SESSION 2: ACID RAIN:
  VISUALIZATION OF THE PROBLEM

 

 

SESSION OBJECTIVES:

To implement the knowledge accumulated in SESSION 1

To develop you creative thinking skills

To visualize the effects of acid rains on the environment

To comprehend the scope of the acid rain problem

To use complexation for quantitative analysis

To use instrumental analysis (spectrophotometry)

EXPLORATION 2A: 

DESIGNING THE EXPERIMENTS ( 30 min)

 

Using your knowledge about acid rain (based on SO2 emissions) propose the set of experiments visualizing in a simple way:

The origin of acid rain

Its chemical properties

Its impact on plants

Its impact on structures

Acid rain prevention methodology

 

The instructor will collect the proposed setups of the experiments.

EXPLORATION 2B: 

EFFECTS OF SO2 AND ACID RAIN ON PLANTS AND STRUCTURE (15 min).

 

REACTANTS AND GLASSWARE:

glass jars with septa, syringe, green leaf (lettuce), a piece of chalk, sodium bisulfite, hydrochloric acid.

 

SAFETY NOTES:

Sulfur dioxide produced in the experiment is toxic and should not be allowed to escape into the lab. Students with allergy to SO2 should not participate in the experiment.

 

Place 10 mg of NaHSO3 in each of two reaction vessels with rubber septa. Introduce a piece of green leaf in one vessel (1) and a piece of chalk in the other (2).

Using syringe inject 0.1 mL of HCl. Do it carefully in order to avoid the direct contact of acid with the objects placed in the jars.

Using a chemical equation describe what happened when you added HCl

Observe the effect of released gas on the leaf and chalk. Record your observations every half hour.
                                                        

time (hr)

jar (1)

leaf

jar (2)

chalk

0.5

    

1

    

1.5

    

2

    

CREATING THE CONTEXT 2: 

DETERMINATION OF SO2 IN AIR: WEST-GAEKE METHOD

One of the methods to determine SO2 in the atmosphere is the procedure proposed by West and Gaeke (P. W. West and G. C.Gaeke, Analytical Chemistry 1956, 28, 1817-1819). This method is sensitive in the concentration range 0.005-0.2 ppm. The system obeys Beer’s law up to about 0.6 ml of SO2 per mililiter of absorbing solution. The air is bubbled through 0.1 M sodium tetrachloromercurate solution and the stable, nonvolatile dichlorosulfitomercurate is formed. The sulfur dioxide is then determined by spectrophotometric measurement of the red-purple pararosaniline methylsulfonic acid formed when acid-bleached pararosaniline and formaldehyde are added to the scrubbing solution.

Main reaction:

[HgCl4]2- + 2SO2 ---> [Hg(SO3)2]2- + 4Cl- + 4H+

REACTANTS AND GLASSWARE:

0.1 M solution of sodium tetrachloromercurate (II), hydrochloric acid-bleached p-rosaniline solution (0.04%), formaldehyde solution (0.2 %), 100 mg/mL standard solution of SO2 (sodium bisulfite).

scrubber (200 mL cylinder with appropriate tubing), crucible, 100 ml volumetric flasks, pipettes (10 ml), rubber pumps, air pump, plastic sample container, cuvettes, spectrophotometer.

 

SAFETY NOTES:

Sodium tetrachloromercurate is highly poisonous; if spilled on skin, flush off with water immediately.

EXPLORATION 2C: 

WEST-GAEKE METHOD: calibration curve (60 min)

Pipet exactly 10 mL of standard sulfite solution into a 100-ml volumetric flask and dilute to the mark with absorbing reagent (0.1 M sodium tetrachloromercurate). The final solution contains 3.5 ml SO2 per milliliter.

Add accurately 0.5, 1.0, 1.5 and 2.0-mL portions of the dilute standard sulfite solution to a series of testing tubes. Mark them as (1), (2), (3) and (4).

Add accurately the following volumes of 0.1 M sodium tetrachloromercurate to the corresponding tubes:

(1) 9.5 mL

(2) 9.0 mL

(3) 8.5 mL

(4) 8.0 mL

Add 10 mL of absorbing regent (0.1 M sodium tetrachloromercurate) to the tube marked (0). This is going to be your blank solution.

To each tube add accurately 1 mL of acid-bleached pararosaniline solution and 1 mL of formaldehyde solution and mix using a glass rod. Hold for 20 min at room temperature. (while waiting for color development you can start EXPLORATION 2D)

Look to appendix IV of your CHEM 104.1 lab manual to see how to use the spectrophotometer

Measure the absorbance at 57O nm against the zero standard.

Plot the absorbance as the ordinate against the micrometers of SO2 per 10 mL of absorbing solution. Compute the slope and intercept.

 

Calibration solutions

TESTING TUBE #

mL OF STANDARD SO2 SOLUTION

CONCENTRATION OF SO2 [mL/10ml]

ABSORBANCE AT 570 nm

1

0.5

    

2

1.0

    

3

1.5

    

4

2.0

    

 

y= ax +b

a=

b=

EXPLORATION 2D: 

WEST-GAEKE METHOD: evaluation of SO2 content in Air (45 min)

Pipet exactly 10 mL of the absorbing solution into the scrubber (absorber).

Connect the air pump to the scrubber

Aspirate the air sample through the absorber at a rate 3.5 L/min. The location of your sampling will be given by the instructor. Stop when your total air volume is 38.2 L.

Transfer the absorbing solution to the plastic sample container.

Repeat the sampling for another 10 mL of absorbing solution.

Transfer both absorbing solutions to the testing tubes marked "1" and "2".

Add 10 mL of absorbing regent (0.1 M sodium tetrachloromercurate) to the tube marked (0). This is going to be your blank solution.

Add 1 mL of acid-bleached pararosaniline solution and 1 mL of formaldehyde for each tube. Wait 20 min for color development.

Measure the absorbance at 570 nm.

 

Sampling

time

volume of air [L]

absorbance at 570 nm

1

      

2

      

CALCULATION:

Convert the volume of air sampled to the volume at standard temperature and pressure. Compute the microliters of SO2 in the sample by dividing the absorbance by the slope of the calibration plot (assume that your experimental conditions are STP conditions). Then the concentration by  volume is:

 

EXPLORATION 2E: 

WORKING WITH INFORMATION/MAKING THE LINK: Home assignment

Calculate the average percentage of SO2 in air. Identify the possible sources of sulfur dioxide.

What happened with the piece of chalk? Explain the chemical process? Do you know the name used for this process?

What is the effect of SO2 on plants? Is it limited to only one particular plant (lettuce)?

Is the amount of sulfur dioxide calculated from your data reasonable compared to the concentration of SO2 in unpolluted air?

 

LABORATORY REPORT:

Prepare a detailed report containing:

An Introduction with the objective of the experimental work

Experimental procedure and analytical methods used

Results of the experiments

Discussion with answers to all questions asked during the SESSION 2.

 

 

                                               

                                                   Back to SESSION 1