NaOH is a strong alkali and HCl acid is a strong acid respectively. To identify the equivalence point in the titration, we use titration curves and indicators. According to the concentration of acid and base solutions, we have to choose correct curve and indicator.
In this tutorial we will learn, how to obtain titration curves and decide which indicator should be used.
HCl and NaOH are strong acid and strong base respectively and their titration curves are similar (shape of curve) in different concentrations.
Both reactants and products are in aqueous state.
Indicators are used to find pH value in equivalence point. Colour of indicator is changed at one range of pH. That colour changeing point is called "end point".
If colour change of indicator is occurred at pH=7 in strong acid - strong base titration, its end point and equals to the equivalence point.
|Indicator||pH of end point||pH range||Colour 1||Colour 2|
|Methyl orange||3.7||3.2 - 4.2||Red||Yellow|
|Bromophenol blue||4.0||2.8 - 4.6||Red||Blue|
|Bromocresol green||4.7||3.8 - 5.4||Yellow||Blue|
|Methyl red||5.1||4.2 - 6.3||Red||Black|
|Bromothymol blue||7.0||6.0 - 7.6||yellow||blue|
|Phenol red||7.9||6.8 - 8.4||yellow||red|
|Phenolphthalein||9.3||8.2 - 10.0||colourless||red|
|Thymophthaline||9.7||8.3 - 10.0||colourless||blue|
There are three sections of titration curve of when strong base is added to the strong acid.
0.1 moldm-3 HCl 25cm3 is placed at a beaker. 0.1 moldm-3 NaOH solution is added to the HCl solution drop by drop slowly.
In the titration curve, first pH increases slowly, then has a steep increment and then again increases slowly.
Check the pH range which has a steep increment (3-11). Then check pH range of indicator. When this steep increment occurs, indicator should change its colour. Lets look which indicators are suitable for this titration.
0.01 moldm-3 HCl 25cm3 is placed at a beaker. 0.01 moldm-3 NaOH solution is added to the HCl solution drop by drop.
0.001 moldm-3 HCl 25cm3 is placed at a beaker. 0.001 moldm-3 NaOH solution is added to the HCl solution drop by drop.
In all beakers, aqueous HCl acid solution is present and aqueous NaOH solution is added to HCl solution.
So we are done with titration curve when HCl is in the flask. Now we are going to focus on how pH curve will vary when flask holds NaOH and adding HCl.
So pH of NaOH is higher than 7 and with addition of HCl, pH decreases of the solution.
As mentioned earlier, due to 1:1 stoichiometry ratio between NaOH and HCl, they react in same amount.
|NaOH Concentration (mol dm-3)||pH|
We take three flasks which have three different concentrations of NaOH. Then we add dilute HCl to react with NaOH and calculate pH of the solution to obtain three titration curves.
There is no certin values for NaOH and HCl concentrations. If you want to make a certain pH value NaOH solution, you have to measure the volume and concentrations of HCl solution.
No. In a titration, we are going to determine the concentration of HCl or NaOH or other used acid or base. One known concentration solution is used to titration.
Bromothymol blue is not titrated in NaOH and HCl titration. When pH value is changed, colour of bromothymol blue is changed.