thyroid-protection agent in radiological emergencies, and a key reagent in synthetic and analytical chemistry.

Chemical Formula

KI

Molecular Weight

166.00 g/mol

CAS Number

7681-11-0

Appearance

White crystalline powder or colorless crystals

Solubility in Water

Highly soluble (~140 g/100 mL at 20 °C)

Melting Point

681 °C

pH of Aqueous Solution

Approximately neutral (6–8 for fresh solutions)

Physicochemical Properties

Potassium iodide (KI, CAS 7681-11-0) is an ionic compound with a molecular weight of 166.00 g/mol. In its pure form it appears as white crystalline powder or colorless cubic crystals with a saline taste. It is highly hygroscopic and should therefore be protected from atmospheric moisture to prevent caking.

Key physicochemical data:

• Density: approximately 3.13 g/cm³
• Melting point: 681 °C
• Boiling point: 1330 °C
• Water solubility: ~140 g/100 mL at 20 °C; solubility increases markedly with temperature
• Solubility in ethanol: slightly soluble; sparingly soluble in acetone
• pH (5% aqueous solution): approximately 6–8

On prolonged exposure to air, light, or moisture, KI undergoes slow oxidation: iodide ions are oxidised to molecular iodine (I₂), causing the material to turn yellow or brown. Addition of a small quantity of sodium thiosulfate or storage under inert atmosphere retards this degradation.

Laboratory Applications

Potassium iodide serves diverse roles across biochemistry, analytical chemistry, organic synthesis, and radiobiology:

1. Lugol's Iodine and Starch Detection: KI is combined with molecular iodine (I₂) to form Lugol's solution, a classical reagent used to detect starch via the formation of a deep blue-black iodine–starch complex. This is routine in cell biology, histology, and food science.
2. Halide Source in Organic Synthesis (Finkelstein Reaction): KI provides iodide ions for halide-exchange reactions, converting alkyl chlorides or bromides to the corresponding alkyl iodides in acetone. This is exploited widely in medicinal chemistry and total synthesis.
3. Iodometric Titrations: In analytical chemistry, KI is used as a reducing agent to liberate iodine from oxidising species (e.g., hydrogen peroxide, chlorine, permanganate), which is then titrated with sodium thiosulfate using starch as an indicator.
4. Photography and Silver Halide Preparations: KI is employed in the preparation of silver iodide (AgI) emulsions in photographic and radiographic research applications.
5. Cell Biology and Biochemistry: Potassium iodide solutions are used in protein solubilisation, certain enzyme assays, and as part of denaturing gel electrophoresis protocols requiring chaotropic iodide salts.
6. Radiological Protection Research: KI is investigated as a thyroid-blocking agent in studies of radioiodine uptake; at saturated solution concentrations (SSKI), it saturates thyroid iodide uptake and is used as a reference compound in relevant in vitro studies.

Safe Handling and Hazard Information

Although potassium iodide has relatively low acute toxicity compared with many laboratory reagents, correct handling practice is essential to protect personnel and maintain reagent quality.

Hazard Classification

• KI is not classified as a flammable, explosive, or highly toxic substance under standard GHS criteria at typical laboratory concentrations.
• It may act as an irritant to eyes, skin, and the respiratory tract upon prolonged or repeated exposure to dust.
• Ingestion of large quantities may cause gastrointestinal disturbance and thyroid disruption (iodism).

Personal Protective Equipment (PPE)

• Wear safety spectacles or goggles when handling powders or concentrated solutions.
• Use nitrile or latex gloves to prevent skin contact.
• Work in a well-ventilated area or fume hood when handling fine powder to avoid inhalation of dust.
• Lab coat and closed-toe footwear are standard requirements.

First Aid Measures

• Eye contact: Flush with copious water for at least 15 minutes; seek medical advice if irritation persists.
• Skin contact: Wash thoroughly with soap and water.
• Inhalation: Move to fresh air; seek medical attention if symptoms develop.
• Ingestion: Do not induce vomiting; seek immediate medical attention.

Consult the current Safety Data Sheet (SDS) for full regulatory and emergency information before use.

Storage, Stability, and Quality Guidelines

Proper storage of potassium iodide is critical to maintaining its chemical integrity and ensuring reproducible experimental results.

Recommended Storage Conditions

• Store in a tightly sealed container, preferably amber glass or opaque packaging, to exclude light and moisture.
• Keep at room temperature (15–25 °C) in a cool, dry, well-ventilated area away from direct sunlight.
• Isolate from strong oxidising agents (e.g., permanganates, peroxides, halogens) and strong acids.
• Hygroscopic nature requires desiccant storage (e.g., silica gel) if humidity levels are high.

Stability Considerations

• Pure, anhydrous KI stored under recommended conditions is stable for extended periods (typically several years).
• Yellowing or browning of the solid or solution indicates oxidation to I₂ and the material should be assessed before use.
• Aqueous solutions should be prepared fresh when high purity is required; for longer-term storage of solutions, maintain in amber vessels at 2–8 °C and add a stabiliser such as sodium thiosulfate (0.01–0.05% w/v) if appropriate for the application.

Quality and Purity Grades

• For most analytical and biochemical applications, reagent-grade KI (≥99% purity) is suitable.
• For trace-metal-sensitive analyses or ICP work, use trace-metal-grade material and confirm specifications with the supplier certificate of analysis (CoA).
• Always verify the CoA for lot-specific assay values, heavy-metal content, and water content before use in critical assays.

Preparation of Common KI Solutions

The following protocols outline preparation of the most frequently used KI solutions in research laboratories. All procedures should be performed using freshly opened or verified reagent-grade KI. Use deionised or ultrapure water unless otherwise specified.

1. Lugol's Iodine Solution (Standard)

1. Weigh 10 g of potassium iodide and dissolve in approximately 100 mL of deionised water with stirring.
2. Add 5 g of iodine (I₂) to the KI solution; iodine dissolves readily due to triiodide complex formation (I₃⁻).
3. Make up to 500 mL with deionised water. Store in a sealed amber glass bottle at room temperature, protected from light. Stable for approximately 12 months under these conditions.

2. Saturated Potassium Iodide Solution (SSKI, ~100% w/v)

1. Add approximately 100 g of KI to 100 mL of warm deionised water (40–50 °C) and stir until fully dissolved.
2. Allow to cool to room temperature. The solution will remain saturated at room temperature.
3. Filter if particulates are present. Store in an amber bottle at room temperature. Prepare fresh monthly or when discolouration is observed.

3. 10% w/v KI Stock Solution

1. Weigh 10.0 g of reagent-grade potassium iodide.
2. Dissolve in approximately 80 mL of deionised water in a 100 mL volumetric flask.
3. Adjust volume to exactly 100 mL. Mix thoroughly and transfer to an amber glass bottle.
4. Label with preparation date and lot number. Store at 2–8 °C; use within 1 month or until discolouration occurs.

Note: All KI solutions should be inspected visually before use. Discard any solution showing yellow or brown colouration indicative of iodine liberation unless I₂ is intentionally present (e.g., Lugol's).

Frequently asked questions

Why does my potassium iodide solution turn yellow or brown?

Yellowing or browning indicates oxidation of iodide ions (I⁻) to molecular iodine (I₂). This is accelerated by exposure to light, air, heat, or acidic conditions. Discard discoloured solutions for sensitive applications, improve storage conditions (amber vessel, 2–8 °C, sealed), and consider adding a trace amount of sodium thiosulfate as stabiliser.

What purity grade of potassium iodide should I select for analytical applications?

For most standard analytical and biochemical work, reagent-grade KI (≥99% purity) is sufficient. For trace-element analysis, ICP-MS, or applications where even ppb-level metal contamination is critical, select trace-metal or ultra-pure grade and confirm the certificate of analysis for heavy metal specifications before use.

Can potassium iodide be mixed with oxidising agents in the laboratory?

No. Potassium iodide should never be mixed with strong oxidising agents such as permanganates, peroxides, chlorates, or concentrated acids outside of a controlled, intended reaction. Such mixtures can produce elemental iodine rapidly and, in extreme cases, may generate sufficient heat or reactive intermediates to pose a safety hazard. Always consult the SDS and relevant literature before combining KI with other reactive chemicals.

How should I dispose of potassium iodide waste?

Dispose of KI waste in accordance with local, national, and institutional regulations. Dilute aqueous solutions of KI may be permissible for drain disposal in some jurisdictions after pH adjustment and confirmation of concentration limits, but always verify with your institution's environmental health and safety office. Solid waste should be collected in labelled chemical waste containers.

Is potassium iodide the same as potassium iodate?

No. Potassium iodide (KI) contains iodide ions (I⁻) and is a reducing agent, whereas potassium iodate (KIO₃) contains iodate ions (IO₃⁻) and acts as an oxidising agent. They have different chemical reactivity, applications, and safety profiles. They should never be substituted for each other without careful review of the intended chemistry.

References

1. Haynes, W.M. (Ed.) CRC Handbook of Chemistry and Physics, 97th Edition. CRC Press, Boca Raton, FL.
2. Sigma-Aldrich / Merck. Potassium Iodide Safety Data Sheet. Version current at time of publication.
3. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th Edition. Wiley-Interscience, New York. (Finkelstein Reaction reference)
4. World Health Organization (WHO). Iodine and Health. WHO Technical Report. Geneva.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.