How do acid base indicators work

Other common indicators include: bromothymol blue thymol blue methyl orange bromocresol green methyl red phenol red. How do acid base indicators work? Chemistry Acids and Bases Acids and Bases. Apr 12, Other common indicators include: bromothymol blue thymol blue methyl orange bromocresol green methyl red phenol red Hope this helps! Related questions How do resonance structures affect acidity? How does carbon dioxide change the acid-base condition of a solution?

Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators , which do respond to a change in the hydrogen ion concentration.

Most of the indicators are themselves weak acids. The most common indicator is found on "litmus" paper. It is red below pH 4. Other commercial pH papers are able to give colors for every main pH unit. Universal Indicator, which is a solution of a mixture of indicators is able to also provide a full range of colors for the pH scale.

A variety of indicators change color at various pH levels. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample. An indicator is usually some weak organic acid or base dye that changes colors at definite pH values.

The weak acid form HIn will have one color and the weak acid negative ion In - will have a different color. The weak acid equilibrium is:. Phenolphthalein is an indicator of acids colorless and bases pink. Red cabbage can also be used as an acid-base indicator. If the indicator is a weak acid, the acid and its conjugate base are different colors. If the indicator is a weak base, the base, and its conjugate acid display different colors.

For a weak acid indicator with the genera formula HIn, equilibrium is reached in the solution according to the chemical equation:.

HIn aq is the acid, which is a different color from the base In - aq. An example of a weak acid indicator is phenolphthalein, which is colorless as a weak acid but dissociates in water to form a magenta or red-purple anion. In an acidic solution, equilibrium is to the left, so the solution is colorless too little magenta anion to be visible , but as pH increases, the equilibrium shifts to the right and the magenta color is visible.

The equilibrium constant for the reaction may be determined using the equation:. The color change occurs at the point where the concentration of the acid and anion base are equal:. A particular type of acid-base indicator is a universal indicator , which is a mixture of multiple indicators that gradually changes color over a wide pH range.

The indicators are chosen so mixing a few drops with a solution will produce a color that can be associated with an approximate pH value. Several plants and household chemicals can be used as pH indicators , but in a lab setting, these are the most common chemicals used as indicators:. The "acid" and "base" colors are relative. Also, note some popular indicators display more than one color change as the weak acid or weak base dissociates more than once.

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.