Because acetic acid is a weak acid, Chris has decided to titrate it with an aqueous solution of sodium hydroxide because this is a strong base.
Chris has the following acid-base indicators currently available in the lab:. For an acid-base titration, the pH of the final solution depends on the relative strength of the acid and the strength of the base :. Ideally, indicator's colour change at the end point should occur at the same pH as the equivalence point of the neutralisation reaction. In practice, pH equivalence occurs within the pH range of the indicator:.
Since acetic acid is a weak acid and sodium hydroxide is a strong base, that is, base is stronger than acid:. Phenolphthalein would be the best choice because its whole pH range is greater than 7, that is, its pH range is 8. Work backwards: Assume we use phenolpthalein, what sort of acid and base would be used in the titration?
Since the relative strength of the acid and base determined by working backwards agrees with the information given in the question we are reasonably confident that our answer is plausible. At the equivalence point, 1 equivalent of acid neutralises 1 equivalent of base.
You probably won't be using "equivalents" as a measure of quantity in your high school chemistry course, but it is useful to understand where the term "equivalence point" comes from. Do you know this? Do you understand this? Can you apply this? Take the exam now! State the Question.
What is the question asking you to do? Yes, we have named one of the indicators given. Is your answer plausible? State the Solution Chris should use the phenolphthalein indicator. Bromothymol Blue mL strong acid added to strong base. Red litmus paper turns blue in the presence of a base. Blue litmus paper turns red in the presence of an acid. Litmus paper does not indicate the pH of a solution, only whether it's an acid or base. To determine pH, use pH paper, universal indicator solution, or a pH meter.
This type of precision is usually reserved for advanced science, college, or research coursework. Most general chemistry labs use pH paper or universal indicator solution instead of meters.
These papers display a unique color for each pH unit and come with their own color chart. Universal indicator solutions work similarly and can be the most cost-effective option because only 1 drop is usually required. Bogen universal indicator solution , which contains the indicators methyl red, phenolphthalein, and bromthymol blue, measures pH over the range 4 to Another popular universal indicator solution is also natural, red cabbage juice.
For a fun activity, measure the pH of common household liquids with a pH indicator of your choice. The results may surprise you see Table 2!
Table 2. To see indicators in action, try this fun activity:. We use cookies to provide you with a great user experience. By using our site, you accept our use of cookies. You can review our cookie and privacy policy here. Login or Register My Account Login or register now to maximize your savings and access profile information, order history, tracking, shopping lists, and more.
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As you will see below, that isn't true for other indicators. Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange is yellow and the structure is:. Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively charged oxygen. That's the obvious place for it to go.
Not so! In fact, the hydrogen ion attaches to one of the nitrogens in the nitrogen-nitrogen double bond to give a structure which might be drawn like this:. Note: You may find other structures for this with different arrangements of the bonds although always with the hydrogen attached to that same nitrogen. The truth is that there is delocalisation over the entire structure, and no simple picture will show it properly.
Don't worry about this exact structure - it is just to show a real case where the colour of a compound is drastically changed by the presence or absence of a hydrogen ion.
You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You should be able to work out for yourself why the colour changes when you add an acid or an alkali.
The explanation is identical to the litmus case - all that differs are the colours. Note: If you have problems with this, it is because you don't really understand Le Chatelier's Principle. Sort it out! In the methyl orange case, the half-way stage where the mixture of red and yellow produces an orange colour happens at pH 3. This will be explored further down this page. In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless.
Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 9. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy!
Note: If you are interested in understanding the reason for the colour changes in methyl orange and phenolphthalein, they are discussed on a page in the analysis section of the site about UV-visible spectroscopy. This is quite difficult stuff, and if you are coming at this from scratch you will have to explore at least one other page before you can make sense of what is on that page.
There is a link to help you to do that. Don't start this lightly! Think about a general indicator, HInd - where "Ind" is all the rest of the indicator apart from the hydrogen ion which is given away:. Because this is just like any other weak acid, you can write an expression for K a for it. We will call it K ind to stress that we are talking about the indicator. Note: If this doesn't mean anything to you, then you won't be able to understand any of what follows without first reading the page on weak acids.
Use the BACK button on your browser to return to this page. Think of what happens half-way through the colour change. At this point the concentrations of the acid and its ion are equal.
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