Introduction
Titration
The volumetric analytical  technique called an acid-base titration is commonly used to determine the acid or base content in a solid  sample or solution. In titration, the volume of  a solution,  necessary to completely react with a  given mass or volume  of the  substance being titrated is precisely measured.   The addition of the  titrant is done using  an accurately calibrated  piece of equipment, a buret. The completion of the reaction, the end point, may be determined by the color change of a third substance added to the analyte before titration,  a  pH indicator. The end point, as signaled by the color change of the indicator, is achieved by adding  just one additional  drop of the titrant, after the equivalence point has been reached.

Determining the molarity of NaOH solution
This is done by using the NaOH solution (titrant) to titrate a given amount of a primary standard.  The primary standard must meet certain requirements: it must be a solid with known composition,  must be extremely pure, be stable upon heating and  exposure to air, and cannot be hygroscopic.
A pre-weighed amount of the primary standard, which in this experiment is an acid,  potassium hydrogen phthalate (abbreviated as KHP, M.M.  204 g/mol), is dissolved in water and titrated against  the pH indicator, phenolphthalein  with the NaOH solution The reaction equation:

indicates that there is a 1: 1 molar ratio of KHP and NaOH. Therefore, the reaction reaches its equivalence point when all moles of  KHP present in the dissolved sample (n _KHP) have been neutralized by the equivalent number of moles of NaOH (n _NaOH ) present in the delivered volume of the solution (V _NaOH).  The  number of moles of NaOH (n _NaOH = n _KHP ) and the volume of the NaOH solution (V _NaOH) used to titrate the KHP sample, one can calculate the  molar concentration of NaOH solution (M _NaOH =n _NaOH / V _NaOH ).

Buret
A buret is a long tube calibrated in ml, fitted with a stopcock that allows for precise control of the titrant flow . The total capacity of a buret used in this experiment is 50.00 ml, with 0 marked on the top and  the volume increasing down the buret to the mark of 50  ml. The lowest mark on the buret scale indicates 0.10 ml.
The buret is filled and the initial titrant  volume read (at the bottom of the meniscus) and recorded. The titration is performed by slowly adding the titrant,  manipulating the stopcock with left hand while using right hand to swirl the flask with the analyzed solution  After the end-point has been reached, the final  volume of the titrant is read again and recorded.  The volume of titrant used for the reaction is calculated by subtracting the initial volume from the final volume recorded.

PROCEDURE
I. Titration  of  the NaOH solution
1. Preparation of  the buret
1. Obtain a ring stand and attach a buret clamp to it.

2.  Secure a buret  in the clamp.  The buret should be clean but always check if it is.

3. With the buret secured in a clamp and the stopcock closed, place a small funnel in the top opening. Obtain a clean and dry  50 mL beaker  and fill it with the provided NaOH solution.  Pour about  5 ml of the  solution from the beaker into the buret.

4. Take the buret off the stand and tilt it so that the  walls came in contact with the NaOH solution.

5. Place the buret in the clamp again and drain the rinse solution through the stopcock into a waste beaker. If the buret was wet to start with, repeat rinsing process with another 5 ml of NaOH.

6. With the stopcock closed, fill the buret with NaOH a little above the 0 mark.

7. Open the stopckock promptly and let the titrant flow for a moment to  fill the tip. To  remove the air bubble, you may need to turn  the stopcock back and forth in sharp, quick  movements while  tapping the tip with your fingers. Record the initial volume of titrant to 0.1 ml (it does not have to be 0.0).

2. Titration
Refer to the slide show: Weighing Using a Top-Loading Balance , to review weighing. Use multiple clicks of  the Back button in the upper left-hand corner of your screen to return to this document.

8. Using a spatula, transfer  0. 10 g  (within ± 0.01 g range)  of potassium hydrogen phthalate (KHP) onto a folded weghing paper, tared on the balance. Record the actual mass of KHP.

9. Transfer the KHP sample to a  50 or 125 ml Erlenmeyer flask and add about 15 ml of distilled water, and   2 drops  of  0.1% phenolphthalein solution. Swirl the flask to dissolve KHP. Not all of the KHP will dissolve at this point, but  it will eventually,  as titration progresses.

10. Position the flask with KHP under the buret and raise the flask until  half of the  buret’s tip is inserted into the flask. Place a sheet of white paper underneath the flask. Titrate, adding NaOH in small increments while swirling the flask.  Slower the addition of the titrant when the pink color appearing after each addition  dissipates at a slower rate (you  may need to slow down  to one drop per addition).
The titration end-point is indicated by an extremely pale pink color of the solution that persists for more than 15 seconds.
Record the final volume of the titrant in the buret.

11. Repeat steps  8-10 two more times using a new sample of KHP and a clean flask each time.

 BURET CLEANUP
 LEAVE THE BURET SECURED TO THE STAND.   DRAIN THE BURET THROUGH THE TIP and INTO A CLEAN BEAKER.
 DISPOSE OF THE NaOH SOLUTION IN A WAY INDICATED BY THE INSTRUCTOR.
 CLOSE THE STOPCOCK , FILL THE BURET WITH TAP WATER and DRAIN THROUGH THE TIP (leave it  on the stand).
 FILL THE BURET WITH TAP WATER AND DRAIN AGAIN.
 ADD ABOUT 10 ML OF DISTILLED WATER, REMOVE THE  BURET FROM  THE STAND AND TURN IT AROUND, TILTING   SO  THAT THE WATER COULD REACH THE TOP.
DRAIN THE WATER THROUGH THE TIP. LEAVE THE STOPCOCK IN THE OPEN POSITION.
RETURN  TO  THE INSTRUCTOR’S CART: THE BURET,  PIPET, BULB AND ANY OTHER PIECE OF EQUIPMENT THAT  DID NOT COME FROM YOUR  DRAWER .

DATA SHEET

	Titration 1	Titration 2	Titration 3
Mass of KHP, grams
Initial volume of NaOH, ml
Final volume of NaOH,  ml

CALCULATIONS AND RESULTS

1. number of moles of KHP:  n = mass KHP x (1 / 204 g/mol)

n ₁   =  ______________        n₂   = _____________        n₃   =___________________

2. number  of moles of NaOH =  number of moles of KHP

n ₁   = __________________        n₂ = ___________________       n₃   =______________________

3.  Volume of NaOH used  in each titration = final volume of NaOH - initial volume of NaOH

V ₁ =   _________________        V₂  = ___________________        V₃   =_______________________

4. Molarity of NaOH solution = n/V

M ₁ =   _________________        M₂ = ___________________       M₃   =______________________

2. Average molarity of NaOH:

 M = (M₁ +  M₂  + M₃) / 3  =  ________________________________________

PRELAB

POSTLAB
Students performing vinegar analyses made several mistakes in their  experiments. Explain how each error affected the calculated molarity of the NaOH solution.  Hint: start with deciding how the volume of the titrant was affected by each error

1. Student A   did not fill the tip of the buret with titrant before starting the NaOH standardization, so the tip contained air when the titration was started, but  was filled  after the first  titration.
 
 
 
 
 
 
 

2. Student B overtitrated the KHP solution, but  ignored  it , and continued on with calculations.
 
 
 
 
 
 

3. Student C was in a hurry to leave the lab . He finished the  titration without waiting for the pink color to persist for 15 seconds. He recorded the volume of NaOH used, just to  notice that the pink color disappeared. He ignored this observation and proceeded with calculations.