NaOH and HCl Titration Curves | Selecting Indicators

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.


Titration curve of strong acid and strong base

HCl and NaOH are strong acid and strong base respectively and their titration curves are similar (shape of curve) in different concentrations.


NaOH + HCl = NaCl + H2O

Both reactants and products are in aqueous state.


Important factors and equations of HCl + NaOH reaction and its titration curve.

  • HCl and NaOH reacts in 1:1 ratio (in same amount).
  • To decide required amount (mol) and volume, the relationship between amount (mol), volume and concentration is used. concentration = amount / volume


Equivalence point

  • Amount of titrant added is enough to completely neutralize the analyte solution. That means, all reactants (here HCl and NaOH) are reacted and no remaining reactants in the solution. Due to no HCl and NaOH, solution is neutral.

Indicators

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.


pH ranges, pH of end point, and colours of indicators

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


Obtaining Titration Curve when flask holds HCl and adding NaOH

There are three sections of titration curve of when strong base is added to the strong acid.

  1. pH increases slowly
  2. Steep increment of pH due to unreacted of NaOH, no HCl exist furthermore in the aqueous solution. Solution become neutral or become basic.
  3. Slow increment of pH
Titration Curve of adding NaOH to HCl

0.1 mol dm-3 HCl and 0.1 mol dm-3 NaOH Titration Curve

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.

0.1 moldm-3 HCl and 0.1 moldm-3 NaOH titration curve

In the titration curve, first pH increases slowly, then has a steep increment and then again increases slowly.


Explanation to titration curve of 0.1 mol dm-3 HCl and 0.1 mol dm-3 NaOH

  • In the initial step, there is 0.1mol dm-3 HCl in the flask. So pH of the initial aqueous HCl solution is 1. If you are not familiar with calculating pH of HCl, refer this tutorial.
  • Initial HCl amount of 25 cm3 is 0.0025 mol. To neutralize the all HCl amount, same amount of NaOH is required because HCl and NaOH reacts according to the 1:1 ratio.
  • To provide 0.0025 mol of NaOH, 25 cm3 of aqueous NaOH solution is need. At that point, solution does not have more HCl or added NaOH. Only NaCl is present with water.
  • NaCl is stable in the water. So it is a stable salt and at that point pH of the solution is 7.
  • After addition of 25 cm3 of NaOH, more NaOH is added drop by drop. So now there is hydroxyl ions are present in the solution. Therefore solution starts to get basic and pH value will be over 7 now.
  • When add more aqueous NaOH solution, it will be more basic solution and pH value will increase furthermore.


Choose correct indicator

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.

  • Phenolphthalein: colour change of Phenolphthalein is occurred between 8.2 - 10 . So Phenolphthalein can be used as an indicator to identify equivalence point.
  • Bromothymol blue: Bromothymol blue is also a suitable indicator because its colour is changed between 6.0 - 7.6 .


0.01mol dm-3 HCl and 0.01mol dm-3 NaOH

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.

0.01 moldm-3 HCl and 0.01 moldm-3 NaOH titration curve

0.001mol dm-3 HCl and 0.001mol dm-3 NaOH

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.

0.001 moldm-3 HCl and 0.001 moldm-3 NaOH titration curve

Overall Explanation of the titration curves of HCl and NaOH reaction

In all beakers, aqueous HCl acid solution is present and aqueous NaOH solution is added to HCl solution.

  • At the beginning (no addition of NaOH) pH is depends on the concentration of NaOH and when concentration of HCl decreases pH value increases.


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.



Titration Curves when adding HCl to the aqueous NaOH solution

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.


pH variation of NaOH with concentration


NaOH Concentration (mol dm-3) pH
0.1 13
0.01 12
0.001 11


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.


0.1 mol dm-3 NaOH 25 cm3 with 0.1 mol dm-3 HCl

titration curve of 0.1moldm-3 NaOH and 0.1 moldm-3 HCl

Suitable concentration for a NaOH and HCl?

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.



is it important that the HCl and NaOH be similar in concentration?

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.




If bromothymol blue is titrated in NaOH and HCl, what is the ph at equivalence point?

Bromothymol blue is not titrated in NaOH and HCl titration. When pH value is changed, colour of bromothymol blue is changed.