MadSci Network: Chemistry

Re: How can I calculate the pH of the titration of citric acid with NAOCl?

Date: Wed Apr 7 20:22:10 1999
Posted By: Charlie Crutchfield, , Retired, Retired
Area of science: Chemistry
ID: 923379737.Ch

I believe you mean to use NaOH [sodium hydroxide] in your titration, not 
NaOCl, which is sodim hypochlorite [i.e. Clorox bleach]

Titration with  standatrd NaOH to a Phenolpthtalein end point is the 
correct method to determine the concentration of citric acid in these 
juices. Sorry, but you cannot calculate the pH of the juice from this. 
Solutions of 1 Molar hydrochloric acid, and 1 Molar acetic [or citric]acid 
will cgive the same titration, but the pH of the origonal solutions can be 
different. To measure the pH you would need a pH meter, or several 
indicators that change color in the range of pH 1.5 - 4.0.

Since you know that [almost] the only acid here is citric, you can 
calculate the pH from the concentrations that you obtain from the 
titrations of the different juices. The equation for this is:

       [H]   =   square root of the quantity[MK]

         [H] is the concentration of Hydrogen ions in Moles/liter
         M is the concentration of citric acid in moles per liter
         K  is the first ionization constant of Citric acid 

  "M" you found from your titration, K is found in texts or reference books. 
 One of my books shows K to be [at 20 C] 7.2 x [0.0001], or 0.00072.

Now, multiply M by K, and the square root of this product gives the 
[H] ion concentration in moles/liter. Convert this to pH.

I am lazy, and I prefer to use logarithms all the way. As follows:

        pH  = 1/2pK  - 1/2 pM

   pH is :the log. of the reciprocal of the {H} concentration
   pK is :the  ""    """       ""     ""   Ionization Constant
   pM is :the  ""    """       ""     ""    M,  the Citric concentration
     By the way, pK is 3.14 [at 18 C]in my book 

  This way, you only have to do one calculation, converting M to pM 
For your general information, here is some rough data that might help you 
-I do strongly urge that you verify the lemon and pineapple acid data.  Do 
not trust books completely.
Lemon juice:  pH  2.2 - 2.4,   Citric acid  6 - 9 % by weight
Lime juice: pH 1.8 - 2.0
Orange juice: pH 3.0 - 4.0
Grapefruit juice: pH 3.0 - 3.3
Pineapple juice:               Citric Acid 0.4 - 1 %

I hope this is of use to you.  Contact me if you need more info

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