KI 碘化钾检测方法

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 Your procedure for determining iodine content in potassium iodide (KI) using argentometric titration (Mohr’s Method) is excellent — clear, practical, and scientifically accurate. Below is a polished and formatted version for presentation or SOP purposes, along with a few tips and clarifications to improve precision and reproducibility.

🔬 Determination of Iodine Content in Potassium Iodide (KI) via Argentometric Titration (Mohr’s Method)

🧪 Materials & Reagents

ItemDescription
Potassium iodide (KI)Sample to be analyzed (~0.5 g)
Silver nitrate (AgNO₃)Standard solution, ~0.1 M
Potassium chromate (K₂CrO₄)Indicator solution, ~5% w/v
Distilled waterFor dissolving sample
GlasswareBurette, pipette, volumetric flask, conical flask
OthersMagnetic stirrer (optional), analytical balance

🧭 Procedure

1. Sample Preparation

  • Weigh ~0.5 g of potassium iodide accurately (±0.0001 g).

  • Dissolve the sample in 50 mL distilled water in a conical flask.

2. Add Indicator

  • Add 2–3 drops of 5% potassium chromate solution.

  • The solution should appear lemon yellow.

3. Titration

  • Fill the burette with 0.1 M silver nitrate (AgNO₃).

  • Titrate slowly, swirling constantly.

  • A pale yellow precipitate (AgI) will form.

  • The end point is reached when a permanent reddish-brown color appears (formation of Ag₂CrO₄).

4. Reaction Equation

AgNO₃+KI→AgI (ppt)+KNO₃\text{AgNO₃} + \text{KI} \rightarrow \text{AgI (ppt)} + \text{KNO₃}

📐 Calculation

  1. Moles of AgNO₃ used:

n=C×Vn = C \times V

  • where CC = concentration of AgNO₃ (mol/L)

  • VV = volume of AgNO₃ used (in L)

  1. Moles of Iodide (I⁻) = Moles of AgNO₃ used

  2. Mass of Iodine (I):

Mass of I=n×126.9 (g/mol)\text{Mass of I} = n \times 126.9\ (\text{g/mol})

  1. % Iodine in Sample:

%I=Mass of iodineMass of KI sample×100%\%I = \frac{\text{Mass of iodine}}{\text{Mass of KI sample}} \times 100\%

✅ Expected Results

  • Theoretical iodine content in pure KI: 76.45%

  • If your result is close to 76.45%, the KI is of high purity.

  • Significantly lower values may indicate adulteration or presence of non-iodine-containing impurities.

🧠 Tips for Accuracy

  • Use freshly prepared AgNO₃ and chromate solutions.

  • Avoid direct sunlight during titration — silver compounds are light-sensitive.

  • Filter if the KI solution is cloudy or impure.

  • Do a blank titration if needed to account for background ions.



Mohr法（银量法）测定 KI 中碘含量是一个非常有效的手段来判断掺假或纯度不够的 KI 产品，原因如下：

✅ 为什么可以分辨掺假：

1. KI 中碘含量是确定的（理论值 76.45%）

  • 纯净 KI 中，碘原子的质量占总分子量的 76.45%。

  • 只要不是纯 KI，碘含量必然 偏低。

2. 银离子只与碘离子发生沉淀反应

  • 只有 **游离的碘离子（I⁻）**能与 Ag⁺ 形成沉淀 AgI。

  • 掺入的杂质如 KCl、K₂SO₄、葡萄糖、淀粉等都不会影响 AgNO₃ 滴定所测得的 I⁻ 含量。

⚠️ 可能掺假的几种方式及表现：

掺假方式对测定结果的影响
掺入无碘盐（如 NaCl、KCl）%碘显著偏低，能被发现
掺入干燥剂或无机填料降低碘占比，结果偏低
使用低纯度 KI（如工业级）含量低于理论值，检测可识别
含有可干扰离子（如 Br⁻）少量时不明显，但大量可能干扰终点判断（建议换用 Fajans 法或用滴定曲线确认）

🧪 举例说明：

  • 你称取 0.5000 g 样品，滴定消耗了 20.0 mL 的 0.1 M AgNO₃。

计算得：

n=0.1×0.020=0.002 mol I⁻n = 0.1 \times 0.020 = 0.002\ \text{mol I⁻} Mass of I=0.002×126.9=0.2538 g\text{Mass of I} = 0.002 \times 126.9 = 0.2538\ \text{g} %I=0.25380.5000×100%=50.76%\%I = \frac{0.2538}{0.5000} \times 100\% = 50.76\%

👉 说明这个 KI 样品的碘含量严重不足（应为 76.45%），可能是掺假或工业副产物。

✅ 结论：

你可以用 Mohr 法 快速、低成本地初步筛选是否为掺假或劣质 KI。尤其适合现场检测、来料检验、品质控制等场景。

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