Why is phenolphthalein a good indicator

Titration : Titration of an acid-base system using phenolphthalein as an indicator. 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. Answer and Explanation: Methyl orange is an acid - base indicator. The solution used for titrations typically is a 0.

When methy orange is added to a solution of NaOH then it will impart a yellow colour. Methy orange generally changes from yellow to red in acidic medium. A natural indicator is obtained from natural sources while synthetic indicators are man made and do not occur in nature. Turmeric,red cabbage, flowers of hydrangea etc are natural indicators. Methyl orange and phenolphthalein are synthetic indicators and are prepared in laboratory. The pH range of phenolphthalein is about 8.

For a strong base-weak acid titration, the equivalence point is probably near pH 9. Phenolphthalein is great for this titration. A strong acid is an acid which is completely ionized in an aqueous solution. Hydrogen chloride HCl ionizes completely into hydrogen ions and chloride ions in water.

Litmus is not used in titrations because the pH range over which it changes colour is too great. Universal indicator which is actually a mixture of several indicators displays a variety of colours over a wide pH range so it can be used to determine an approximate pH of a solution but is not used for titrations. A double indicator titration is the process used in chemistry experiments to determine and analyze the amount and concentration of acids or bases in a solution at two end points.

Methyl orange is an indicator with a pH range of 3. In classical term a base is defined as a compound which reacts with an acid to form salt and water as depicted by the following equation.

As an indicator of a solution's pH , phenolphthalein is colourless below pH 8. First of all, solid NaOH absorbs water from the air, so accurately weighing a sample during the preparation of a solution is impossible. Once the NaOH solution has been standardized and its concentration is well-known, it can be used to titrate other acid solutions such as HCl. End Point. It is colourless in a neural and acidic solution. Carolina is your quality source for a well-equipped lab.

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Carolina understands. Acid-base indicators are weak organic acids. Unlike most acids, however, the acid and base forms of indicators are different colors. Since the color of the indicator depends on the pH of the solution, indicators find wide use in applications that involve pH changes, such as titrations, pH testing, and science demonstrations.

The most important property of an indicator is its pH range , which is dependent on the indicator's acid strength. An indicator's pH range is the range of pH values over which the indicator changes colors from its acid form to its base form. It extends from the highest pH at which only the acid form is visible to the lowest pH at which only the base form is visible.

The indicator is not sensitive to pH changes outside its range because the indicator does not change color over these pH values.

For example, methyl red does not change color over the pH values 1 to 4 or 6 and above. If you aren't happy about either of these things, you must follow these links before you go any further. Litmus is a weak acid. It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can be given away to something else. The "Lit" is the rest of the weak acid molecule.

There will be an equilibrium established when this acid dissolves in water. Taking the simplified version of this equilibrium:. Note: If you don't understand what I mean by "the simplified version of this equilibrium", you need to follow up the weak acids link before you go any further.

Now use Le Chatelier's Principle to work out what would happen if you added hydroxide ions or some more hydrogen ions to this equilibrium. Note: If you don't understand Le Chatelier's Principle , follow this link before you go any further, and make sure that you understand about the effect of changes of concentration on the position of equilibrium.

At some point during the movement of the position of equilibrium, the concentrations of the two colours will become equal. The colour you see will be a mixture of the two. The reason for the inverted commas around "neutral" is that there is no reason why the two concentrations should become equal at pH 7.

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.