美国药典34重金属231中英文

231HEAVY METALS

This test is provided to demonstrate that the content of metallic impurities that are colored by sulfide ion, under the specified test conditions, does not exceed the Heavy metals limit specified in the individual monograph in percentage (by weight) of lead in the test substance, as determined by concomitant visual comparison (see Visual Comparison in the section Procedure under Spectrophotometry and

Light-Scattering 851) with a control prepared from a Standard Lead Solution. [note—Substances that typically will respond to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum.]

Determine the amount of heavy metals by Method I, unless otherwise specified in the individual monograph. Method I is used for substances that yield clear, colorless preparations under the specified test conditions. Method II is used for substances that do not yield clear, colorless preparations under the test conditions specified for Method I, or for substances that, by virtue of their complex nature, interfere with the precipitation of metals by sulfide ion, or for fixed and volatile oils. Method III, a wet-digestion method, is used only in those cases where neither Method I nor Method II can be used.

Special Reagents

Lead Nitrate Stock Solution— Dissolve 159.8 mg of lead nitrate in 100 mL of water to which has been added 1 mL of nitric acid, then dilute with water to 1000 mL. Prepare and store this solution in glass containers free from soluble lead salts.

Standard Lead Solution— On the day of use, dilute 10.0 mL of Lead Nitrate Stock Solution with water to 100.0 mL. Each mL of Standard Lead Solution contains the equivalent of 10 μg of lead. A comparison solution prepared on the basis of 100 μL of Standard Lead Solution per g of substance being tested contains the equivalent of 1 part of lead per million parts of substance being tested.

Method I

pH 3.5 Acetate Buffer— Dissolve 25.0 g of ammonium acetate in 25 mL of water, and add 38.0 mL of 6 N hydrochloric acid. Adjust, if necessary, with 6 N ammonium hydroxide or 6 N hydrochloric acid to a pH of 3.5, dilute with water to 100 mL, and mix.

Standard Preparation— Into a 50-mL color-comparison tube pipet 2 mL of Standard Lead Solution (20 μg of Pb), and dilute with water to 25 mL. Using a pH meter or short-range pH indicator paper as external indicator, adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, dilute with water to 40 mL, and mix.

Test Preparation— Into a 50-mL color-comparison tube place 25 mL of the solution prepared for the test as directed in the individual monograph; or, using the designated volume of acid where specified in the individual monograph, dissolve in and dilute with water to 25 mL the quantity, in g, of the substance to be tested, as calculated by the formula:

2.0/(1000L)

in which L is the Heavy metals limit, as a percentage. Using a pH meter or short-range pH indicator paper as external indicator, adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, dilute with water to 40 mL, and mix.

Monitor Preparation— Into a third 50-mL color-comparison tube place 25 mL of a solution prepared as directed for Test Preparation, and add 2.0 mL of Standard Lead Solution. Using a pH meter or

short-range pH indicator paper as external indicator, adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, dilute with water to 40 mL, and mix.

Procedure— To each of the three tubes containing the Standard Preparation, the Test Preparation, and the Monitor Preparation, add 2 mL of pH 3.5 Acetate Buffer, then add 1.2 mL of thioacetamide–glycerin base TS, dilute with water to 50 mL, mix, allow to stand for 2 minutes, and view downward over a white surface*: the color of the solution from the Test Preparation is not darker than that of the solution from the Standard Preparation, and the color of the solution from the Monitor Preparation is equal to or darker than that of the solution from the Standard Preparation. [note—If the color of the Monitor Preparation is lighter than that of the Standard Preparation, use Method II instead of Method I for the substance being tested.]

Method II

note—This method does not recover mercury.

pH 3.5 Acetate Buffer— Prepare as directed under Method I.

Standard Preparation— Prepare as directed under Method I.

Test Preparation— Use a quantity, in g, of the substance to be tested as calculated by the formula:

2.0 / (1000L)

in which L is the Heavy metals limit, in percentage. Transfer the weighed quantity of the substance to a suitable crucible, add sufficient sulfuric acid to wet the substance, and carefully ignite at a low temperature until thoroughly charred. (The crucible may be loosely covered with a suitable lid during the charring.) Add to the carbonized mass 2 mL of nitric acid and 5 drops of sulfuric acid, and heat cautiously

until white fumes no longer are evolved. Ignite, preferably in a muffle furnace, at 500to 600, until the carbon is completely burned off. Cool, add 4 mL of 6 N hydrochloric acid, cover, digest on a steam bath for 15 minutes, uncover, and slowly evaporate on a steam bath to dryness. Moisten the residue with 1 drop of hydrochloric acid, add 10 mL of hot water, and digest for 2 minutes. Add 6 N ammonium hydroxide dropwise until the solution is just alkaline to litmus paper, dilute with water to 25 mL, and adjust with 1 N acetic acid to a pH between 3.0 and 4.0, using short-range pH indicator paper as an external indicator. Filter if necessary, rinse the crucible and the filter with 10 mL of water, combine the filtrate and rinsing in a 50-mL color-comparison tube, dilute with water to 40 mL, and mix.

Procedure— To each of the tubes containing the Standard Preparation and the Test Preparation, add 2 mL of pH 3.5 Acetate Buffer, then add 1.2 mL of thioacetamide–glycerin base TS, dilute with water to 50 mL, mix, allow to stand for 2 minutes, and view downward over a white surface*: the color of the solution from the Test Preparation is not darker than that of the solution from the Standard Preparation.

Method III

pH 3.5 Acetate Buffer— Prepare as directed under Method I.

Standard Preparation— Transfer a mixture of 8 mL of sulfuric acid and 10 mL of nitric acid to a clean, dry, 100-mL Kjeldahl flask, and add a further volume of nitric acid equal to the incremental volume of nitric acid added to the Test Preparation. Heat the solution to the production of dense, white fumes; cool; cautiously add 10 mL of water; and, if hydrogen peroxide was used in treating the Test Preparation, add a volume of 30 percent hydrogen peroxide equal to that used for the substance being tested. Boil gently to the production of dense, white fumes. Again cool, cautiously add 5 mL of water, mix, and boil gently to the production of dense, white fumes and to a volume of 2 to 3 mL. Cool, dilute cautiously with a few mL

of water, add 2.0 mL of Standard Lead Solution (20 μg of Pb), and mix. Transfer to a 50-mL

color-comparison tube, rinse the flask with water, adding the rinsing to the tube until the volume is 25 mL, and mix.

Test Preparation— Unless otherwise indicated in the individual monograph, use a quantity, in g, of the substance to be tested as calculated by the formula:

2.0/(1000L)

in which L is the Heavy metals limit, as a percentage.

If the substance is a solid— Transfer the weighed quantity of the test substance to a clean, dry, 100-mL Kjeldahl flask. [note—A 300-mL flask may be used if the reaction foams excessively.] Clamp the flask at

an angle of 45, and add a sufficient quantity of a mixture of 8 mL of sulfuric acid and 10 mL of nitric acid to moisten the substance thoroughly. Warm gently until the reaction commences, allow the reaction to subside, and add portions of the same acid mixture, heating after each addition, until a total of 18 mL of the acid mixture has been added. Increase the amount of heat, and boil gently until the solution darkens. Cool, add 2 mL of nitric acid, and heat again until the solution darkens. Continue the heating, followed by addition of nitric acid until no further darkening occurs, then heat strongly to the production of dense, white fumes. Cool, cautiously add 5 mL of water, boil gently to the production of dense, white fumes, and continue heating until the volume is reduced to a few mL. Cool, cautiously add 5 mL of water, and examine the color of the solution. If the color is yellow, cautiously add 1 mL of 30 percent hydrogen peroxide, and again evaporate to the production of dense, white fumes and a volume of 2 to 3 mL. If the solution is still yellow, repeat the addition of 5 mL of water and the peroxide treatment. Cool, dilute cautiously with a few mL of water, and rinse into a 50-mL color-comparison tube, taking care that the combined volume does not exceed 25 mL.

If the substance is a liquid— Transfer the weighed quantity of the test substance to a clean, dry, 100-mL Kjeldahl flask. [note—A 300-mL flask may be used if the reaction foams excessively.] Clamp the flask at

an angle of 45, and cautiously add a few mL of a mixture of 8 mL of sulfuric acid and 10 mL of nitric acid. Warm gently until the reaction commences, allow the reaction to subside, and proceed as directed for If the substance is a solid, beginning with ―add portions of the same acid mixture.‖

Monitor Preparation— Proceed with the digestion, using the same amount of sample and the same procedure as directed in the subsection If the substance is a solid in the section Test Preparation, until the step ―Cool, dilute cautiously with a few mL of water.‖ Add 2.0 mL of Lead Standard Solution (20 μg of lead), and mix. Transfer to a 50-mL color comparison tube, rinse the flask with water, adding the rinsing to the tube until the volume is 25 mL, and mix.

Procedure— Treat the Test Preparation, the Standard Preparation, and the Monitor Preparation as follows. Using a pH meter or short-range pH indicator paper as external indicator, adjust the solution to a pH between 3.0 and 4.0 with ammonium hydroxide (a dilute ammonia solution may be used, if desired, as the specified range is approached), dilute with water to 40 mL, and mix.

To each tube add 2 mL of pH 3.5 Acetate Buffer, then add 1.2 mL of thioacetamide–glycerin base TS, dilute with water to 50 mL, mix, allow to stand for 2 minutes, and view downward over a white surface*: the color of the Test Preparation is not darker than that of the Standard Preparation, and the color of the Monitor Preparation is equal to or darker than that of the Standard Preparation.

* In those countries or jurisdictions where thioacetamide cannot be used, add 10 mL of freshly prepared hydrogen sulfide TS to each of the tubes, mix, allow to stand for 5 minutes, and view downward over a white surface.

Auxiliary Information— Please check for your question in the FAQs before contacting USP.

<231> 重金属

本试验系在规定的试验条件下，金属离子与硫化物离子反应显色，通过制备的标准铅溶液目视比较测定，以确证供试品中重金属杂质含量不超过各论项下规定的限度（以供试品中铅的百分比表示，以重量计）。（见分光光度法和光散射项下测定法目视比较法<851>）[ 注意:对本试验有响应的典型物质有铅、汞、铋、砷、锑、锡、镉、银、铜和钼等 ]。

除各论另有规定外，按第一法测定重金属。第一法适用于在规定试验条件下，能产生澄清、无色溶液的物质。第二法适用于在第一法规定试验条件下不能产生澄清、无色溶液的物质，或者适用于由于性质复杂，易干扰硫化物离子与金属离子形成沉淀的物质，或者是不易挥发的和易挥发的油类物质。第三法为湿消化法，仅用于第一法、第二法都不适合的情况。

特殊试剂

硝酸铅贮备液—取硝酸铅159.8mg，溶于100ml水中，加1ml硝酸，用水稀释至1000ml。制备和贮存本溶液的玻璃容器应不含可溶性铅。

标准铅溶液—使用当天，取硝酸铅贮备液10.0ml， l标准铅溶液制备的对照溶液，μg的铅。按每克供试品取100μ用水稀释至100.0ml。每1ml的标准铅溶液含相当于10 相当于供试品含百万分之一的铅。

方法 I

pH3.5醋酸盐缓冲液—取醋酸铵25.0g溶于25ml水中，加6N盐酸液38.0ml，必要时，用6N氢氧化铵液或6N盐酸液调节pH至3.5，用水稀释至100ml，混匀。

g的Pb），μ标准溶液准备—精密量取标准铅溶液2ml，（相当于20 置50ml比色管中，加水稀释至25ml，以精密pH试纸作为外指示剂，用1N醋酸液或6N氢氧化铵液调节pH至3.0~4.0，用水稀释至40ml，混匀。供试品溶液制备—取各论项下规定的供试品溶液25ml，置50ml比色管中，或用各论项下规定用量的酸溶解样品，再用水稀释至25ml，供试品以g计，按下式计算：

2.0/（1000L）

式中L是重金属限度（%）。以精密pH试纸作为外指示剂，用1N醋酸液或6N氢氧化铵液调节pH至3.0~4.0，用水稀释至40ml，摇匀。

对照溶液制备—取供试品溶液制备项下的溶液25ml，置50ml比色管中，加标准铅溶液2.0ml，以精密pH试纸作为外指示剂，用1N醋酸液或6N氢氧化铵液调节pH至3.0~4.0，用水稀释至40ml，摇匀。

测定法—在上述三试管中，分别加入pH3.5的醋酸盐缓冲液2ml，然后再加硫代乙酰胺—甘油试液1.2ml，用水稀释至50ml，混匀，放置2分钟，自上向下观察：供试品溶液产生的颜色与标准品溶液产生的颜色相比，不得更深。对照溶液产生的颜色比标准溶液深或相当。[注意：如果对照溶液的颜色比标准溶液浅，*在白色平面用方法II代替方法I测定供试品]。

方法II

pH3.5醋酸盐缓冲液—按方法I配制。

标准溶液准备—按方法I配制。

供试品溶液制备—供试品以g计，按下式计算：

2.0/（1000L）

式中L是重金属限度（%）。取供试品适量，称重，置适宜的坩埚中，加适量的硫酸使湿润，低温小心灼烧，灼烧，?直至全部炭化，（在炭化过程中坩埚不可盖严），加硝酸2ml和硫酸5滴至炭化物上，小心加热直到白烟不再逸出，置马富炉中500~600 直至完全灰化，放冷，加6N盐酸液4ml，加盖，置蒸气浴上加热15分钟，去盖，在蒸汽浴上慢慢蒸发至干，用1滴盐酸湿润残渣，加热水10ml，蒸煮2分钟，滴加6N氢氧化铵液，直到溶液对石蕊试纸呈碱性，用水稀释至25ml，以精密pH试纸作为外指示剂，用1N醋酸液调节pH至3.0~4.0，必要时，滤过，用10ml水洗涤坩埚和滤器，合并滤液和洗液，置50ml比色管中，用水稀释至40ml，摇匀。

测定法—在上述二试管中，分别加入pH3.5的醋酸盐缓冲液2ml，然后再加硫代乙酰胺—甘油试液1.2ml，用水稀释至50ml，混匀，放置2分钟，自上向下观察：供试品溶液产生的颜色与标准品溶液产生的颜色相比，不得更深。*在白色平面

方法III

pH3.5醋酸盐缓冲液——按方法I所示的方法配制。

的铅），混匀，移入50mL比色管中，用水洗涤烧瓶，洗液并入比色管中，并稀释至25mL，混匀。μ标准溶液的制备——取硫酸8mL和硝酸10mL的混合液，置洁净干燥的100mL凯氏烧瓶中，再加硝酸适量，加入量与供试品溶液中加入的硝酸量相当。加热使产生浓的白烟，冷却，小心加水10mL，若处理供试品需用过氧化氢，则加30%过氧化氢适量，加入量相当于供试品中消耗的过氧化氢量。缓缓煮沸至产生浓的白烟，再冷却，小心地加水5mL，混匀，缓缓煮沸至产生浓的白烟，浓缩至体积2~3mL，冷却，小心加水数毫升稀释，加标准铅溶液2.0mL（20

供试品溶液的制备——

角，加入硫酸8mL和硝酸10mL的混合液适量，其量应足以使样品完全湿润，缓缓加热，至反应开始后停止加热，待反应平息，再分数次加入上述剩余的酸混合液，每次加酸后再加热，直至18mL酸混合液全部加完。继续加热至微沸，直至溶液变黑，冷却，加硝酸2m?若供试品为固体——按各论中的规定称取供试品适量，置洁净干燥的100mL凯氏烧瓶中[注意——若反应泡沫过多，可用300mL的烧瓶]，夹住烧瓶使成45 L，再加热至溶液变黑。继续加热，再加硝酸，直至溶液不再变黑，然后加强热使产生浓的白烟，冷却，小心地加入水5mL，缓缓加热至产生浓的白烟，继续加热直至体积仅剩数毫升，冷却，小心地加水5mL，观察溶液颜色，若呈黄色，则小心地加入30%的过氧化氢1mL，再蒸发至产生浓的白烟且体积仅剩2~3mL，若溶液仍呈黄色，可重复加水5mL及过氧化氢处理。冷却，小心地加水数毫升稀释，并洗入50mL比色管中，注意合并洗液后的体积不得超过25mL。

角，小心地加入硫酸8mL与硝酸10mL的混合液数毫升，缓缓温热至反应开始，待反应渐止，按固体样品项下自“分数次加入上述相同的酸混合液”起，同法处理。?若供试品为液体——取各论中规定量的供试品，置一洁净干燥的100mL凯氏烧瓶中[注——若反应泡沫过多，可用300mL烧瓶]，夹住烧瓶使成45

检查法——供试品溶液及标准品溶液制备均按以下方法处理：用氢氧化铵调节pH值为3.0~4.0，用精密pH 试纸为外指示剂（当接近规定的pH值时可用稀氨溶液），然后用水稀释至40mL，混匀。

每支比色管中加入pH3.5的醋酸盐缓冲液2mL，然后加硫乙酰氨——甘油碱性试液1.2mL，再加水稀释至50mL，混匀，静置2分钟，置白色平面上自上向下观察，供试品溶液的颜色与标准品溶液的颜色相比，不得更深。