When using phenolphthalein indicator what should be seen

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.

In that case, they will cancel out of the K ind expression. You can use this to work out what the pH is at this half-way point. If you re-arrange the last equation so that the hydrogen ion concentration is on the left-hand side, and then convert to pH and pK ind , you get:.

That means that the end point for the indicator depends entirely on what its pK ind value is. For the indicators we've looked at above, these are:. Indicators don't change colour sharply at one particular pH given by their pK ind.

Instead, they change over a narrow range of pH. Assume the equilibrium is firmly to one side, but now you add something to start to shift it. As the equilibrium shifts, you will start to get more and more of the second colour formed, and at some point the eye will start to detect it.

For example, suppose you had methyl orange in an alkaline solution so that the dominant colour was yellow. Now start to add acid so that the equilibrium begins to shift. What is the unknown concentration of a Titrations are reactions between specifically selected reactants—in this case, a strong base and a weak acid. A titration curve reflects the strength of the corresponding acid and base, showing the pH change during titration.

The titration curve demonstrating the pH change during the titration of the strong base with a weak acid shows that at the beginning, the pH changes very slowly and gradually. This indicates the formation of a buffer system as the titration approaches the equivalence point. At the equivalence point and beyond, the curve is typical of a titration of, for example, NaOH and HCl. Titration of a weak Acid with a strong base : This figure depicts the pH changes during a titration of a weak acid with a strong base.

The initial pH of the solution at the beginning of the titration is approximately that of the weak acid in water. However, the pH at the equivalence point does not equal 7.

This is due to the production of conjugate base during the titration. The resulting solution is slightly basic. The endpoint and the equivalence point are not exactly the same: the equivalence point is determined by the stoichiometry of the reaction, while the endpoint is just the color change from the indicator.

This conjugate base reacts with water to form a slightly basic solution. Recall that strong acid-weak base titrations can be performed with either serving as the titrant. An example of a strong acid — weak base titration is the reaction between ammonia a weak base and hydrochloric acid a strong acid in the aqueous phase:.

The acid is typically titrated into the base. A small amount of the acid solution of known concentration is placed in the burette this solution is called the titrant. A known volume of base with unknown concentration is placed into an Erlenmeyer flask the analyte , and, if pH measurements can be obtained via electrode, a graph of pH vs.

In the case of titrating the acid into the base for a strong acid-weak base titration, the pH of the base will ordinarily start high and drop rapidly with the additions of acid. As the equivalence point is approached, the pH will change more gradually, until finally one drop will cause a rapid pH transition through the equivalence point.

If a chemical indicator is used—methyl orange would be a good choice in this case—it changes from its basic to its acidic color. Titration of a weak base with a strong acid : A depiction of the pH change during a titration of HCl solution into an ammonia solution.

The curve depicts the change in pH on the y-axis vs. In strong acid-weak base titrations, the pH at the equivalence point is not 7 but below it. Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule. Monoprotic acids are acids able to donate one proton per molecule during the process of dissociation sometimes called ionization as shown below symbolized by HA :.

Common examples of monoprotic acids in mineral acids include hydrochloric acid HCl and nitric acid HNO 3. On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group, and sometimes these acids are known as monocarboxylic acid.

Polyprotic acid are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Certain types of polyprotic acids have more specific names, such as diprotic acid two potential protons to donate and triprotic acid three potential protons to donate. For example, oxalic acid, also called ethanedioic acid, is diprotic, having two protons to donate.

The color change in phenolphthalein is a result of ionization, and this alters the shape of the phenolphthalein molecules. Phenolphthalein pronounced fee-nawl-thal-een , often abbreviated to phph, is a weak acid. This mildly acidic compound is a white to yellow crystalline solid. It easily dissolves in alcohols and is slightly soluble in water. Phenolphthalein is a large organic molecule with the chemical formula of C 20 H 14 O 4. Ionization occurs when a molecule gains or loses electrons, and this gives the molecule a negative or positive electric charge.

Ionized molecules attract other molecules with the opposite charge and repel those with the same charge. Phenolphthalein is a weak acid and is colorless in solution although its ion is pink.

Adding hydroxide ions OH - , as found in bases will change the phenolphthalein into its ion and turn the solution pink. The phenolphthalein indicator has two different structures based on whether it is in an alkali pink or acid colorless solution. Pure water is neutral, and so is paraffin. Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory.

Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions, and purple in neutral solutions.