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
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
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