To study the action of a buffer solution

EXPERIMENT ( 14 )

Topic : To study the action of a buffer solution

Introduction : In this experiment, you are to compare the effects of adding small amounts of acid and alkali to buffered

and unbuffered solutions of the same pH.

You are provided with a buffer solution designed to maintain a pH of 7.0 at 25OC and some pure water

which, if it is pure enough, should also have a pH of 7.0 at 25OC.

To samples of these 2 liquids, you add small measured amounts of 0.1M NaOH and 0.1M HCl, measuring

the pH at each addition.

By comparing the pH changes in the 2 solutions, you can demonstrate the action of a buffer solution.

Chemicals : 0.1M HCl(aq) , 0.1M NaOH(aq) , Distilled water , Buffer solution (pH 7.0) ,

Pure water (freshly-boiled distilled water)

Procedures : Part A : Effect of NaOH on pH of buffer solution (pre-set at pH 7)

Fill a burette with 0.1M NaOH.
Using a measuring cylinder, put 25 cm3 of the buffer solution in a 50 cm3 beaker.
Rinse the pH meter electrode with distilled water from a wash-bottle, and put it into the beaker, making sure that the glass bulb is completely immersed. Set the meter to read 7.0 .
Place the beaker under the burette containing NaOH and, making sure the alkali does not fall directly on to the electrode, add 1 drop of 0.1M NaOH.

Stir gently to ensure thorough mixing & record the pH in Table 1.

Add more NaOH to make the total volume added 1.0 cm3. Measure & record the pH in Table 1.
Add more NaOH to make the total volume added 5.0 cm3. Measure & record the pH in Table 1.

Rinse the electrode in distilled water & stand it in a flask of distilled water.

Part B : Effect of HCl on pH of buffer solution (pre-set at pH 7)

1. Repeat the similar procedures as in Part A by using HCl instead of NaOH

Measure & record all the pH values at 0 cm3 , 1 drop , 1.0 cm3 and 5.0 cm3 in Table 1.

Rinse the electrode in distilled water & stand it in a flask of distilled water.

Part C : Effect of NaOH and HCl on pH of pure water

Put 25 cm3 of pure water in the beaker and, keeping its exposure time to air as short as possible,

measure its ...

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Part B : Effect of HCl on pH of buffer solution (pre-set at pH 7)

1. Repeat the similar procedures as in Part A by using HCl instead of NaOH

Measure & record all the pH values at 0 cm3 , 1 drop , 1.0 cm3 and 5.0 cm3 in Table 1.

Rinse the electrode in distilled water & stand it in a flask of distilled water.

Part C : Effect of NaOH and HCl on pH of pure water

Put 25 cm3 of pure water in the beaker and, keeping its exposure time to air as short as possible,

measure its pH. When you have a pH between 6.0 and 7.0 for the “pure” water, measure & record

the pH changes on addition of 0.1M NaOH (0 cm3 , 1 drop , 1.0 cm3 and 5.0 cm3) in Table 1.

Rinse the electrode in distilled water & stand it in a flask of distilled water.

2. Repeat the similar procedures as in step 1 of Part C by using HCl instead of NaOH.

Measure & record all the pH values at 0 cm3 , 1 drop , 1.0 cm3 and 5.0 cm3 in Table 1.

Rinse the electrode in distilled water & stand it in a flask of distilled water.

Part D : Effect of air exposure on pH of pure water

Put 25 cm3 of pure water in the beaker and, keeping its exposure time to air as short as possible,

measure & record its pH in Table 2.

2. Leave the pure water to stand open to the air for 10 minutes, and measure & record its pH in Table 2.

Results : Table 1

Table 2

Questions :

Explain, by using suitable equations, the effect of NaOH on pH of buffer solution.

After adding NaOH into buufer solution, ethanoic acid will react with OH- to form salt and water. The equation is:

CH3COOH + OH- CH3COO- + H2O

The pH of the buffer solution only slightly rises from 7.0 to 7.4. The pH value can be calculated by this equation: pH = pKa + log [A-]/[HA]

Since Ka is the equilibrium constant of the reaction, the pH value is only affected by the ratio of concentration of CH3COO- and CH3COOH. Because the ratio is in log form, the pH value will have a smaller change.

Explain, by using suitable equations, the effect of HCl on pH of buffer solution.

After adding HCl into buufer solution, ethanoate will react with H+ to form ethanoic acid and water. The equation is:

CH3COO- + H+CH3COOH + H2O

The pH of the buffer solution only slightly drops from 7.2 to 6.4. The pH value can be calculated by this equation: pH = pKa + log [A-]/[HA]

Buffer solution has a buffering capacity. What is meant by this term ?

Buffering capacity can be defined in two main ways:

(1) Maximum amount of either strong acid or strong base that can be added before a significant change in the pH will occur.

(2) Quantity of strong acid or base that must be added to change the pH of one liter of solution by one pH unit.

Total concentration of the buffer, Cbuf, is given by

Cbuf = [HA] + [A-]

State 3 examples of daily applications of buffer action.

We can use buffer in our daily life very widely.

(1) Food preservation

During the fermentation of alcohol, the buffer solution is necessary to maintain pH value since the enzyme will be denatured in an unsuitable pH environment.

(2) Blood

Our body needs a stable pH values range between 7.35-7.4. Otherwise, cells will suffer from acidic or alkaline blood and we will die.

(3) Electroplating

As mentioned in step 1 of Part C, if the distilled water is absolutely pure, then its pH will be 7.0.

But it is difficult to achieve this.

If the pH drops below 6.0, what can we do to achieve the working range pH 6.0 to 7.0 ?

As we need to keep the working range of buffer from pH 6.0-7.0, we can add certain amount of basic buffer solution which have basic property to resist pH changes in a basic medium. After adding buffer solution, the pH will not have any big changes.

Why does the pH of pure water drops when exposed to the air ?

Carbon dioxide in the air will dissolve in the pure water to form carbonic acid. The equation is as below:

CO2 + H2O H2CO3

As carbonic acid is a weak acid, the pH drops slightly and below 7.0

Why is a pH 8.8 buffer less stable over a period of time than many other buffers ?

It is because at room temperature, the equilibrium of the reaction of buffer

formation shifts to the left and more reactants is formed. So the concentration of

NH3 increases which make the alkali buffer unstable.

If the buffer is diluted with pure water, explain whether there is a pH change.

(1) The acidic buffer:

The effect of dilution on the buffer is that the more the dilution, the higher the pH. The reason that the pH increases is the water added will lower the concentration of acid in the buffer which increased the pH.

(2) The basic buffer:

The effect of dilution on the buffer is that the more the dilution, the lower the pH. The reason is that

the water added will lower the concentration of alkali in the buffer which decreased the pH.

Suggest how could you make a buffer of pH of 5.2 by using the following chemicals.

(Given : 1.0M CH3COOH, Ka=1.8x10-5 ; NaOH(s) r.a.m : Na=23, O=16, H=1)

Firstly, add solid NaOH into 1.0M CH3COOH. Then, CH3COO- and H2O will be formed as a product.

NaOH + CH3COOH CH3COO-Na+ + H2O

Secondly, mix the sodium ethanoate solution and ethanoic acid together into a beaker.

Put the data into this equation:

pH = pKa + log [CH3COO-]/[ CH3COOH]

5.2 = -log (1.8x10-5) + log [CH3COO-]/1.0

5.2-4.74 = log [CH3COO-]

0.46 = log [CH3COO-]

[CH3COO-] = 2.85M

If 1 dm3 of the buffer solution is produced, the number of mole that CH3COO- needed =2.85(1) = 2.85 mole.

Number of mole of CH3COO- = Number of mole of NaOH

So the mass of NaOH that needed to use for making pH 5.2 buffer is

2.85 X (23+16+1) = 114g

Explain how could you measure Ka of 1.0M CH3COOH by using the following :

(Given : 1.0M CH3COOH(aq) , 1.0M NaOH(aq) , pH meter)

Firstly, add 1.0M NaOH(aq) into 1.0M CH3COOH. Then, CH3COO- and H2O will be formed as a product.

NaOH + CH3COOH CH3COO-Na+ + H2O

Secondly, mix the sodium ethanoate solution and ethanoic acid together into a beaker. The mixed solution is the buffer solution that we want to find its pH.

Finally, put the electrode that belongs to pH meter into the buffer solution. The reading on the pH meter shows the pH of this buffer solution.