EXPERIMENT 4

 

ANALYSIS OF NICOTINIC ACID TABLETS (NIACIN) BY POTENTIOMETRIC TITRATION

 

Objective: To determine the amount of drug present in the sample tablets and to evaluate the ionization constant of an organic acid

 

Introduction

Niacin is a weak organic acid.  Therefore, when niacin is  titrated with a strong base (for example NaOH in this experiment), the conjugate base of the weak acid is formed.   By performing such a titration it is possible to quantitate the amount of niacin in a tablet.

 

The purpose of Part A of this experiment is  to standardize the NaOH solution that will be used to titrate against the weak acid- nicotinic acid.    A known weight of KHP is titrated using:

NaOH  +   KHP  ®    H2O   +  Salt

 

It is now possible to calculate the molarity of the of NaOH (Please refer to the calculations section for the exact approach.)

 

The equivalence point in the titration is determined by the first derivative method.   The first derivative method involves graphing DpH/DV versus the volume of NaOH added. The inflection point is indicative of the pKa. 

 

The purpose of Part B of this experiment is to determine the amount of niacin in the sample tablets,  Standardized NaOH is added stepwise to dissolved nicotinic acid tablets, while a change in pH is noted using an electrode.  By performing the appropriate calculations it is possible to determine the amount of drug present in the sample tablets.

 

 

Experimental

 

Reagents

0.1  M NaOH

KHP (potassium acid phthalate)

buffers (pH 4.0 and pH 7.0)

Niacin tablets (100 mg)

 

Equipment

250 ml Erlenmeyer flasks (2)

250 ml beakers (2)

pH meter

1 50-ml buret

 

Procedure

Part A Standardization of 0.1 M NaOH

 

1)      Accurately weigh two portions of  0.5XXX g of KHP (potassium acid phthalate) into

two 250 ml Erlenmeyer flasks.  Add about 75 mLs of distilled water to each and dissolve completely by the aid of heat.  Cool the solutions.

 

2)      Using the elctrode supplied by the instructor potentiometrically titrate with approx.

0.1 N NaOH solution. 

 

3) Prepare a table containing the following information: mls of NaOH, pH, DVol,  DpH, and DpH/DV Use the data to plot the potentiometric titration curve and locate the equivalence point.  From the first derivative plot (need to explain) determine the volume of NaOH (V) at the equivalence point and convert ml to L.

 

 

Part B Determination of Niacin in Tablets

 

1)      Select 6 of the Niacin Tablets and divide them into two portions.  Weigh each tablet

separately and record the weights. Determine the average weight of one tablet.

 

2)      Reduce one tablet sample (3 tablets) to a fine powder and weigh it.  Do the same with

the second sample.

 

3)      Transfer each portion of the fine powder to a 250 ml beaker, and dissolve each

portion in about 100 ml of H2O. 

 

4) Perform a potentiometric titration of each sample with the 0.1 N NaOH using the first derivative method for locating the equivalence point.

 

5) Record in the lab notebook volume of titrant (V) and pH.  Using Sigma Plot, calculate DV, D pH, D pH/D V for each set of data. (refer to Appendix for Sigma Plot Tutorial.)

 

6) Plot pH (y-axis) vs. Volume of NaOH (x-axis) for each set of data.  Prepare first derivative plot by plotting D pH/DV of NaOH(y-axis) vs. Volume of NaOH (x-axis).  Determine the volume of NaOH (V) at the equivalence point and convert ml to L.

 

Calculations:

1)      Number of moles of KHP

            ­­­­ KHP sample weight, g     x         1 mol of KHP     =  # mol of KHP

     204.22 g 

 

2)      Molarity  of NaOH

           g KHP                  x    1 mol of KHP x  1 mol of NaOH  =    mol NaOH

          vol of NaOH (L)           204.22 g KHP           1 mol KHP                 L of NaOH

 

 

3)      Weight of Nicotinic acid

Vol of NaOH x  (mol of NaOH)  x mol of Nicotinic Acid x 123.11 g Nic acid

                                       vol of NaOH           mol of NaOH                1 mol of Nic acid

 

4)      % Nicotinic Acid per sample as follows:

    weight of nicotinic acid                  x   100

                        weight of sample (after crushing)

 

 

Mg of Nicotinic Acid per Tablet as follows:

 

mg/Tablet  =  % Nicotinic Acid  x   Average Weight of Tablet, g  x 1000 mg   

                                                                                                             1 g     

 

pKa of Nicotinic Acid

 

Determine the pKa of Nicotinic Acid from the pH vs. V plot, by recording the pH at half equivalence point.

 

Since  pH = pKa + log [A-] at half of equivalence point pH= pKa.

                                      HA]
DATA SHEET                                                                                      

ANALYSIS OF NIACIN TABLETS

 

 

Part 1  NaOH Standardization

(20 points)

 

   KHP Mass 1(g)                               Volume of NaOH used to titrate 1 (ml)         

   Molarity of NaOH             

  

   KHP Mass 2 (g)                              Volume of NaOH used to titrate 2 (ml)       

   Molarity of NaOH             

 

                                                Average Molarity of NaOH (+/- std)                       

 

 

PART 2 Titration of Niacin tablets

(50 points)

Weight of each Tablets (g)                                                                           

                                                                                                                       

                                                                                                                       

 

Weight of Nicotinic Acid used (g)                                                               

                           (after crushing)

Volume of                   M NaOH (ml)                                                            

            Wt of Nicotinic Acid Found   (g)                                                                   

% Nicotinic Acid                                                                                           

             mg of Niacotinc Acid/Tablet                                                            

           

Using optimal results 

Nicotinic acid pKa                                                                           

                                                            Ka                                                                   

 

Paste the following six graphs in your notebook.

Part 1 – Standardization of NaOH

pH vs. V for both sets of data

D pH/D V vs. V for more accurate set of data

Data values for first derivatives graphs must be presented in tabular form

 

 

Part 2 – Determination of Niacin in Tablets

pH vs. V for both sets of data

D pH/D V vs. V for more accurate set of data

Data values for first derivative graphs must be presented in tabular form.

 

Notebook Grade:                                                                   Final Grade: