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Kenneth Kang

Wednesday, May 14, 1997

Chemistry

Titration Lab

Determining pH is important in various chemical experiments. This lab uses two methods, one with an electronic probe and the other, titration neutralization. Our results (2.339) contradicted the probe's reading of 2.76. Errors included the qualitative observation of pinkness.

Introduction

Measuring pH using a neutralization reaction was one of the goals. We also learned how to properly titrate a solution. Other challenges included measurements to the 0.01 mL. An indicator was added to the solution so that we could see when it was neutralized. Our investigation used the H+ ion and OH- ion combination into H2O.

Materials

  1. 2 burets
  2. 0.1M NaOH
  3. Vinegar (Acetic Acid HC2H3O2)
  4. phenolphthalein solution
  5. Erlenmeyer flask
  6. distilled water

Procedure

  1. fill one half of acid buret with vinegar
  2. fill base buret with NaOH
  3. read burets to 0.01 mL
  4. using the acid buret add 3mL to the Erlenmeyer flask
  5. add 50mL of distilled water
  6. add two drops of phenolphthalein solution

Titration

  1. add NaOH slowly while stirring
  2. wait for a color change (pink) and stop
  3. read final buret levels
  4. repeat three times

Data


Trial   Acid Level   Base Level   

1       21.96        3.835        

        25.18        42.015       

2       15.85        1.79         

        18.95        36.75        

3       18.95        0.88         

        21.82        33.59        



Table 1: Readings

It should be noted that the burets were labeled with zero on the top and 50 mL on the bottom. It will not affect our readings because we can just subtract the readings to find out how much liquid actually came out.

Analysis


Trial      Change in acid (mL)  Change in base (mL)  Normality   pH       

1          3.22                 38.18                1.857       2.236    

2          3.1                  34.96                1.128       2.394    

3          2.87                 32.7                 1.39        2.307    

Average                                              1.1509      2.339    



Table 2: Calculated Data

The following equations were used:

  • NAVA=NBVB, used to find the normality
  • , used to find the concentration of hydrogen ions
  • Logarithms were used to convert the hydrogen ion concentration to the pH numbers.
  • Determining the color change became harder as the solution approached neutrality. The pink started appearing at first at the contact point between the stream of incoming base solution and the dilute vinegar solution. It would then spread and we would have to wait, shaking the flask to make the pink go away. We would wait about thirty seconds for the pink to leave. It could be possible that the pink dissipate after 30 seconds and thus our solution would not have been neutral. Carrying an increase through the equations shows that we would have generated a bigger pH number.

    Conclusion

    Our measurements gave us the correct pH within one pH of the actual reading. Qualitative errors of pinkness impeded a more accurate reading. Changing the indicator or using dual indicators would increase accuracy. A colorimeter coupled with a titration machine could quickly be able to determine pHs of various substances including those that are too dangerous to read using an electronic pH meter.