Introduction

Learning Goals

Develop proper titration technique.
Perform a chemical analysis to determine the concentration of an unknown solution of hydrochloric acid by indicator titration.

Titrations

Titration is an analytical method used to determine the concentration of a solution. Titrations are commonly performed by starting with a fixed amount of reactant in a flask (known as the analyte) then adding a solution of a second reactant (known as the titrant) from a buret. While titrations are typically set up such that the titrant is a solution of known concentration and the analyte is a solution of unknown concentration, that is not always the case. In fact, in this experiment, your analyte will be a known amount of sodium carbonate (Na₂CO₃) dissolved in water, and your titrant is a solution of HCl of an unknown concentration. The goal of a titration is to monitor the amount of titrant needed to fully react with the analyte. The recorded volume and stoichiometric calculations can then be used to determine the concentration of the unknown solution.

To visualize when a reaction is complete, indicators are often used in titrations. Indicators are chemicals that have a minimal effect on the reaction to which they are added, and change color based on the progress of the reaction. In this experiment, you will be using thymol blue, which is blue under basic conditions, but turns green, then eventually yellow, and finally red as the solution becomes more acidic. Indicators are chosen based on different factors, such as the type of reaction or the expected pH associated with the point at which the analyte is fully reacted, known as the end point of a titration.

Before the end point, the solution is blue. At the end point, the solution is a faint grey. Past the end point, the solution is yellow.

Left to right: Flasks showing the color of thymol blue at the beginning of the titration, at the end point, and past the end point.

As mentioned above, stoichiometry plays an important role in analyzing the data from a titration. The carbonate ion, CO₃^2-, in sodium carbonate can accept up to two protons. Although sodium carbonate can react with up to two equivalents of HCl to form carbonic acid (H₂CO₃), (in a 1:2 ratio), you will be reacting sodium carbonate and HCl in a 1:1 ratio instead. In this process you will produce sodium bicarbonate (HNaCO₃) instead of carbonic acid, and you will need to use a 1:1 conversion ratio for your calculations.

N A 2 C O 3, aqueous, and H C L, aqueous, yield H N A C O 3, aqueous, and N A C L, aqueous.

Dilutions

In addition to creating a range of concentrations, dilutions are also used to reduce the concentration of potentially hazardous chemicals, or create reactant solutions of comparable/specific concentrations. In the process of diluting a solution, although more solvent is added, the number of moles of solute remain the same. Ultimately, this leads to the following relationship between the concentration and volumes of a solution before and after dilution.

M concentrated times V concentrated equals M dilute times V dilute.

Quality of Data

By the end of this experiment you will have three or more trials worth of data to analyze. Two basic ways to describe the quality of a data set are accuracy and precision. While these terms are often used interchangeably, they have two completely distinct meanings in scientific analysis. Accuracy represents how close a data point or the average of a data set is to a given reference or literature value (in other words, the supposed correct or true value), whereas precision represents how close data points within a data set are to each other (in other words, how spread out the data set is). You can comment on the accuracy of a data set only if you have a reference value for comparison, but you can always comment on your data's precision, assuming you have multiple data points. Good precision means that you have consistent technique and skills when you perform your experiments.

There are many ways of quantifying precision, but for this experiment you will be using percent difference. The percent difference is a value calculated for each trial and expresses the difference between the result of that data point and the mean of the data set as a percentage of the average. Overall, lower percent differences indicate greater precision compared to higher values.

$Percent difference equals open parenthesis, the fraction, the numerator is the absolute value of x minus average, the denominator is the average, close parenthesis, times 100 percent.$

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