Why Does The Pink Color Which Forms At The Point

Introduction Titration is a process of neutralization and is commonly used to determine the concentration of an acid or base in a solution. This process involves a solution of known concentration delivered from a buret into the unknown solution until the substance being analyzed is just consumed. In this lab, experimenters using hydrochloric solution to react with the solution of NaOH according to this equation: HCl (aq)+ NaOH(aq) → NaCl(aq) + H2O(l) HCl is a strong acid, NaOH is also a strong base, therefore they will dissolve completely. By adding NaOH into HCl, the hydrogen ions neutralized with hydroxide ions, the theoretical endpoint and then the solution will be weak basic solution. The endpoint
could be determined by phenolphthalein indicator. Once the hydrogen ions has been neutralized with OHcompletely, the solution will be basic and the color of solution will turns from colorless into a faint pink. Then the experimenters should record the volume of NaOH and calculate the concentration of unknown solution.

Purpose
Calculate the concentration of the unknown solution by using the equation and the details in the introduction.

Materials Equipment
NaOH solution (0.1M)
Beaker, 250ml
Funnel
Erlenmeyer flask, 250ml
HCl solution (unknown concentration)
Buret, 25ml
Ring stand
Phenolphthalein indicator
Buret Clamp
White Paper graduated cylinder

Safety Consideration
HCl and HaOH are caustic and experimenters cannot contact them with eyes and skin.
Wash hands immediately if contact HCl or NaOH.
Wash all equipment before and after the lab.

Procedure
1.Use flow water to wash the buret, beaker , funnel and erlenmeyer flask.
2.Use the buret clamp to hold the buret, the buret should in a proper hight.
3.Put the funnel on the buret and pour 15 ml of NaOH into the buret and drip into the empty beaker, then using new 15 ml of NaOH to repeat this step twice more.
4.Pour NaOH into the buret until the meniscus is approach 0 ml.
5.Use graduated cylinder to measure 5 ml HCl and pour into the Erlenmeyer flask.
6.Add two drops of Phenolphthalein indicator into the Erlenmeyer flask.
7.Put the Erlenmeyer flask on a piece of white paper right under the tip of the buret
8.Drip NaOH into HCl slowly, shake the Erlenmeyer flask every time when pink appears. Stop until the pink won’t disappear.
9.Stop and count the volume of NaOH added into HCl and clean the Erlenmeyer flask.
10.Repeat steps #4-9 twice more.
11. After finished the lab, clean all equipment and disposed remaining solution with tap water.

Data

Calculation
1.The number of moles of NaOH used.
Trial 1: n=CV=0.1x0.0085=0.00085 moles
Trial 2: n=CV=0.1x0.0127=0.00127 moles
Trial 3: n=CV=0.1x0.0156=0.00156 moles

2. The concentration of HCl
Trial 1: C= n/V=0.00085/0.005= 0.17M
Trial 2: C= n/V=0.0127/0.008= 0.16M
Trial3: C= n/V=0.00156/0.0010=0.16M

3.
The concentration that we get is much bigger than the accepted concentration of HCl. It illustrates that we made some errors during the lab, the possible causes are water left in the flask and buret which makes the concentration of NaOH decreased, the inaccurate measurement of the volume of NaOH and HCl used.

Questions
Why does the pink color, which forms at the point where the NaOH comes into contact with the solution in the flask, disappear more slowly near the endpoint?
The reason is that the Phenolphthalein indicator slightly dissolve in HCl solution, the OH-ions reacted with Phenolphthalein indicator first then titrates with H+ ions. As the NaOH solution dripped into the flask, the OH-ions react with H+ions and the pink color disappear. However the H+ is limited, as adding more OH-, all H+ ions will reacted with OH- ions. So that the pink color will disappear slower and finally never disappear around the endpoint.

Why is it a good idea to carry out titrations in triplicate(three times)?
Carry out titrations in triplicate is to eliminate the differences caused by some error during the experiment like the water left in the buret and to get a closer answer to the precise concentration of the
solution.

Would the addition of several milliliters of distilled water to the Erlenmeyer flask during the titration affect the results of the titration? Explain your answer.
Obsoletely yes, since the water left in the erlenmeyer flask will mix with the solution which will decrease the concentration of the solution.
For instance, if there is some water left in the buret, while we add the 0.1M NaOH solution into the buret, the water will mix with the solution therefore the concentration of NaOH will change to less than 0.1M.

Errors
1. When experimenters filled the buret into 0 ruling and then began drop NaOH into HCL solution, if the color of the first time doesn't change into pink and the experimenters added more NaOH into the buret after the meniscus drop below 25 ml mark, then the volume of NaOH has used will not accurate. When the color doesn't change, experimenter should make sure that the meniscus drop below that 25 ml ruling, the right way is to add more NaOH solution when the meniscus approaches 25 ml mark precisely.

2. Experimenters might miss the exact end point and drop more NaOH solutions in the flask before stop the experiment. So that the volume of NaOH added in the Erlenmeyer flask might more than it really need. The concentration will effect by these wrong volumes. To avoid this error,experimenter should drip less NaOH solution each time slowly and always shake the flask which can help experimenter to find the real end point and eliminate the
differences.

Conclusion
Through this experiment, we followed the procedure and finally we achieved the goal of this experiment and get the concentration of NaOH. However, there are still some mistakes involved , for instance, the inaccurate volume of NaOH and water has left in the flask and buret. We repeated the experiment for three times to trying eliminate the differences.
However, our result still has an obvious difference with the accepted concentration, which means we still need some improvement. The first is when we prepare the experiment, we should clean the equipment and left enough time to dry it to avoid that the water left mix with the solution and makes the result incorrectly. Another one is try to measure the solution as accurate as we can and we should look at the meniscus horizontally in front of the buret. Last one is to roll the flask frequently between each time to find when the color does not disappear. During the calculation part, I compared my answer with my group member, we all get the same answer, which means the methods is right but the result is different with other groups’ answer, which means we might made some mistakes during measurement. Overall, we achieved the goal and calculated the concentration of HCl, but it has deviation with the accepted concentration, so this experiment still has some errors involved, we need improve it in the future lab.