The Concentration Of An Unknown Solution Of Hydrochloric Acid

The aim of this experiment was to calculate the concentration of an unknown solution of Sodium Hydroxide (NaOH) by titrating it with Hydrochloric Acid (HCl) and to identify any possible sources of error. A colour change from pink to yellow signified the end of each individual titre as the NaOH had been neutralised.
Experimental:
Method:
Firstly two solutions were prepared to show the colour of the Phenol Red indicator in acid and alkaline conditions. These colours were then used to determine the end point of each titre. In the experiment 0.100M HCl (acid) was added, from a burette, to a solution of 25cm3 of NaOH (base) and several drops of Phenol red indicator. This was done until the base had been neutralised (signified by a colour change from pink to yellow). A magnetic stirrer was used to ensure even mixing of the acid being added.
Problems:
Firstly, it took time for the tap on the burette to be closed, so even though the titration was complete more acid was being added to the conical flask, this resulted in the reading on the burette higher than usual. Also each titration was carried out by a different person, the reaction times of different individuals will vary and so the tap would be closed at a faster/slower rate depending on the individual. Additionally, different individuals may spot the colour change at different times. Once this problem was noticed the remaining titrations were carried out by the same person to have more consistency in the experiment. Furthermore parallax became an issue when measurements were taken from the burette, if the reading were took at different angles then the values appeared to be different. To ensure that effects of parallax were minimised readings were taken standing directly in front of the burette and verified by another