What is the pKa for phenolphthalein?
What is the pKa for phenolphthalein?
9.3
Phenolphthalein has a pKa of 9.3. When conducting a titration of an acid analyte with a base titrant using phenolphthalein as the indicator, the pH will be low at the beginning, so the phenolphthalein will be mostly in the un-ionized or “protonated” “acid” form (we’ll refer to as HIn).
What is pKa value of indicator?
Consequently, the pKa of an indicator corresponds to the pH of the solution at the inflection point in a plot of absorbance as a function Page 2 of pH as shown in Figure 1.
What is the pH range for phenolphthalein?
As an indicator of a solution’s pH, phenolphthalein is colourless below pH 8.5 and attains a pink to deep red hue above pH 9.0.
How is pKa related to pH when looking at the best pH indicator for titration?
It is usually assumed that at pH = pKa – 1 the acid form dominates whereas at pH = pKa + 1 the basic form dominates. Therefore to have the colour change when the equivalence is reached the pH then must be as close as possible of the pKa of the indicator.
How do you calculate pH from pKa?
When the moles of base added equals half the total moles of acid, the weak acid and its conjugate base are in equal amounts. The ratio of CB / WA = 1 and according to the HH equation, pH = pKa + log(1) or pH = pKa.
What is pH range of indicator?
pH indicators are specific to the range of pH values one wishes to observe. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly. Usually, universal pH indicators are in the paper strip form.
How does phenolphthalein act as a pH indicator?
How does Phenolphthalein Indicator Work? The pH scale has a marking from 0 to 14 fro detecting acid and base. Also, it turns in pink colour in an alkaline solution or base. Moreover, the compound remains colourless in acids but starts to turn pink on pH 8.2 and continues to turn bright purple in strong bases.
Is phenolphthalein indicator affected by pH?
A strong acid- strong base titration is performed using a phenolphthalein indicator. Phenolphtalein is chosen because it changes color in a pH range between 8.3 – 10. It will appear pink in basic solutions and clear in acidic solutions.
What is pH range of an indicator?
pH indicators are specific to the range of pH values one wishes to observe. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly.
Are pKa and pH directly related?
pKa (acid dissociation constant) and pH are related, but pKa is more specific in that it helps you predict what a molecule will do at a specific pH. Essentially, pKa tells you what the pH needs to be in order for a chemical species to donate or accept a proton.
What is the pKa of phenolphthalein?
The pKa of phenolphthalein is 9.3 and may be usefully employed over the pH range 8.2 – 10.0. In the titration of a strong acid with a strong base there is a change from pH3 to pH 11 with the addition of 1 drop of reagent and consequently the colour of the phenolphalein will change as well.
Can phenolphthalein indicator be used for titration?
Hence, indicators have different regions of operation. As the change in pH is usually large at the equivalence point this means that provided the pH change takes place through the pKa of the indicator then it can be used for a titration. The pKa of phenolphthalein is 9.3 and may be usefully employed over the pH range 8.2 – 10.0.
Is phenolphthalein an acid base indicator?
An acid-base indicator which is colorless in acid solution, but turns pink to red as the solution becomes alkaline. It is used medicinally as a cathartic. Phenolphthalein is an organic compound used as a laboratory reagent and pH indicator.
How do you select an indicator based on PKA?
How do you select an indicator based on pKa? It makes sense to select an indicator with a pKa right in the middle of the pH change at the equivalence point. In practice, there are few indicators in common use. For strong acids and weak bases methyl orange is the indicator of choice.
