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102-Lect-3_Analysis_of_Vinegar_and_Antacids

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					                Analysis of Vinegar and Antacids



Purpose:
  1. To determine the percent by mass of acetic acid in vinegar
  2. To determine the neutralizing effectiveness per gram of a
     commercial antacid



Theory:
Volumetric analysis: is a technique for determining the amount or
concentration of a certain substance by doing a titration.

Titration: involves the delivery (from a buret) of a measured
volume of a solution of known concentration into a solution
containing the substance to be analyzed.

Acid- base titration involves the complete reaction between an
acid and a base. (Neutralization reaction)
           Acid + base → Salt + water

     HCl(aq)+ NaOH(aq) → NaCl(aq) + H2O(l)

             H+(aq) + OH ‾ (aq) → H2O(l)

Equivalence point
Is the point at which all the acid has completely reacted or has
been neutralized with the base, it is a theoretical concept.

End point of a titration:
Is the point in a titration when a physical change (change in color)
occurs that is associated with the condition of chemical
equivalence, is determined experimentally.
Indicators are used to detect the end point.
They are substances that have distinctly different colors in
acidic and basic media, they are weak organic acids or bases that
change their color at the equivalence point:
                   HIn + H2O ⇌ H3O+ + In‾
                  (acidic)           (Basic)

The acid base indicators used in the experiment are:
Indicator           Color in acid Color in base pH range
Bromophenol blue Yellow            Bluish purple 3.0-4.6
Phenolphthalein     colorless      pink           8.3-10.0

Part1. Vinegar analysis:
   House hold vinegar is a dilute solution of acetic acid
    CH3COOH in Water, it contains 4-5% by mass acetic acid
   The amount of acetic acid is determined by an acid-base
    titration

Acid- base titration:
     CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)

At the equivalence point: Complete neutralization
                 n of moles H+ = n of moles OH¯
For a monoprotic acid:
              n of moles of acid = n of moles of base
          ⇨n of moles of CH3COOH = n of moles of NaOH

             n of moles of NaOH = Molarity x volume

       mass of CH3COOH = n of moles of CH3COOH x Molar mass

         % by mass of CH3COOH = mass of CH3COOH x 100
                                  mass of vinegar
Part 2: Antacid Analysis
   Antacids are used to neutralize the excess H+ in the
     stomach to relieve acid indigestion.
   The neutralizing effectiveness per gram of an antacid is
     determined by an acid –base titration.

pH is a measure of acidity

            pH = - log [H3O+]
At 25oC;
        Acidic solutions: [H3O+] > 1.0 x 10-7   ⇨   pH < 7
                                +          -7
        Basic solutions:   [H3O ] < 1.0 x 10    ⇨   pH > 7
        Neutral solutions: [H3O+] =1.0 x 10-7   ⇨ pH = 7

   pH of gastric juice = 1.0-2.0, Hydrocloric acid HCl is
    secreted while eating and is important for digestion
   If pH < 1.0, there is excess of acid and this leads to
    heartburn.

The most common bases used for over the counter antacids are:
     Hydroxides: Al(OH)3 and Mg(OH)2
     Carbonates: CaCO3      and MgCO3
     Bicarbonates: NaHCO3 and KHCO3

They neutralize hydronium ions H3O+ according to the following
reactions:
     Mg(OH)2 (s) + 2H3O+ (aq) → Mg2+ (aq) + 4 H2O (l)

      CaCO3 (s) + 2H3O+ (aq) → Ca2+ (aq) + CO2 (g) + 2H2O (l)

      NaHCO3 (s) + H3O+ (aq) → Na+ (aq) + CO2 (g) + 2H2O (l)
To decrease the possibility of the stomach becoming too basic
from the antacid, buffers are added as part of the formulation
of some antacids.
    A buffer solution is made of a weak acid and its salt or a
     weak base and its salt
    A buffer solution has the ability to resist changes in pH
     upon the addition of small amounts of either acid or base.

Antacids that contain HCO3- or CO32- buffer system react quickly
with H3O+ as follows:

      CO32- (aq) + H3O+ (aq) → HCO3- (g) + H2O (l)
      HCO3- (aq) + H3O+ (aq) → CO2 (g) + 2H2O (l)

Experiment: Strong Acid- Base Titration
   Excess of hydrochloric acid, HCl, is added to the solution to
     destroy the buffer
   The antacid solution is heated to remove carbon dioxide.
   The excess HCl is then titrated with a standardized sodium
     hydroxide, NaOH, solution.
   This analytical technique is referred to as called “back
     titration”

Back titration is an indirect method of volumetric analysis, it
involves the deliberate addition of an excess of a reagent to the
test sample. After reaction is complete, the amount in excess is
determined by titration.


         molesantacid + moles   NaOH   = moles HCl

         ⇨   molesantacid = moles HCl - moles NaOH

Determine the number of moles of base / g of antacid.
Procedure: Vinegar Analysis
1. Preparation of the vinegar sample:
      - Clean at least two 250mL Erlemmeyer flasks, when dry label
        each flask and measure its mass (±0.01g)
     - Add approximately 10 mL of one brand of vinegar to the
         flask weigh and record the mass.
     - Add 2 drops of phenolphthalein
     - Add 20 mL of distilled water

2. Preparation of the Buret and titration setup
   - Rinse a clean 50 mL buret with the standardized 0.5M NaOH
     solution, making certain no drops cling to the inside wall.
   - Fill the buret with the standardized NaOH solution eliminate
     all air bubbles in the buret tip, and after 30 seconds, read
     and record the initial volume (± 0.02ml )
   - Place a white tile under the flask containing the vinegar
     sample.

3. Titration of the vinegar sample.
   - Slowly add the NaOH solution from the buret to the acid,
     swirling the flask (with the proper hand) after each addition.
   - Continue addition of the NaOH titrant until the endpoint is
     reached. The endpoint should be within one half drop of a
     slight pink color. Be careful not to surpass the endpoint. The
     color should persist for 30 seconds.
  - Record the final volume (± 0.02 mL)of NaOH titrant in the
      buret

4. Repeat with the same vinegar
   Refill the buret and repeat the titration at least once more
   with another sample of the same vinegar
5. Perform two analyses of a different vinegar sample and
  determine its acetic acid content.
Procedure: Antacid Analysis
1. Preparation of the antacid for analysis
  - Grind the antacid tablet with a mortar and pestle
  - Weigh around 0.20g of antacid into a 250 mL Erlenmeyer
     flask (± 0.01g)
  - Pipet 25.00 mL of standardized 0.1M HCl solution into the
     flask and swirl
  - Heat the solution to a gentle boil and maintain the heat for 1
     minute to remove dissolved CO2
  - Add 4-8 drops of bromophenol blue indicator
  - If the solution is blue pipet an additional 10.00 mL of 0.1M
     HCl and boil again
  - Repeat as often as necessary
  - Record the total volume of HCl added that is added to the
     antacid

2. Preparation of the Buret and titration setup
    Same as for vinegar analysis but using 0.1M NaOH

3. Titration of the antacid sample.
 - Slowly add the 0.1M NaOH solution from the buret to the
    antacid, swirling the flask (with the proper hand) after each
    addition.
 - Continue addition of the NaOH titrant until the endpoint is
    reached. The endpoint should be within one half drop of a
    blue color. Be careful not to surpass the endpoint. The color
    should persist for 30 seconds.
 - Record the final volume (± 0.02 mL)of NaOH titrant in the
    buret.

4. Repeat with the same antacid

5. Repeat with another antacid.

				
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posted:11/29/2011
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