Acid Rain Experiment and Construction of a Simple Turbidity Meter Eric A. Betterton Department of Atmospheric Sciences, University of Arizona, Tucson, AZ 85721 I t is the author's experience that there is a dearth of nuhlished lahoratorv exnerimenta that are suitable for a . freshman or sophomore course in atmospheric chemistry and that can he offered without the need for expensive instrumentation or materials. This note describes the construction of an inexpensive turbidity meter, and its use in an experiment that simulates the formation of acid rain from the combustion of a svnthetic sulfur-containing "fossil fuel". Commercial turbidity meters normally cost on the order of U.S. $1000, yet here we descrihe a simple instrument that can he assembled from readily available components for approximately $30. The "fossil fuel" is methanol containing a small amount of thioacetamide or dimethvlsulfoxide (DMSO) as a sodrce of sulfur that, when burned,;~ oxidized to SOz and, in a separate step, is converted to BaSOd. In southeastern Canada, the northeastern United States, and much of northern Eurooe, forests are declining, - lakes and rivers are becoming acidic, and aquatic organisms are being adversely impacted due to the effects of acid deposition. Unpolluted rainwater is normally mildly acidic (pH 5.7) due to the uptake of atmospheric carbon dioxide that dissolves to form the weak acid, carbonic acid, H Z C O ~ . ' , ~ (This assumes that the rain or cloud has not intercepted sources of alkalinity such as ammonia or crustal material containing carbonate.) The pH values of rainwater in the northeastern United States are now about pH 4.2-4.5, which is 15 to 30 times more acidic than unpolluted rain. At nresent. the west coast of the United States does not aooear to he as'hadly affected, but in one instance an acidic figwas collected near Los Angeles that had a DHof 1.7-the current record for acidic hydrometeors. h he two majoranthropogenic acids in acid rain are sulfuric (HzS04) and nitric (HNOd acid. In southern California the two acids are found in approximately equinormal ratios, but in the northeastern United States and Scandinavia sulfuric acid dominates hv a factor of 2-3 due to the increased levels of H2SO4 precursors, notablv The SO" that is released to the atmosohere durinithe Eombustioi of sulfur compounds in fossil 'fuels, especially some types of coal, yields HzSOa, while NO, (i.e., NO NO2), that is also released as a combustion product, yields HN03. NO, is composed of "fuel NO,", i.e., the oxidation products of nitrogenous compounds contained in the fuel, and "thermal NO,", i.e., the products of the high-temperature reaction of atmospheric nitrogen and oxygen in the comhustion zone. Emissions of fuel NO- can in nrincinle he avoided by selecting fuels that are low in organic nitrogen, e.g., natural gas, but the formation of thermal NO, is largely unavoidable. Once released to the atmosphere, NO, is subsequently oxidized to nitric acid almost exclusively in a gas-phase
.
+
Finlayson-Pins. B. J.; Pins, J. N. Atmospheric Chemistry Funda. menials andExperimenta1Techniques; Wiley: New York, 1986. 2Trace Atmospheric Constituents. Adv. Environ. Sci. Techno!. 1983, 12.
Sefnfeld, J. H. Atmospheric Chemistry and Physics of Air Po111+ tion: Wiley: New York. 1986. 254
Journal of Chemical Education
reaction by the hydroxyl radical-arguably the most important scavenger in the troposphere (lower atmosphere): NO,
+O H
-
HNO,
The reaction is rapid: the lifetime of NO2 with respect to oxidation by OH. is on the order of 1day. By contrast the lifetime of SO2 with respect to oxidation by O H is approximately 2 weeks. The gas-phase reaction of SO2with OH. is not completely understood, but i t is thought to involve the formation of the HOSO; radical in the first step followed by a number of subsequent steps to yield H2SOa: SO,
+ OH.
HOSO;
-
--
HOSO;
H,SO,
The atmospheric oxidation of SO2 occurs not only in the gas phase hut also in the aqueous phase(in clouds, fog, dew, etc.). SO2 is a fairly soluble gas with a high Henry's law constant, and therefore i t readily dissolves in cloud water, etc.. to form what is sometimes called "sulfurous acid" ( ~ ~ 5 hut 0 ~which ) is more correctly called hydrated SO2 (H90.SO~).This molecule mav be involved in two hvdrolvsis &;ilibriiupon dissolution yiklding bisulfite and shfite," SO,(g)
+ H 2 0 = H20.S0,
H,O.SO, r HSO; HSO;
+ SO:-
+ H+
+ Ht
and therefore the effective Henry's law constant of SOz, and concomitantly, the total dissolved S(IV), i.e., Hz0.S02 + HSO; SO?. increases dramaticallv with uH-an examnle of ~ e ~ h a t e f i e rprinciple. 's Due to the colhhined effectsof high solubility and rapid aqueous phase oxidation reactions, the aqueous phase oxidation of SO2 is often a more important route than eas ohase oxidation hv OH.. There are sev&lcompeting oxidants for S(IV) in natural systems, including atmospheric oxygen (catalyzed by transition metal ions such as Mn(I1) and Fe(III)), ozone, and hydrogen peroxide. The pH of the system and the relative concentrations of the competing oxidants determines which is the most important oxidant under the prevailing conditions. For example, the dominant oxidant is Hz02 from pH 0 to 5, while a t pH > 5, O3becomes more important for typical atmospheric concentrations. Hz02 is normally the most important oxidant over the pH range that is applicable to most hydrometeors. The reaction proceeds very rapidly yielding sulfate and the hydronium ion as the sole products thereby acidifying the water droplet:
+
Experlrnent In this experiment we simulate the formation of acid rain by burning an organic, sulfur-containing fuel (methanol with added thioacetamide or DMSO) and collecting the combustion gases (including SO21 in water that contains
Figure 3. Circuit diagram fw the turbidity meter. Part numbers for the light emining diode and phototransistor are from Radio Shack. Figure t . Equipmem setup f a the acid raln experiment
Figure 2. Calibration curve obtained with me turbidity meter described here using three separate sets of standards.
H?02.The resulting "acid rain" is analyzed for sulfate content by precipitating with BaCI9and measuring the resulting turbidity with the turbidity meter. A 76-mm-diameter metal funnel is used to collect the combustion gases (see Fig. 1). I t is soldered to one end of a 75-cm length of 6.4-mm-i.d. copper tubing that has been bent into an inverted U shape. The funnel stem is cut if necessary to obtain the desired fit. The other end of the tubing is passed through a rubber stopper and nearly to the bottom of a 500-mL filtration flask containing 100 mL 0.01 M H202. The funnel is positioned over a 120-mL alcohol burner (Wheaton Scientific) that contains 40-50 mL methanol with 10 mM thioacetamide or DMSO. The funnel and tubing are supported with a clamp and stand. The volume of methanol in the burner is measured by transferring to a graduated cylinder after the wick is saturated. The methanol is returned to the burner, the side arm of the filtration flask is connected to an as~irator,and air is drawn rapidly but smoothly through th;~202 solution. If necessary, the rate can be adjusted with a hose clamp. After any fuel spills have been cleaned up, the burner is lit. Caution: methanol burns with an almost invisible blue flame, and i t is therefore easy to burn oneself, particularly when the copper tubing and funnel also become hot. After approximately 10 mL of fuel have been burned (15 min), the flame is extinguished, and the burner is capped to reduce evaporative losses. The apparatus is -~ disconnected after it has cooled, and 10 drops 0.1 M BaC12 are added to the peroxide soluti& to form &oluble BaS04 that is manifested as a slight turbidity. A portion of ~
~
~~
~
~
~
Flgrre 4 LayoUt 01 the turbidity meter viewed from the hoot of the instrument. ( I t Wooden block. 121Samp e companment (31 Phototransistor. 141Fu l-scale potentiometer. (5)Toggle switch. (6) Mi Ilammeter. (7) lnssument h0.s ng. ( 8 ) iigm-mining diode. (9) Light path.
the turbid sample is poured into a sample vial, and the turbidity is measured with the turbidity meter. The volume of alcohol that remains in the burner is also measured. The concentration of BaSOl is read from the linear calibration curve that is constructed using a blank (distilled water) and three BaS04standards (58,117,175 mg L-') that are freshly prepared by adding excess BaC12 to 0.25, 0.50, 0.75 mM Na2SOa solutions (see Fig. 2). T o calibrate the meter, the highest standard is set a t approximately 0.9 mA using the potentiometer, and then the other standards are measured without anv further adiustments. Students are required to calculate the overall efficiency of conversion and collection of fuel sulfur as sulfate eiven the concentration of thioacetamide or DMSO in the Fuel. The overall efficiencv 20-3070, but this varies mark. is twicallv .edly with air flow rate, etc. if desired,the experiment can be extended bv requiring students to also determine the acidity of the acid iainby ti6ation or using a pH meter. Appendix. Construction of a Turbidity Meter A battery-powered, portable turbidity meter was constructed fromparts that are all available from electronicsupply houses. Since it was our intention to minimize costs, we deliberately kept the circuitry as simple as possible, and therefore the instrument described here could easily be upgraded if necessary. For example, an Volume 68 Number 3 March 1991
255
ohvious upgrade would be to include a zero offset capability, which would greatly facilitate the use of the full scale of the meter. The circuit diagram is shown in Figure 3. I t incorporates an infrared light-emitting diode (Radio Shack 276-143A) as the light source and an infrared-sensitive phototransistor (Radio Shack 276-1451 as detedor. Surprisingly, the infrared system appeared to show a better response than a prototype system based on thevisible spectrum. An LF351 operational ernplifier is used to amplify the signal from the phototransistor, and a single-turn 5 kfl potentiometer is used to set the full-scale reading. Two 9-V dry-cell batteries act as the power supply. A spring-loaded double-pole, double-throw toggle switch prevents accidentally leaving the instrument turned on and thereby draining the batteries. A 0-1-mA dc analogue rnilliammeter provides the signal output. All the components are mounted in a 129- X 67- X 41-mm plastic box with aluminum base (Radio Shack 2702. RR .,.I ~
Thecomponcnt layout end the optical system arerhown in Fipurr 4. The source and detector arc rnounced at rrght angles in hder drilled in a block of wood (35 X 29 X 38 mm) that has also been drilled out to receive a sample cuvette. We used a 10-mL cylindrical (55-mm X 15-mm-diameter) borosilieate glass sample vial with a screw-on cap as a cuvette and masked that portion of the vial that protruded from the instrument and was exposed to room lights with a hand of black electrical insulating tape. A square of black tape was
256
Journal of Chemical Education
also placed inside the box a t the bottom of the sample compartment to minimize stray light. When the turbidity meter has been assembled, the positions of the source and detector in the wooden block are adjusted (by pushing them closer to, or pulling them farther away from, the cuvette) so that the largest working range on the milliammeter is obtained. In practice this means alternately inserting a hlank (deionized water) and a full-scale sample (175 mg L-' RaSO*) and adjusting the relative positions of the two components until the water readsabout 20% (0.2 mA) and the RaSOlreadsabout 90% (0.9 mA) of full scale. This procedure takes only a few minutes, but it could probably be largely avoided by using a circuit that included a zero offset. We have found that this turbidity meter is linear over the range from 0 to 175 mg L-' BaSOd.
Acknowledgment
.
T h e initial imoetus for t h i s oroiect came from heloful discussions with M. Bonner Denton a n d his research group in t h e Institute for Chemical Education at t h e Universitv of Arizona. The a u t h o r would also like to acknowledge 'the assistance of M a r k E. H o m a n a n d Andrew M. H u d o r i n designing t h e electronic circuit and t h e technical skills of Richard E. Milliron in designing and assembling t h e turhidity meters.
