Environmental Chemistry 3

changes in atmospheric CO₂ concentrations

On first impression, the climate might not seem like chemistry.  But the absorption of energy from the Sun and the emission of radiation back into space is determined by the chemical substances in the atmosphere and on the Earth's surface.  Typical substances are transparent to some wavelengths of sunlight while absorbing at other wavelengths.  And the surfaces of materials are often reflective under some conditions but not others.  Finally, substances often release energy at a different wavelength than they absorb.  This might all be of little concern to humans except that human activities have been unintentionally changing the balance of these processes in ways that may be undesirable.

Archaeologists studying ancient civilizations have noticed that human civilizations did not always get progressively better but sometimes declined and occasionally totally collapsed.  They have suggested than the declines of a number of human civilizations were at least partially due to failing to correctly understand and adjust to changes in their environment.  If that is true, then it may be imperative that we try to understand what changes we are making in our environment and make any adjustments that are appropriate.

There is evidence that the Sun has emitted a steady rate of energy (at 1.99 calories/cm²/minute = 1.73 x 10¹⁷ Watts at a distance of one astronomical unit) for many centuries and will continue doing so (except for a 3% increase every 11 years related to the sunspot cycle).  That energy is spread over various wavelengths (or colors) of light in a nearly bell shaped curved (more precisely, a Maxwell-Boltzmann distribution tailing off to right) centering on visible light.  The air surrounding the earth is transparent to the visible wavelengths allowing much of the light into the earth.  Ozone, O₃, and other molecules in the upper atmosphere block nearly all the short wavelength ultraviolet light which otherwise would harm life.  Other molecules absorb light at particular colors making the intensity of light reaching the earth pocked by dips (as shown at right).  About 25% of the incident solar radiation is absorbed by the atmosphere and turned to heat.

In addition the water droplets in clouds refract visible light scattering much it of back towards space.  Another 25% of the incident solar radiation is reflected back into space.  Light colored soil and ice fields reflect about 5% of the incident radiation with the remaining 45% absorbed by the oceans, land, and plants.  Only 0.02% is actually captured by photosynthesis of plants and utilized by life.  Deforestation, creation of smoke, and other human activities can influence how much of the incident solar radiation is immediately reflected back into space.  All the absorbed energy eventually warms various parts of the earth.

Each warm object eventually releases the energy, emitting infrared light as governed by the object's temperature.  But unlike most of the directly reflected light, some of the infrared light can be recaptured by the atmosphere.  This effect was first proposed nearly two Centuries ago by Fourier.  Greenhouse gases such as CO₂ operate by the same mechanism as a greenhouse.  Such a building lets visible light in through the window glass which is clear to visible colors but opaque to infrared radiation.  Thus the energy is captured inside, warming the building.  Likewise the greenhouse gases are transparent to visible light but absorb infrared radiation, warming the earth.  In 1896 Svante Arrhenius published a paper On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground presenting calculations on the effect of atmospheric CO₂ concentrations on temperature as a function of latitude.  He proposed a decline on CO₂ concentration could cause an ice age.  But this caused little concern for six decades.  Charles David Keeling (1928-2005, photo above) was recruited by Scripps Institute of Oceanography in 1956 to help monitor CO₂.  In 1958 he started measured CO₂ on top of Mauna Loa on the island of Hawaii, in the middle of the Pacific Ocean to establish a baseline of CO₂ in the earth's atmosphere.  He discovered a seasonal variation in concentration (correlated with photosynthesis variations).  But his persistence in continuing measurements (for over 40 years) led after a couple years to the far more important discovery that there is a steady annual increase in the earth's CO₂ concentration.  There is now little doubt that the burning of fossil fuels to power the industrial age is changing the earth's temperature.  Keeling's pioneering work fundamentally changed the way we view our role on planet Earth.

In 1973 at the Mitre Corporation I received a call from the National Science Foundation asking me to conduct a study under the recently authorized Research Aimed at National Needs program.  The study was to develop a research program to foster the use of solar energy.  My report, Energy Use and Climate, NSF-RA-N-75-052, was published in April 1975.  This was the first U.S. government report stating that the use of solar energy instead of stored energy sources (both fossil and nuclear) would avoid the problem of ... global temperature increase. ... My conclusion was that the use of fossil fuels would have to be sharply curtailed by 2025 and eliminated by 2100 to avoid temperature increases of 2 to 3°C globally and 10°C at the poles.  The world is rapidly moving towards those numbers. ... Despite the billions of dollars invested, we have essentially lost 30 years in implementation of significant amounts of solar energy production. ... the only way to make solar energy cost-effective is to legislation a carbon tax or other method to sharply curtail the use of fossil fuels.

Update

There has been much evidence gathered suggesting that the Earth's temperature continues to warm as the levels of carbon dioxide increase.  But controversy remains as to how much of the temperature change is due to the CO₂ concentration increase and how much is due to other causes.  Water vapor, H₂O, is responsible for roughly 95 per cent of the total greenhouse effect due to all atmospheric gases.  The proportion due to water varies with season, weather, temperature, and other local differences.  But many scientists believe that proportion is only temperature related when averaged over the entire Earth.  The other greenhouse gases, carbon dioxide, methane (CH₄), nitrogen dioxide (NO₂), and various others including various halogenated organic molecules, contributed the remaining 5% of the greenhouse effect, with CO₂ being the greatest contributor at 3.6%.  Human-related methane, nitrogen dioxide and ChloroFluoroCarbons (CFCs, predominately escaping after use as the working fluid in refrigeration devices) contribute 0.066%, 0.047% and 0.046% respectively.  Thus with the other factors having tiny effects or relatively constant effects, the increase in carbon dioxide concentration is thought to govern global temperature rise with the temperature effect on water vapor magnifying the warming.

There is also controversy as to whether CO₂ emissions should be immediately curtailed.  Some people propose that funds should be spent to maximize benefits, to current funding for research with anticipation that more economical solutions will be found.

Theory and Mathematics

In pure water near room temperature, the concentration of H⁺¹ is about 1 x 10^-7M so that the pH of the water is about 7.  Consider vinegar, an acid with concentration of H⁺¹ much GREATER, perhaps 10^-2M, the pH is about 2.  In household ammonia, a strong base with concentration of H⁺¹ much LOWER, nearly 10^-14M, the pH is nearly 14.  So small differences in pH numbers represent much larger differences in H⁺¹ concentration.  Because of the negative sign in the definition of pH, a DECREASE in pH by one represents a TEN-fold INCREASE.  And a LOWER pH by 3 represents 1000 ( = 10³) times MORE H⁺¹ concentration.

Because of the substances dissolved in the oceans, the pH of surface sea-water is about 8.2 pH.  And due to the increasing CO₂ in the atmosphere, more CO₂ has dissolved in the oceans lowering over two centuries the pH from 8.3, a 30% increase in H⁺¹ concentration.  That increase may be enough to modify the balance of life in the ocean, for example greatly reducing the viability of the great coral reefs.  Continued use of fossil fuels may decrease pH 0.5 by the end of the century.

Experiment

To develop a feel for the effect of CO₂ on H⁺¹ concentration in water, we need a method of detecting pH differences.  At higher concentrations our human tongues detect H⁺¹ as a tangy sour taste.  But at the concentration of tap water or sea-water, our senses need assistance.  Fortunately there are chemicals called indicators which change color when immersed in water of different pH.  While it is possible to purchase pH indicators, it is reasonably easy to extract natural indicators from fruits such as blueberries, blackberries, purple grapes, flowers such as rose petals and leaves such as purple cabbage.  While you might want to experiment with some of other indicators, the directions below explain how to obtain a low cost natural indicator.  The procedure that follows will extract anthocyanin pigments from the purple cabbage leaves to serve as indicators of pH changes.  But the procedure could be used to extract other natural indicators as well.

1. Obtain and prepare a device or substances capable of distinguishing the anticipated situations.  Specialized pH meters can be used, but the description below provides a versatile, inexpensive procedure using chemical indicators which will often be adequate.
2. Standardize the readings of the equipment or the chosen substances by comparing using substances of known values.  Here too pH standards can be purchased or obtained from others, but the procedure below describes how readily available substances of known pH can be used.
3. Once the apparatus or material is standardized, it can be used for making the desired comparisons.  In this experiment a procedure describes how pH sensitive chemical indicators might be used to investigate the effect of dissolved CO₂ on water pH.  But a wide range of experiments can be done using standardized pH indicators or equipment.

Procedure

Step I: Preparation of an Indicator

• several red cabbage leaves (or equivalent of flower petals, blue berries, purple grapes, or blackberries. Shown at right is a mix of immature and ripe blackberries used to make the sample chart shown in Step II below.)
• 1/2 cup (~125 mL) alcohol (rubbing alcohol, isopropanol, or ethanol)
• 1/2 cup (~125 mL) water
• apparatus for grinding leaves (chemists use a mortar and pestle, but ancients successfully used hand sized rocks)
• filter or strainer (coffee filter, cloth, or layers of porous paper such as paper towels)
• storage container with lid

1. Tear or cut the thin, highly colored sections of several cabbage leaves into small pieces.
   
   
2. Grind to tear open the plant cells to release the pigments.  Adding a little clean sand might help.
   
   
3. Cover with a 50:50 mixture of water and alcohol.  (Rubbing alcohol typically already contains both.)
   
   
4. Stir or grind a bit then let stand, covered, until solution is deeply colored.  Overnight extraction might be a reasonable time period.
   
   
5. Filter or strain to obtain the colored indicator liquid.
   
   
6. Store for later use.  The indicator might be kept covered in a refrigerator for months.

Step II: Standardization of the Indicator

• indicator from Step I
• variety of solutions of known pH.  A list of food pH is provided here

The concept used in this procedure is to determine the colors of the pigment when a small amount is mixed in each of a series of water solutions of various known pH.  Colored substances selectively absorb a portion of incident light, reflecting (or transmitting) the remaining which constitutes their apparent color.  Each indicator molecule typically appears one color a bit above a specific pH and a second color a bit below that pH.  In a solution close to that specific pH the indicator exists as a mixture of the two colored forms, resulting in an intermediate color.  The actual color of the mixture depends on the relative proportions of the two forms as well as the composition of the incident light.  (For example, Litmus is an indicator derived from lichens.  Around pH 7 it reacts with H⁺ ions changing form and resulting in a color change.  In more acidic solutions it absorbs a Hydrogen ion appearing red when illuminated by white (all colors) light.  In more alkaline (basic) solutions it releases the Hydrogen ion appearing blue in color.  It is an intermediate color at pH 7.  Because Litmus changed color at pH 7, it is has been used for distinguishing acids and bases for centuries.  As perhaps the best known indicator, the term litmus test is now often loosely used for any clearly distinguishing tool!)

Many plants contain a combination of several colored pigments each of which changes colors at at different pH values.  Thus, unlike solutions made with a single indicator, these natural indicators often change to a variety of different colors over a broad range of H⁺ concentrations. 

Once we know the indicator's color at various H⁺ concentrations, small amounts of indicator can be added to solutions of unknown concentrations to provide useful information.  Generally a single indicator will only reveal if the H⁺ concentration is above or below the pH at which the indicator changes color, unless we are lucky and the pH is within the narrow range where the indicator is a mix of its two colors.  To determine more about the H⁺ concentration, we usually need a number of indicators either for separate tests, or mixed together providing a color remaining after the light absorption of all the indicators present.  Consider the indicators A and B shown below:  If indicator A is placed in an unknown liquid and appears blue, we can conclude the pH > 7.  But if both indicators are added and the liquid is blue, we know 7 < pH < 9.

1. Select a set of water solutions which have the range of pH desired for standardizing the indicator extracted in Step I (or other indicator or equipment you wish to standardize).  Recall many foods are mostly water.  A useful list is provided.  Try to choose substances with little color of their own, and only a small natural ranges of pH.  Other water based solutions may have lower or higher pH values, but for investigating CO₂ effects, you will only need to measure pH near 7.  Caution: Both acids and bases can damage eyes, sometimes beyond repair; wear chemical splash goggles.  If a corrosive substance does get in an eye (it often stings AT FIRST), IMMEDIATELY rinse with comfortable temperature water for 15 minutes THEN get prompt medical attention.  For any other SMALL spill, dilute with lots of water.
   
   
2. Put a little of your indicator in samples of each different pH, recording the color.  More indicator and less sample will cause a more intense color.  Usually only a couple drops are needed if the indicator is sufficiently concentrated.  Every indicator has acid/base properties, so it changes the pH of the sample.  Using as little indicator as possible and at least several mL of sample minimizes any error caused by the acidity of the indicator.
   
   

Step III: Use Indicator to Investigate CO₂ Effect on Concentration of water pH

• indictor solution from Step I
• chart of standardized colors from Step II
• 1/2 cup (~125 mL) water
• optional: salt, preferable sea salts
• straw

1. Create or obtain a sample of sea-water.
   
   
2. Remove a sample and test its pH.
   
   
3. Blow your breath through the straw into the water depths for some time.  Animals such as us make CO₂ which we add to the air we exhale.  Blowing into the water should increase the concentration of CO₂ in the water.
   
   
4. Test the pH of the water occasionally during the process.
   
   
5. Were you able to measurably change the pH of the water?  If so, how much?  Try to explain why this happens.
   
   
6. Assuming your exhaled breath may be 1% CO₂, calculate how much (molar concentration?) CO₂ might have been added to the water.  Consider that much of the CO₂ may have remained in the bubbles and not dissolved.
   
   
7. Consider the magnitudes of experimental errors in this investigation.  How could the experiment be improved?

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.

Other Activities

1. Consider that the concentration of CO₂ in the atmosphere has increased from about 270 ppm in the in the 1940s to 380 ppm now, that the earth has a radius of approximately 6400 km and an atmosphere tapering over 80 km from 1 atm to 0 atm, estimate the mass of CO₂ increase.  (Recall 1.00 mole of gas occupies 22.4 L at STP and CO₂'s molecular mass)
   
   
2. Many of the suggestions for dealing with the greenhouse effect involve reducing combustion of carbon fuels.  But another proposal is to grown plants such as trees, then store the removed carbon as lumber or some equivalent product.  Estimate how much lumber (≈ cellulose ≈ glucose, CH₂O, 0.8 g/cm³) would be necessary to restore CO₂ concentration to earlier levels.  Is this a reasonable proposal?
   
   
3. Investigate the effect of the salt(s) on the experiment results.  Would the effects be the same on fresh water as on sea water?

References

• Bjorn Lomborg, An Economist's View of Saving the World, TED talk (ideas worth listening to), Feb 2005
• solar spectra from OceanOptics
• Roger Revelle's interest in atmospheric CO₂, NASA
• Charles David Keelings measurements in CO₂, NOAA
• Richard S. Greeley, letter, Chemical & Engineering News, (Vol 83, #27), July 4, 2005
• Preparation of Cabbage Juice Indicator, Glenn Crosby director, NWW, Operation PROGRESS, 1987-1996
• Acid-Base Indicators Extracted from Plants, Bassam Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol 3, U. Wisconsin Press, 1989
• Global Warming News, Rudy Baum, Editor-in-chief, Chemical & Engineering News, 13 Feb 2006