PH Indicator and Endpoint Observations

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In summary, when a weak base is titrated with a strong acid, the endpoint volume will be lower than the actual equivalence point volume. This can lead to the mistaken assumption that the endpoint volume is the equivalence point volume, resulting in a lower calculated number of moles of strong acid and a lower molarity of the weak base. While it is possible to conduct a titration in the opposite direction and stop at the first color change, this method is not accurate due to the gradual nature of color change. It is best to follow accepted titration procedures using appropriate indicators.
  • #1
i_love_science
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2
Homework Statement
see below
Relevant Equations
titration curve
A weak base is titrated with a strong acid.
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Untitled 2.png


I think that both B and C are correct. The endpoint volume will be lower than the actual equivalence point volume. When the student mistakenly assumes the endpoint volume is the equivalence point volume, the number of moles of strong acid is seen as equal to the number of moles of weak base. The calculated number of moles of strong acid would be lower (because of the lower volume), and the molarity of the weak base would also be lower.

The answer says only B is correct. Could anyone explain why, and where I went wrong? Thanks.
 
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  • #2
I see nothing wrong in your explanation, I would choose B and C as well.

Funny thing is, this titration is still doable, you just have to do it in the opposite direction and stop at the first color change. That's how titrations against phenolphthaleine are done :smile:
 
  • #3
Borek said:
Funny thing is, this titration is still doable, you just have to do it in the opposite direction and stop at the first color change. That's how titrations against phenolphthaleine are done :smile:
Thanks, though I don't really understand the above part. If you do it in the opposite direction, the color change is still not at the equivalence point...
 
  • #4
i_love_science said:
Thanks, though I don't really understand the above part. If you do it in the opposite direction, the color change is still not at the equivalence point...
Which is why you stop at the first sight of color change. Color change is not instantaneous, it takes place gradually over at least 2 pH units. In the case of phenolphthalein it means the color change (when titrating strong acid with a strong base) starts quite closely to the equivalence.

Compare http://www.titrations.info/acid-base-titration-indicators
 

FAQ: PH Indicator and Endpoint Observations

What is a pH indicator?

A pH indicator is a substance that changes color in response to changes in the acidity or basicity of a solution. It is used to determine the approximate pH of a solution by comparing the color change to a color chart.

How do pH indicators work?

pH indicators work by undergoing a chemical reaction with hydrogen ions (H+) or hydroxide ions (OH-) in a solution. This reaction causes a change in the molecular structure of the indicator, resulting in a change in color.

What are the most commonly used pH indicators?

The most commonly used pH indicators are litmus, phenolphthalein, methyl orange, and bromothymol blue. These indicators cover a wide range of pH values and are easily obtainable.

What is an endpoint in pH indicator observations?

An endpoint is the point at which the color change of a pH indicator is completed and the final color is observed. It indicates the pH of the solution and can be used to determine the acidity or basicity of the solution.

What factors can affect pH indicator observations?

The accuracy of pH indicator observations can be affected by various factors such as temperature, concentration of the indicator, and the presence of other substances in the solution. It is important to control these factors to obtain reliable results.

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