TITRATION OF ACIDS & BASES Name EXTENSION Soil pH & Plant Growth Section Date Th
ID: 1055870 • Letter: T
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TITRATION OF ACIDS & BASES Name EXTENSION Soil pH & Plant Growth Section Date The color of flowers might seem fixed: any pre-school student will tell you violets are blue, for example. However, the color of flowers depends on a lot of environmental factors, including how acidic or basic the soil is. One flower that shows a very clear difference is the hydrangea. In the eastern US where soil is acidic, hydrangeas are blue while in the Midwestern states, where soil is basic or alkaline, hydrangeas are pink. The chemicals responsible for this change are known as anthocyanins. The structures shown here provide the chemical structure of an anthocyanin in an acidic environment (pH 3) compared to the structure present in a basic environment (pH I1). Notice that in acidic environments, there is a protonated oxygen (-OH) that is deprotonated in basic environments to a charged oxygen HO On Acidic Soil, pH3 Basic Soil, pH> Blee 1. If you want to test the acidity of the soil without waiting for hydrangea growth, you can add vinegar (acetic acid) and see if there is any reaction. Is the soil acidic or basic when you observe a reaction? Explain why. 2. If you treat your soil with lime (Ca(OH)), which color of hydrangea do you expect to grow? Why? 103Explanation / Answer
1. A reaction takes place only when we have the basic soil. Acetic acid dissociates in solution as
CH3COOH (aq) ------> CH3COO- (aq) + H+ (aq)
The protons from acetic acid can protonate the charged oxygen in the basic soil to form hydroxde group. The reaction can be monitored by a colour change of an acid-base indicator.
The acidic soild will not react with acetic acid since there is no site for protonation in the acidic soil.
2. If the soil is treated with lime, Ca(OH)2, the hydrangea will shown pink colour. This is because lime is basic in nature and the soil cannot remain protonated in a basic medium. Therefore, we will have the basic form of the soil with the charged oxygen atom. Thus, hydrangea will be pink in colour.
3. At the end of the titration of a strong acid with a strong base, the pH indicator derieved from the anthocyanins will be pink in colour. This is because the solution is neutral and the neutralization has been brought about by adding the strong base to the strong acid in presence of the pH indicator. Just past the equivalence point, if a drop of base is added, the solution will show the pink colour characteristic of an alkaline solution.
This can be tested by titrating a solution of HCl with strong NaOH. The HCl is taken in a Erlenmeyer flask and NaOH is added from a burette. The solution will show a pink end point.
4. Let humic acid be denoted as H2A (diprotic acid = 2 protons). Write down the balanced chemical equation for the reaction:
H2A (aq) + 2 NaOH (aq) -------> Na2A (aq) + 2 H2O (l)
The molar ratio of reaction is 1:2.
Find out the moles of NaOH added = (volume of NaOH added)*(concentration of NaOH in mol/L) = (70 mL)*(1 L/1000 mL)*(1.0*10-3 mol/L) (1 mM = 10-3 M) = 7.0*10-5 mol.
Humic acid neutralized = (7.0*10-5 mol NaOH)*(1 mole humic acid/2 mole NaOH) = 3.5*10-5 mol.
Concentration of humic acid = (moles of humic acid neutralized)/(volume of solution in L) = (3.5*10-5 mol)/(0.05 L) = 7.0*10-4 mol/L = 0.7 mM (ans).
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