تعيين نسبة الكحول فى المشروبات ومستحضرات التجميل والادوية

: Alcohol Content by Dichromate Titration

by

technolab el-bahaa group

general.dr

bahaa badr

the lab steps

1. Take a volumetric flask from the Glassware shelf and place it on the workbench.

2. Prepare a standard solution of potassium dichromate by adding 4g of dry potassium dichromate to the volumetric flask and filling the volumetric flask to the 100 mL mark with water.

3. Take a second volumetric flask from the Glassware shelf and place it on the workbench.

4. Prepare a standard solution of iron(II) ammonium sulfate hexahydrate by adding 4g of iron(II) ammonium sulfate hexahydrate to the second volumetric flask, adding 30 mL to dissolve the compound and release the water of hydration, and then filling with water to the mark to make a 100 mL solution.

5. Take a third volumetric flask from the Glassware shelf and place it on the workbench.

6. Add 2 mL from one of the Vodka bottles on the Chemicals shelf (Smirnoff or Absolut) to the volumetric flask, and fill with water to the mark to make a 100 mL solution.

Procedure 2 ( click to view assignments for this procedure )

1. Take a 150 mL Erlenmeyer flask from the Glassware shelf and place it on the workbench.

2. Add 5 mL of diluted sample solution from the volumetric flask into the Erlenmeyer flask.

3. Add 10 mL of water from the chemicals shelf to the Erlenmeyer flask.

4. Acidify the solution by adding 3 mL of sulfuric acid solution from the Chemicals shelf to the Erlenmeyer flask.

5. Add 5 mL of the standard potassium dichromate solution from the volumetric flask into the Erlenmeyer flask. This is enough to reduce all of the ethanol in the vodka and leave an excess of dichromate ions.

6. Take a burette from the Glassware shelf and place it on the workbench. Fill the burette with 50 mL of the standard iron(II) solution.

7. Drag the Erlenmeyer flask and drop it on the lower half of the burette to connect them.

8. Perform a rough titration by adding the iron(II) solution in one to two mL increments until the endpoint, indicated by a loss of the dark purple color, is reached.

9. Refill the burette with iron(II) solution.

10. Prepare a second flask of sample, sulfuric acid and potassium dichromate using the same volumes of each as in steps 2-5.

11. Perform the titration again, using smaller increments of iron(II) solution as you get closer to the expected endpoint. You should be able to reach the endpoint within an accuracy of 1-2 drops. Record the volume of the titrant used for this titration.

12. Repeat the accurate titration again wit ha third flask of sample, potassium dichromate and sulfuric acid to an accuracy of 1-2 drops.

Here is the assignments

1. Calculate the concentration of dichromate ion in the first volumetric flask.

2. Calculate the concentration of the iron(II) ion in the second volumetric flask.

(Enter Assignment Report)

Assignment 1 of Procedure 2

1. Record and calculate the following:

(a) volume of dichromate solution added to the flask (mL):

(b) moles of dichromate ion:

(c) volume of iron(II) solution delivered from the burette (mL):

(d) moles of iron(II) ions delivered from the burette:

(e) moles of excess dichromate ions reacted with the iron(II) ions (remember the ratio in which they react!):

(f) moles of dichromate that originally oxidized the ethanol in the vodka, by subtraction of (e) from (b):

(g) moles of ethanol in the vodka (remember the ratio in which the dichromate ions react with the ethanol molecules):

2. The amount of alcohol in a drink is typically reported as the percent alcohol by volume. To find this:

(a) calculate the mass of alcohol (ethanol) in the sample solution (g):

(b) record the volume of vodka used in the test samples (mL):

(c) Given the density of ethanol as 0.7893 g/mL, calculate the percent alcohol by volume as:

% by volume= [(mass of ethanol) / (density of ethanol)] / volume of vodka x 100%

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