Analytical

Acid-base titrations ( strong acid vs. strong base)

In this article we will discuss the problematics of acid-base titrations. As it is far more diffcult and complex to talk about weak acid/base titrations we´ll take  the other case – strong acid vs. strong base. It´s easier to work with and do the calculations because both analyte and titrant are strong acid or strong base and are completely ionized. Before we start with our lab work we should get familiar with the words like titration curve,  equivalence point, titrant and analyte. The titrant is the solution with known concentration, usually standardized. The analyte is the solution with unknown concentration we want to determine. The equivalent point is the point at which a stoichiometric amount of titrant is added to analyte.  It can be detected from the titration curve by several methods  (this one looks simply  click here  . The titration curve is constructed by plotting the pH of the solution as function of the volume of titrant added. 

To detect the endpoint of the reaction it´s convenient to use an indicator which is always weak acid or base. The difference in color between ionized and un-ionized form allows us to assume whether the reaction is complete or not. The color of indicator changes with changing pH value of the solution. Usually a transition from one to another color  takes about two pH units. For our titration of hydrochloric acid solution we will use phenophtalein as indicator with transition range between 8,2-10 (colorless to fuchsia)   and 0,1 M solution of KOH. For 250 cc of 0,1 M solution of KOH we will need 1,122 grams of KOH..

koh

 When the solution is prepared, put 10cc of your analyte into a 100cc titration flask and add few drops of phenolphtalein indicator solution. Start the titration and go on until  the solution stays pinkish. Then mark the volume of the titrant used and repeat the whole procedure twice again. Create a table with data measured and do the calculations to determine the concentration of your analyte…

titration2

If we know the concentrations of both the analyte and the titrant and the amount of the analyte used, we can easily make up our own titration curve (to see full document click here titration curve).

curve