[angelku]

A Known excess amount of sodium hydroxide is mixed with weighed mass of aspirin, the amount of  hydoxide can be determined by a back titration with H2SO4.The mass of aspirin can be determined by calculations.


If a base CaCO3 present in the tablet,what is the difference betwwen the calculated value with the actual value?

 I have no idea about that.Would CaCO3 present in the tablet can allow smaller amount of NaOH to hydrolysis the ester?So that more amount of H2SO4 needed to used to reach the end point.The lower mass of aspirin may be resulted.

[AWK]

H2SO4 reacts with sodium acetylsalicylate, sodium hydroxide and calcium carbonate. You should specify more details of your method. (If you use phenolpthalein in titration then only excess of NaOH will react and question on CaCO3 is out of science assuming no reaction between aspirin and calcium carbonate took place during storage of aspirin.)

[ARGOS++]

Dear Angelku;

Yes!,   ─   Please, explain more about your “methodic”.

It looks like a good method for training Titration, but also as a bad method for Quality Control.

Good Luck!
                    ARGOS⁺⁺

[angelku]

1.Initiate the hydrolysis of the aspirin by adding NaOH .
2. Titrate the  solution mixture (aspirin and NaOH)diluted by deionized water with H2SO4 using phenol red as indicator.

I also have question why the active ingredient in a tablet produced by drug manufacturer seldom with higher than 90%?Why they dilute the active ingredient?

[ARGOS++]

Dear Angelku;

Thank you very much for the short description!
I will not comment it, because you may have already realised it by yourself, that it is a very unspecific method!

The content of a Tablet can’t be very much above 90% (Tablet weight), because other Ingredients are required, at least one for “blasting” the Tablet (for a higher release rate [DR]), and one for compacting/formulating the Tablet (= for Galenics).

But in case you find equal or less than 90% of the “Nominal Content”, then it will be very bad.
At least in such situations an alternative method like UV/Vis-Spectrometry is indicated, which by correct handling is much more specific and can by used for identification too!

I hope it gives you at least an idea.

Good Luck!
                    ARGOS⁺⁺

[AWK]

1.Initiate the hydrolysis of the aspirin by adding NaOH .
2. Titrate the  solution mixture (aspirin and NaOH)diluted by deionized water with H2SO4 using phenol red as indicator.

I also have question why the active ingredient in a tablet produced by drug manufacturer seldom with higher than 90%?Why they dilute the active ingredient?

What do you mean under hydrolysis?

[angelku]

2-ethanoyloxybenxoic acid which is an active ingredient of aspirin.It can be hydrolysed by using excess  sodium hydroxide into the sodium salts of two weak acids.

After the hydrolysis of aspirin,the reaction mixture was diluted to 250 ml.25ml was titrated against H2SO4 by using phenol red as indicator.

[AWK]

Hydrolysic of acetylsalicylic acid is not quite easy. You should warm solution at lest a part of hour. Then excess of NaOH will catch carbon dioxide from air and your mixture for titration will be qute complex.
Can you write down chemical reaxtions?

[angelku]

C9H8O4+2NaOH-->CH3CO2Na+C7H5O3Na +H20

Does a aspirin tablet  contain CaCO3 will affect the calculated value ?

[ARGOS++]

Dear Angelku;

It should nearly not influence, because with using Phenol Red (pH: 8.0 – 6.6) as Indicator most of the carbonate effect is already “eliminated”.
A little better could be the use of Litmus (pH: 8.0 – 5.0) or Methyl Red (pH: 6.2 – 4.4).

Apropos:   I found the identical Method, except Backtitration with HCl instead of H₂SO₄, in a German Chemistry Forum, ─ with similar problems.

Good Luck!
                    ARGOS⁺⁺

[angelku]

Why the presence of CaCO3 have no influence when using phenol red as an indicator?Do you  mean the calculated value have no difference between the actual value.  ???How the amount of reactants used change?

The drug manufacturer dilute the active ingredient because it require the presence of ingredients that formulating the tablet.Besides,are there any reasons account for this action?

[AWK]

Carbonates start decomposing when pH is lower than 7, and you finish titration at pH 8 in your case

[ARGOS++]

Dear Angelku;

Sorry!,   ─   that my statement is giving a cause for misinterpretation!
(When I answered you I hade (only) the carbonate statement from Mr. AWK in mind, sorry!)

Of course you are right!  In case a manufacturer adds “any” Carbonate to the Tablet you will titrate the contained base too, because at the end point of your Titration, around or below pH 5.5, most or all Carbonate will be “formed” to CO₂.

Such an addition will require more H₂SO₄ and simulate a lower Content.
But that’s also only one indication of an unspecific method.
(To stop Titration on higher pH may also have disadvantages, as collected CO₂.)

Yes!   ─   There are “several” reasons, like Stability (phys. & chem.) of the Formulation for different “Climates” on Earth, Hardness of Tablet, DR (Disolution Ratio), etc.
(Galenics is quite a broad field for R & D.)

Sorry to have given you a wrong cause.

Good Luck!
                    ARGOS⁺⁺

[angelku]

Why more H2SO4  needed to be used when carbonate decompose into CO2?

[AWK]

First of all you should obtain H2CO3 to decompose it. Decomposition of carbonic acid is rather complex problem from scientific point of view (but not practical one).
Concentration of carbonic acid in solution (when formed) depends on concentration of dissolved carbon dioxide in solution together with  partial pressure of carbon dioxide over solution.
You can change the concentration  of CO2 in the solution by
lowering the pressure of CO2 over solution what cause
lowering the concentration of soluble CO2 in the solution.
Moreover lowering of CO2 concentration in solution is possible by adding strong acid or by heating the solution.

Closely to pH 7 eventually hydrogen carbonates may exist in solution. They also may be decomposed by addition of strong acids or by heating the solution.

In my opinion that rather reaction CO2 from air with an excess of NaOH may be a source of an rror in you titration since  reaction:
2NaOH + CO2 = Na2CO3 + H2O
During titration using phenyl red as an indicator you can titrate only half NaOH involved in the above reaction because reaction
CO3^2- + H+ = NaHCO₃
takes place.