Topic 5A: titration

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An indicator: Indicators are weak acids or bases that ionize within a known pH range.

They exist in an equilibrium and have different colors for their dissociated and undissociated states.

This color change is known as the end point, however is not always the same as the equivalence point. The end point is the range over which the indicator will change color.

For optimal accuracy, there should be a clear color difference between the two states of the indicator, and there should be a narrow pH range in which the color changes.

Universal indicators:

can be made from a blend of indicators

Display the entire rainbow of colors from low pH to high pH. Used to make pH paper, which is used to test solutions for their approximate pH.

Other indicators may have just two or less colors, such as litmus

Phenolphthalein is an indicator that changes color when the end point has been reached; This is useful because this indicates that the equivalence point has been reached as well

It is important to pick an indicator with a pK close to the equivalence point. For instance, if you were to titrate a strong acid with a weak base, then you might choose methyl orange as an indicator because its pK is 3.7 and it changes color at a pH range of 3.1 to 4.4

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neutralization: the process by which an acid and base are mixed together to form salt and water.

In this case, salt describes any ionic compound, soluble or insoluble, formed from the reaction.

The classic example is the reaction of hydrochloric acid and sodium hydroxide to form sodium chloride and water. For an acid spill in the lab, you can use sodium bicarbonate or sodium carbonate to neutralize the spill -->

HCl + NaHCO3 → NaCl + H2O + CO2

2HCl + Na2CO3 → 2NaCl + H2O + CO2

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acid-base titration: the addition of some amount of base with a known concentration to a known volume of acid with unknown concentration, until the endpoint is reached.

The concentration of the acid solution can be calculated using the change from the initial and final volumes from the burette.

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titration curves: a plot of the pH measured at various points during an acid or base titration.

Neutralization/equivalence point will be where the moles of titrant (added acid or base of known []) are equal to those of the analyte (the acid or base of unknown [].

Titration of a strong base with a strong acid: The pH stays relatively high until the equivalence point is reached, at which point the slope is the steepest and the pH will be 7.

Titration of a weak base with a strong acid: the pH will start at a lower basic value and the endpoint will be at a pH on the acidic side. You should expect the pH to drop rapidly in this titration.

Due to to the production of the conjugate acid during the titration, neutralization does not occur at pH 7.

example: NH3 + HCl --> Cl- + NH4+ (and then the production of conjugate acid) --> NH4+ + H2O --> H3O+ + NH3

This also causes the endpoint and equivalence points to be different.

Titration of a strong base with a weak acid: the pH starts out very basic and ends with the pH at the equivalence point in the basic range but closer to neutral.

Example: NaOH + CH3COOH --> Na+ CH3COO- + H2O

When a diprotic or polyprotic acid is involved in the titration, there will be more than one equivalence point on the curve.

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Addition of strong base to strong acid: the initial pH is the pH of the acid. The pH will be 7 at the end point because the salt is neutral, so calculate the pH at the midpoint by calculating the molarity of the acid solution based on how many moles of acid are left at the midpoint.

Addition of strong base to weak acid: the initial pH is of the weak acid solution. Calculate the molarity of each component at the midpoint. Plug these values into the Henderson-Hasselbach equation.

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Redox titrations: used to determine unknown amounts of a substance in a solution by finding the equivalence point when the titrant and analyte have reacted stoichiometrically by transferring electrons.

Can accurately determine the concentration of an unknown analyte by measuring it against a standardized titrant.

can be conducted in the same way as acid base titrations (slowly adding the titrant via burette)

redox titrations often do not require indicators because there is often an automatic color change on reaction.

In order to calculate the concentration of the unknown, first balance the half reactions of the redox equation. Then using the concentrations and volumes from the data of the titration, you can calculate the concentration of the unknown.

Common redox reactions:

combination reaction: 2 substances combine to form a 3rd substance -->

CaO (s) + SO2 (g) → CaSO3 (s)

decomposition reaction: a single substance decomposes into 2 or more substances -->

2KClO3 → 2KCl + 3O2

single displacement reaction: an element reacts with an ionic compound and replaces the cation in the compound. The original cation becomes an element -->

Cu + 2AgNO3 → 2Ag + Cu(NO3)2

Combustion reaction: a substance reacts with oxygen to produce 1 or more oxides -->

4Fe + 3O2 → 2Fe2O3