THE DETERMINATION OF ARSENIC WITH IODINE

OBJECTIVES:

1. The student will titrate using iodine as a titrant.
2. The student will use a buffer to control the reaction of the titration.
3. The student will use both acid and base to get the desired pH.
4. The standard and unknown are treated in the same manner.
5. Starch will be used as an indicator.

PROCEDURE:

Preparation of Iodine Solution
Dissolve approximately 25 g of potassium iodide in about 50 mL of water. Add to this 6.5 g of iodine crystals. When the iodine has dissolved completely, dilute the solution to about 500 mL. Do in a 600 mL beaker, not a volumetric flask!

Preparation of Starch Solution:
Dissolve 1 gram of soluble starch in about 5 mL of cold d.i. Slowly add the starch suspension to 95 mL of rapidly boiling water. Boil until the solution clears. Cool to room temperature. NOTE: Starch solutions often do not keep. Make it the day you will need it (if you also will need it the following period try to keep it--if it has a mold growth then it must be discarded).

Standardization of the Iodine Solution
Weigh out into 250 mL Erlenmeyer flasks FOUR samples of pure As2O3 of about 0.20 g each. Add 10 mL of 4% NaOH solution. Swirl until all As2O3 has dissolved. (You may warm it to hasten solution, if necessary.) Dilute to 50 mL. Cool to room temperature and add 1:5 HCl until the solution turns pink with methyl red and cool again. Add SOLID sodium bicarbonate in small portions until you are sure that no more will dissolve; then add about 3 g in EXCESS. If the bicarbonate is dissolved before a titration is completed, add more bicarbonate. Add 3 mL of starch solution and titrate with the iodine solution immediately. Continue the titration to the first permanent blue color. Calculate the normality of the iodine solution.

Titration of Unknown Arsenic
Dry the unknown for 1 hour at 105o C. Do not OVERHEAT. Cool it in a desiccator. Weigh out into 250 mL Erlenmeyer flasks four samples of 0.4 to 0.5 g each. Add 10 mL of 4% NaOH solution. It may be necessary to warm it in order to hasten solution. Dilute to 50 mL. Cool to room temperature and add 1:5 HCl until solution turns pink with methyl red. Cool again. Add bicarbonate in small portions until you are sure no more will dissolve; then add about 3 g in excess. If the bicarbonate is dissolved before a titration is completed, add more bicarbonate. Add 3 mL of starch indicator and titrate IMMEDIATELY with the standard iodine solution. Calculate the percentage of As2O3 in the unknown.

PRECAUTIONS:

1. Arsenic is a very poisonous element. Fatal dose is the amount that you can balance on the head of a pin. Wash carefully before eating.

2. It is important that there be an excess of sodium bicarbonate throughout a titration.

3. Solutions of trivalent arsenic may be oxidized appreciable by air if allowed to stand. The titration should be completed as soon as possible after the sample is dissolved in alkali.

NOTES: Use 250 mL Erlenmeyer flasks for the titrations.
                Do not use aliquots.

The initial sample of arsenic oxide should be dried for one hour at 100o C.

CALCULATION: Calculate the percent of arsenic oxide (As2O3) present in the sample after calculating the molarity of the titrant from the known arsenic titrations. Include a spread sheet and computer print-out for the calculation.

SAFETY AND DISPOSAL INFORMATION: Dispose of all solutions in the "As WASTE" container.  Do not mix the wastes from the I2 and KMnO4 Determinations. CAUTION: Remember to treat all chemicals with respect. Arsenic is BAD!
 

DETERMINATION OF ARSENIC WITH POTASSIUM PERMANGANATE

OBJECTIVE:

1. The student will use the strong oxidant KMnO4 as a titrant.
2. This is a self-indicating titrant.
3. The standard and the unknown are treated the same in this method.
4. The student will use a catalyst in this method.

PROCEDURE:

Preparation of Potassium Permanganate:
    To prepare a 0.02 M solution dissolve 3.2 g of KMnO4 in about 1 liter of water. Heat to boiling and keep hot for about 1 hour. Cover and let it stand over night. Filter the solution through a glass wool filter. Store in a clean BROWN glass bottle. Keep in the dark when not in use.

Standardization of Permanganate Solution:
    Weigh out from 0.18 to 0.20 g of pure arsenious oxide. Dissolve in 10 mL of 3 M sodium hydroxide, and 15 mL of 1:1 hydrochloric acid solution, dilute with 50 mL of distilled water, add 1 drop of 0.002M potassium iodate solution and titrate with permanganate to the first permanent pink color. (Endpoint fades after 15 sec.) Titrate standard and unknown on the same day. Do four standards.

Determination of the Unknown:
    Follow the same method given above for the standard. Use 0.5 g samples. The standardization and unknown sample should be titrated on the same day.
            NOTES: Use 250 mL Erlenmeyer flasks for the titrations.
                                    Do not use aliquots.

The initial sample of arsenic oxide should be dried for one hour at 100o C.

CALCULATION: Calculate the percent of arsenic oxide (As2O3) present in the sample after calculating the molarity of the titrant from the known arsenic titrations. Include a spread sheet and computer print-out for the calculation.

SAFETY AND DISPOSAL INFORMATION: Dispose of all solutions in the "As WASTE" container. Do not mix the wastes from the I2 and KMnO4 Determinations. CAUTION: Remember to treat all chemicals with respect. Arsenic is BAD!

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