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.
NaOH and HCl react 1:1 ratio according to the stoichiometric equation. Therefore, same amount of HCl and NaOH are consumed in the reaction. If same concentration solutions of NaOH and HCl are used, same volumes of NaOH are HCl are consumed too.
Ig your HCl or NaOH is mixed with a un-wanted (dirt) which can react with HCl or NaOH,your experimental ration can be different from 1:1. Otherswise, it should be 1:1.
Think you have a HCl bottle with unknown concentration. You can measure the volume of HCl solution.
Also, there is a NaOH solution with known concentration. Adding this NaOH solution to HCl bottle will neutralize HCl solution. You have to choose correct indicator for this experiement.
When colour of the solution is changed suddenly, it means All HCl are now finished. Now, measure the spent NaOH volume.
From NaOH concentration and volume, you can measure the mole of NaOH. Then, you can determine amount of mol of HCl using stoichiometry of the balanced reaction.
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.
Consider 250C temperature. At 250C temperature, solubility of NaOH is 1000 g for one liter of water. In 1000 g of NaOH, there are 25 moles of NaOH. Therefore, 10M naOH solutions can be exist.
First thing is, there is no K2O solution. K2O immediately converts to KOH when water is added. Because, KOH and HCl are strong base and strong acid respectively, when one component is completely neutralized by another component, pH should be seven at 250C.
Bromothymol blue is not titrated in NaOH and HCl titration. When pH value is changed, colour of bromothymol blue is changed.