Chemistry

Name: ______________________________

C. TITRATION CURVE 

A graph of pH (y axis) vs volume of reagent (titrant) progressively added to the sample.

 Useful for: 1. Determining the equivalence point = midpoint of sharp change in pH (steep vertical portion) 2. Showing the stepwise progress of a reaction involving a polyprotic acid (PP – donates more than one H+) or polybasic (PB – accepts more than one H+) species 3. Determining the volume of titrant required to achieve the equivalence point 4. Selecting a proper indicator = one that changes color over a pH range that includes the endpoint pH value (within the steep vertical portion of the curve) 5. Identifying type of reaction: SA + SB, SA + WB, SB + WA, PP + B, or PB + A Example: Titration of 25 mL of 0.10 mol/L HCl(aq) with 0.10 mol/L NaOH(aq) may yield the following titration curve:

Equivalence Point (pH) = ______ Vol of titrant needed to achieve Equiv. Pt = ______ (Predicted using acid-base stoichiometry – actual is what you read off the graph)

1

Chemistry

Name: ______________________________

Titration Accuracy The standard solution is carefully added to the solution in the flask until the first _________ color change just appears in the indicator. This is called the transition point or endpoint of the indicator. It should signal when the equivalence point in the titration has been reached. The transition point is the point in a titration at which the indicator changes color. At the transition point, the valve on the burette is closed to stop the titration and the volume of standard solution added from the burette is determined. It is important to remember that the pH at the transition point is dependent only on the chemical nature of the indicator and is independent of the equivalence point. The pH at the equivalence point is dependent only on the chemical nature of the reacting species. The pH at which the where the endpoint occurs should be as close as possible to the pH of the solution at the equivalence point. A titration should always be repeated as an accuracy check. _____________ mixing of solutions, incorrect pipetting techniques, or errors made when reading burettes can contribute to inaccuracies in the data collected, particularly for beginning students. Those experienced at performing titrations normally expect that the volumes added from the burette in each trial should agree with each other within 0.02 mL (less than a drop). Students performing titrations for the first time should expect agreement within 0.1 mL. If the volumes delivered from the burette in the first two trials do not agree within the desired uncertainty, then the titration must be repeated until they do. Once agreement between two trials occurs, the volumes added in those trials are averaged, and the other data is discarded. For example, consider the Table below, which lists the volumes of standard solution required to reach the equivalence point in three separate titration trials.

Note that a third trial was _______________ because the first two volumes differed by 0.17 mL, which is beyond the range of acceptable agreement. The volume recorded for trial 3 agrees well with that of trial 2 and so those two volumes are averaged to obtain the correct volume of standard solution, while the data from trial 1 is discarded. (It is common to overshoot a titration on the first trial.)

2

Chemistry

Name: ______________________________

Example: Lactic acid, C2H5OCOOH, is found in sour milk, yogurt, and cottage cheese. It is also responsible for the flavor of sourdough breads. Three separate trials, each using 25.0 mL samples of lactic acid, are performed using a standardized 0.153 M NaOH solution. Consider the following table listing the volume of basic solution required to reach the equivalence point for each trial. Calculate the concentration of the lactic acid.

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