Determination of the Alcohol Lab (1)

AP Chemistry

Determination of the Alcohol Content of Root Beer Lab Background: This analysis determines determines the alcohol content of root beer by a back titration method. The ethanol is oxidized to ethanoic acid by heating with an excess of acidified potassium dichromate solution.

3C2H5H ! 2Cr 2"2# ! $%H! & 3CH3CH ! 'Cr 3! $$H2 ()*n $+ )xcess ,- is added and this reacts with any remaining Cr 2"2# # to produce iodine. Cr 2"2# ! $'H! ! %-# & 2Cr 3! ! "H2 ! 3-2 ()*n 2+ The iodine produced is then titrated with standard sodium thiosulfate solution. -2 ! 2232# & 2-# ! '%2## ()*n 3+ The indicator used in this titration is a $/ solution of starch 0or the determination of alcohol a standardized solution of sodium thiosulfate is re*uired. The sodium thiosulfate solution concentration is determined by titration with a known  potassium iodate solution. The standardization solution is made by adding solid ,- to known concentration solution of sodium thiosulfate. thiosulfate. The following is the the reaction that occurs1 -3 # ! 5-# ! % H! & 3 -2 ! 3 H2 ()*n '+ The iodine is then titrated with the sodium thiosulfate solution to endpoint and the moles of thiosulfate calculated. This known solution is is used to determine the alcohol alcohol content of  the root beer by titrating and using e*uations $ to 3. Solutions:  .5 mol 4#$ potassium dichromate (C6T-7 8 hazardous substance+  5/ sulfuric acid (C6T-7# highly corrosi9e+  .5 mol 4#$ , .$ mol 4#$ sodium thiosulfate  $/ starch solution 8 made by b y adding $m4 of boiling water to $ g of starch powder. Standardization of 0.10 M sodium thiosulfate: $. :ipett :ipettee 5. 5. m4 m4 of the the .$ .$ ; ,- ,-3 solution into a 25#m4 flask. 2. dd 2 g of solid solid ,,- and and $ m4 m4 of of .5 .5 ; H2'. 3. -mmediatel -mmediately y titrate titrate with thiosulf thiosulfate ate until until the solution solution has lost its its initial initial reddish# reddish#  brown color and has become pale yellow. '. dd 2 m4 m4 of starch starch indicator indicator and complete complete the the titratio titration. n. 5. hare the the data data of your your one trial trial with with the other other groups. groups. ll the groups groups need need to calculate the a9erage concentration for the class and use that a9erage concentration to calculate the amount of alcohol in the root beer.

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 Determination of Alcohol Content of Root Beer Lab

Procedure  Record the method used (A or B) and whether the initial sample of root beer is diluted or not in your data table.

Method A: $. :ipette $ m4 of root beer into a 25 m4 9olumetric flask and make up to the mark with water. 2. :lace a 5 m4 ali*uot of the diluted root beer in a $5 m4 )rlenmeyer flask and add $ m4 of .5 mol 4#$ potassium dichromate. lowly add about 2 m4 of 5/ sulfuric acid solution to each flask. WA!"!# 8This solution is 9ery corrosi9e. 3. topper each flask loosely and heat in a water bath> at no more than 5?C> for at least % minutes. '. add a few drops of starch indicator. Continue adding thiosulfate solution until the solution goes a clear> green#blue color. This is the endpoint of the titration# it is not easy to detect> but with  practice it will become easier. $. 7T)1 -t is possible to check whether the endpoint has been reached by holding the solution in the sunlight and adding a drop of starch indicator. The appearance of a dark blue color in the solution (where the drop was added+ indicates iodine is still present. Method B: -f the alcohol sample being in9estigated contains significant *uantities of other  substances that are able to be oxidized by acidified dichromate then the 9alue obtained will be too high. This commonly occurs with alcoholic drinks such as beers and wines  because of the presence of other organic molecules such as secondary alcohols. -n these situations it would usually be necessary to separate the alcohol from the sample by fractional distillation which is difficult and time co nsuming. Howe9er> if the interfering substances are less 9olatile than the alcohol itself the following modification known as 7ichollAs diffusion works ade*uately (as described in CH);7B 7o "3> 7o9 D pp 2$#22+. -n this method> instead of adding the alcohol sample directly to the acidified dichromate mixture it is placed in a small container and suspended abo9e the dichromate solution and left to diffuse into the solution where it is oxidized as pre9iously described. The other components of the alcohol sample remain  behind in the sample container and so do not contribute to the oxidation process.  diagram of the experimental set up is shown at right. Eecause the suspended container is

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 Determination of Alcohol Content of Root Beer Lab

of limited 9olume> smaller *uantities must be used which usually means the original alcohol sample is not diluted before use. Method B: $. :ipette $ m4 of root into a $ m4 9olumetric flask and make up to the mark with distilled water. 2. 6se a graduated pipette to accurately add 2 m4 of the root beer sample into the small test tube> ensure that the test tube will fit into the 25  m4 )rlenmeyer flask.  7T)1 The *uantity re*uired will depend on the sample> e.g. for beers and mixers about 5 m4 of a $/ solution should be appropriate. 3. :ipette $ m4 of the .5 mol 4#$ potassium dichromate. lowly add about 2 m4 of 5/ sulfuric acid solution to each flask. WA!"!# #This solution is 9ery corrosi9e. '. topper each flask loosely and heat in a water bath> at no more than 5?C> for a few hours or o9ernight. 5. add a few drops of starch indicator. Continue adding thiosulfate solution until the solution goes a clear> green#blue color. D. 6sing the titration data determine the number of moles of Cr 2"2# remaining in the flask. Ey subtracting this from the total amount of dichromate originally added to the flask it is possible to determine how many moles of dichromate ha9e reacted with the ethanol present in the original sample. The concentration of ethanol can then be determined in either mol 4#$> g 4#$ or /(V/V).

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