Custom Sensor Solutions, Inc.Custom Sensor Solutions, Inc.
A common zinc-air hearing-aid battery from your corner drugstore can be used as a simple oxygen sensor. In this document, we will describe an experiment to measure the oxygen consumption by a chemical hand-warmer used by hunters and hikers. Only a few inexpensive materials are needed to carry out these experiments.
This demonstration assumes elementary skills in the use of tools and soldering equipment.
The zinc-air battery is commonly used in hearing aids, where weight and low cost are important. Most batteries carry both the oxidant and reductant in one package. The zinc-air battery carries only the reductant, as a lump of zinc metal. The oxidant is oxygen, supplied by the ambient air. The zinc lump is the cathode. The anode is a screen of nickel wire coated with a mixture of carbon and manganese oxides, which catalyze the reduction of oxygen to water. These are immersed in an alkaline electolyte.
Normally, the Zn/air battery is inert and can be stored for several months unused. A strip of tape prevents air from entering the battery. If the tape is removed, air diffuses into the cell through tiny holes, and an "open-circuit" potential of about 1.4 volts develops across the electrodes. This voltage prevents the redox reaction from proceeding. But when current is drawn from the cell, the chemical reactions will attempt to maintain the open-circuit voltage. Zinc will be consumed at the cathode:
Zn0(metal) ---> Zn++ + 2 e-1
and oxygen at the anode:
O2(gas) + 2 H2O + 4 e-1 ---> 4 OH-
In its intended application (a hearing aid), only a tiny current is drawn and the reaction proceeds very slowly. As long as some oxygen is around, the battery can function at full voltage.
On the other hand, if the battery is short-circuited, unlimited current can flow and the reaction will proceed as quickly as it can. After a while, the current stabilizes at its "short-circuit"current, which is about 20 milliamperes for a Type 675 cell. This current is determined by the rate at which oxygen can diffuse into the cell. If the oxygen around the cell increases or decreases, more or less oxygen will diffuse to the anode, and the output current will change accordingly.
In our experiment, we connect a low-value small resistor across a Zn/air cell, small enough to not limit the current, and large enough to allow us to measure a voltage drop. This is how we determine the "short-circuit" current from the cell.
Notes on the parts list:
1. Zn/air batteries are thicker than other types of coin cell, and it is difficult to find a battery holder which will fit one of the available battery types. The Type 675 battery and the Keystone Type 501 battery holder are a perfect match. However, the battery holder may prove difficult to track down. Ordering a $2.00 part from Newark Electronics is not a cost-effective strategy, as they have a minimum order and a shipping charge. Other supply or surplus houses may be easier to buy from.
It would be nice to eliminate this troublesome part, but we have not found a good way. Soldering to a caustic-containing battery is not wise; it is also very likely that the solder will not stick. Taping wires to front and back is frustrating; they will invariably fall off. Conductive epoxy cement may work, but it is more expensive and troublesome than buying the battery holder.
Note: Custom Sensor Solutions will consider putting together a kit of the hard-to-get parts if there is enough expressed interest. Please email us if you would be interested in buying such a kit. We would strive to keep the price under $15.
2. The chemical hand-warmers are a fall and winter item at K-Mart, and usually disappear from the shelves during the fish-and-golf season. On the other hand, I have had no trouble in buying them by asking the clerk to get them from the back. The first ingredient should be iron powder. The supersaturated-salt types, containing sodium acetate or sodium thiosulfate, will not work for this purpose.
3. If you have a chart recorder or electronic datalogger, this is much preferable to the use of a voltmeter. Set the input range for 0-1 V or 0-2 V, and the speed to about 4 inch/hour.
The parts are soldered in place on the perforated board as shown here. This goes very quickly, since the long leads from the resistor can be used to make contact with the leads from the battery holder. In our example, we use a Waldom connector to take the signal from the board, but there is no reason the voltmeter leads cannot be connected directly to the resistor leads.
Top and bottom sides of assembled sensor.
Connect the voltmeter across the resistor and set the voltage range to 0-1 or 0-3 volts. Disable any autoranging or auto-shutoff features if you can.
The battery is inserted by removing the strip of tape to expose the air holes (usually four). As quickly as possible, slip the battery into the holder with the holes up. Look at the voltmeter. You should see a rapidly-rising voltage, topping out over 1.0 V and then slowly falling to a reasonably stable value around 0.8 V. Record these numbers.
Leave the battery for about an hour. The voltage will decline somewhat, due to the accumulation of reaction products in the cell, but the voltage will soon become relatively constant.
Put the sensor into the plastic Zip-Lok bag, being careful not to puncture the bag. Cut open the outer bag of the hand warmer pack, being careful not to penetrate the porous inner bag. Knead the chemical packet to start the reaction. Immediately, put the hand-warmer in the bag with the sensor. Partly inflate the bag by spreading your hand inside (do not use your breath), and seal up the bag around the voltmeter wires. Use transparent tape to hold the bag shut and to seal around the wires so no air can get in or out.
Over a period of hours, the hand-warmer will consume the oxygen in the bag. The sensor signal will fall, quickly at first, then more gradually. The reason for this is that as the oxygen is consumed,the reaction in the hand-warmer also slows down. Many hours are needed to consume all the oxygen.
If time is of the essence, you might use several hand warmers at one time.
The dramatic part of the experiment comes next. When the signal from the sensor has reached a suitably low level in 4 to 16 hours, open the bag and take the sensor out. The voltage will return rapidly to a high level, generally within 60-90 seconds. It may even overshoot the original voltage, then fall back to a signal close to the original pre-experiment voltage.
It is important to remember that the output of an amperometric sensor, such as this one, is measured as current, rather than voltage. Between the voltage measurement and the resistor value, the current can be calculated by Ohm's Law:
Current in amps = voltage / resistance in ohms = V / R
For the hand-warmer experiment, our results are shown below.
A single Zn/air cell can be used for about two days worth of experiments. It can then be discarded ( do not attempt to recharge these cells). Zn/air cells do not contain ingredients that are harmful to the environment.
Other Experiments
There are many experiments that can be performed with this simple sensor system, although it may be necessary to collect some more apparatus to carry them out. The hardware store is a good place to start.
1. Calibration curve. If you can get pure nitrogen or oxygen, you can make accurate dilutions of oxygen in air or nitrogen in air to attain different O2 concentrations. Doing this, you can draw up a curve that can be used in other experiments. Look at our apnote on dilutions
2. Exhaled air. Collect your exhaled air in a plastic bag and measure the oxygen content. Note: Excess carbon dioxide causes more rapid deterioration of the sensor, by dissolving in the alkaline electrolyte. Avoid long or repeated exposures unless you have lots of sensors.
3. Measure oxygen consumed and produced by a plant.
4. Make pure oxygen with drugstore 3% hydrogen peroxide and a fewcrystals of copper sulfate.
5. Measure the effect of temperature on the cell response. Temperature can be used to determine if the sensor is operating under pure diffusion control, where the sensor response is proportional to the square root of the Kelvin temperature. If diffusion control is not operating, the temperature curve will be more complex, and probably steeper.
Current May 26, 1998
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