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Using Volumetric Analysis to Determine Concentration

Instructor: David Skowron
In this lesson we will cover volumetric analysis, or more specifically, titrations. We will look at an in depth example of an acid-base titration to help explain what a titriaton is, and how titrations work.

Measuring Chlorinated Water

Have you ever taken a dink of tap water that tasted of chlorine? If you've drank from a municipal water source in the United States, the answer is probably yes. Chlorine is added to tap water to kill harmful bacteria and microbes.

However, too much chlorine can have negative health effects. In order to properly purify water with chlorine, enough needs to be added to kill harmful bacteria, but not too much, so as avoid negative health effects.

So how can someone test the amount of chlorine in water to ensure proper levels are present?

Titrations

To test chlorine levels in drinking water we need a volume metric analysis, a technique in which a substance of a known concentration is used to determine an unknown concentration. One common method in this case is titration, which involves the slow addition of a solution of a known concentration to an solution of unknown concentration.

In our example, the chlorinated water would be the analyte, or substance of unknown concentration, where our titrant is the solution of known concentration. When we add the titrant to the analyte, the titrant contains some chemical that will react with the analyte in a known and predictable way, such as a color change.

Once enough tritant is added to completely react with all of the active chemical in the analyte, an indicator will mark the end of the titraiton. From there, we can calculate the concentration of the analyte.

So how does this work? Since there are many different types of titrations, it's best to look at a few examples.

Acid-base Titration

Acid-Base Chemistry

Let's take a look at an example of acid-base titration. A strong acid is a chemical that dissociates completely to produce hydrogen (or hydronium) ions, while a strong base is a chemical that dissociates completely to produce hydroxide ions.

Let's work with a common strong acid, hydrochloric acid (HCl), and a common strong base, sodium hydroxide (NaOH). A solution containing HCl is acidic with a pH less than 7, while a solution of NaOH is basic with a pH greater than 7. When HCl and NaOH are combined they chemically react to form sodium chloride (NaCl) and water.

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Let's say we have a solution of HCl of an unknown concentration or molarity. Recall that Molarity, or molar concentration, is a unit of measurement that describes the amount of solute present in a solution. The amount of solute is measured in moles (abbreviated as mol or n), and is divided by the volume of solution, measured in liters (L), to give the Molarity (M).


mol


Setup

We can perform a titration to determine the concentration of the HCl solution. To accomplish this, we first construct our experimental apparatus. We fill the bottom container with our HCl solution of unknown concentration. We'll put the NaOH solution of known concentration into a burette, a glass tube that allows us to accurately measure the amount of titrant.

In a titration experiment, the titrant is put in the burette (the tube-like structure at the top). The analyte fills the container at the bottom. When neutrality is reached, the indicator turns the solution pink.
setup

Slowly add the NaOH solution to the HCl solution until we reach a pH of 7. At a pH of 7, we know that the moles of HCl equal the moles of NaOH.

An indicator is commonly used to determine when the reaction is complete, usually though some type of visual representation. In this case a pink color indicates we are close to a pH of 7 in an acid-base titration, signifying the end of the reaction, or endpoint.

Calculation

So how does the math workout? Well, we now know:

  • the concentration of the NaOH solution
  • how much of the NaOH was added to neutralize the HCl solution (reach a pH of 7)
  • that now the moles HCl = moles NaOH at a pH 7

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