At some point there will be enough of the red form of the methyl orange present that the solution will begin to take on an orange tint. As you go on adding more acid, the red will eventually become so dominant that you can no longe see any yellow. There is a gradual smooth change from one colour to the other, taking place over a range of pH.
As a rough "rule of thumb", the visible change takes place about 1 pH unit either side of the pK ind value. The litmus colour change happens over an unusually wide range, but it is useful for detecting acids and alkalis in the lab because it changes colour around pH 7. Methyl orange or phenolphthalein would be less useful. For example, methyl orange would be yellow in any solution with a pH greater than 4. It couldn't distinguish between a weak acid with a pH of 5 or a strong alkali with a pH of Remember that the equivalence point of a titration is where you have mixed the two substances in exactly equation proportions.
You obviously need to choose an indicator which changes colour as close as possible to that equivalence point. That varies from titration to titration. The next diagram shows the pH curve for adding a strong acid to a strong base. Superimposed on it are the pH ranges for methyl orange and phenolphthalein. However, the graph is so steep at that point that there will be virtually no difference in the volume of acid added whichever indicator you choose.
However, it would make sense to titrate to the best possible colour with each indicator. If you use phenolphthalein, you would titrate until it just becomes colourless at pH 8. On the other hand, using methyl orange, you would titrate until there is the very first trace of orange in the solution. If the solution becomes red, you are getting further from the equivalence point.
This time it is obvious that phenolphthalein would be completely useless. However, methyl orange starts to change from yellow towards orange very close to the equivalence point. This time, the methyl orange is hopeless!
However, the phenolphthalein changes colour exactly where you want it to. The curve is for a case where the acid and base are both equally weak - for example, ethanoic acid and ammonia solution. In other cases, the equivalence point will be at some other pH. You can see that neither indicator is any use. Phenolphthalein will have finished changing well before the equivalence point, and methyl orange falls off the graph altogether. It may be possible to find an indicator which starts to change or finishes changing at the equivalence point, but because the pH of the equivalence point will be different from case to case, you can't generalise.
On the whole, you would never titrate a weak acid and a weak base in the presence of an indicator. This is an interesting special case. If you use phenolphthalein or methyl orange, both will give a valid titration result - but the value with phenolphthalein will be exactly half the methyl orange one. It so happens that the phenolphthalein has finished its colour change at exactly the pH of the equivalence point of the first half of the reaction in which sodium hydrogencarbonate is produced.
The methyl orange changes colour at exactly the pH of the equivalence point of the second stage of the reaction. If this is the first set of questions you have done, please read the introductory page before you start. How simple indicators work Indicators as weak acids Litmus Litmus is a weak acid. After the reaction between the substance and the standard solution is complete, the indicator should give a clear colour change.
When a titration is carried out, the free energy change for the reaction is always negative. In a titration, you determine an unknown concentration of a sample by adding a second reactant of known concentration. In many titrations, you use a chemical called an indicator, which lets you know when the titration finishes. Indicators are substances whose solutions change color due to changes in pH.
They are usually weak acids or bases, but their conjugate base or acid forms have different colors due to differences in their absorption spectra.
The three most common indicators are: Litmus, Methyl orange and Phenolphthalein. The reason for the visible light absorption is the structure of the pink form of the phenolphthalein indicator. Due to ionization, the electrons in the molecule are more delocalized than in the colorless form. Briefly, delocalization is when electrons in a molecule are not associated with a single atom, and instead are spread over more than one atom. An increase in delocalization shifts the energy gap between molecular orbitals.
Less energy is needed for an electron to make the jump into a higher orbital. The absorption of energy is in in the green region, nanometers, of the light spectrum. The human eye perceives a pink hue in the solution. The stronger the alkaline solution is, the more the phenolphthalein indicator changes and the darker the pink hue will be.
The pH scale runs from 0 to 14, with a pH of 7 being neutral. A substance below pH 7 is considered acidic; above pH 7 is considered basic. Phenolphthalein is naturally colorless but turns pink in alkaline solutions. The compound remains colorless throughout the range of acidic pH levels but begins to turn pink at a pH level of 8.
In , the German chemist Adolf von Baeyer discovered phenolphthalein by fusing phenol and phthalic anhydride in the presence of sulfuric acid or zinc chloride, the manufacture process still used today. In the chemistry laboratory, phenolphthalein is mostly used in acid-base titrations.
0コメント