Chem 214
Acid-Base Titration, Determination of Carbonate and Bicarbonate in a water sample
Introduction The purpose of this lab is to determine the concentration of two bases, carbonate and bicarbonate, by using a potentiometric titration. We can determine the concentration of the bases in the reactions ( H+ +CO3- < ==> HCO3- and H+ + HCO3- < ==> H2CO3-) by the way the pH of the solution changes. The way the pH changes when a strong acid is added can be used to determine the concentration of the strong and weak bases that are present in the solution. We can determine this by knowing that when a acid is added to a solution the majority of the hydrogen ion will react with the strongest base. In this experiment the majority of the hydrogen ion will react with the carbonate ion and convert it to the bicarbonate ion.
Data Analysis:
Carbonate and Bicarbonate Titration Curve
Total Alkalinity The total Alkalinity in this experiment is expressed by total moles of base present per liter of solution. In this labe experiment the total amount of base present is equal to the moles of acid needed to reduce the pH of the solution to 4.3.Using my data I found that the total alkalinity to be 2.6 moles of base per liter of solution.
Determination of Carbonate and Bicarbonate Concentrations By using the titration curve I decided that there is both carbonate and bicarbonate in the initial solution because of the amount of acid needed to get to the second equivalent point is double what it took to get to the first equivalent point. The concentration of the carbonate in the initial solution is 0.85 moles per liter, and the concentration of the bicarbonate in the initial solution is 0.9 moles per liter.
Discussion
When a carbonate is the only base stronger than water in a solution the pH is greater than 8.3 and when doing a potentiometric titration the amount of acid needed to reduce the pH from its initial value to 8.3 is equal to the amount of acid need to reduce the solution from 8.3 to 4.3. When there is only bicarbonate ions predominately present the pH is approximately 8. Here, there is a clear indication as to whether there is solely a carbonate or bicarbonate ion predominately present. When there is both a carbonate and a bicarbonate ion in the initial solution the pH will be above 8.3. Because both initial pH’s of carbonate ions and the mixture of carbonate and bicarbonate ions are above 8.3 then there must be a way to differentiate. The main difference in the two is that when there are both (carbonate and bicarbonate) present the amount of acid needed to reduce the pH from 8.3 to 4.3 is greater than the amount of acid needed to reduce the pH to 8.3 from the initial pH. Upon completion of this exercise I found that the amount of acid needed to get the pH down to 8.3 is double compared to the amount of acid needed to get the solution from 8.3 to 4.3. With this information I was able to determine that my initial solution contained both carbonate and bicarbonate ions.
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