Build and experiment with a galvanic concentration cell that produces a small but continuous electrical current flow through an external load resistor for many months without going dead. The purpose of this experiment is to explore galvanic concentration cells driven by stratified electrolytes, a virtually unexplored area of science, meaning these experiments are original, novel and unique rather than the usual cookbook duplication of someone else’s work.
Explore where no one has explored before, design and do experiments no one has done before, and develop ideas that no one has thought of before. With all the possible variations of ideas, materials used, and techniques applied, the possibilities and discoveries are endless.
I hope this information and proof of concept experiment can serve as a starting point for you as experimenters to explore this unexplored area of science, and as the saying goes “write the book” on the subject by posting the details of your own “Stratified Electrolyte Concentration Cell” experiments. How often do you get a chance to explore the unexplored from proof of concept into the vast unknown?
"The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, 'hmm... that's funny...'" - Isaac Asimov
One day back in the mid 1970s, I was playing around with various electrodes (copper wire, pencil lead graphite, aluminum foil, stainless steel butter knives, brass scrap and zinc coated nails). Immersing them, two at a time, into a large glass salad bowl filled with saltwater, while using a voltmeter to measuring the voltage produced across each set of two immersed electrodes. Somehow, I ended up with two approximately identical pieces of copper wire immersed in the saltwater solution when I observed something funny. With one piece of copper wire positioned some distance above the other within the saltwater solution, I observed that a measurable voltage (several millivolts or less) appeared across the two immersed pieces of copper wire. However, with the two approximately identical pieces of copper wire positioned at equal depths within the solution, I observed no measurable voltage. At that time, I did not understand what I was looking at (‘not a clue’ as to what it meant), and I could not find any information about what I was doing or the results I was getting, so I figured my observation was unimportant. I set all of it aside and moved on with my life. In some ways, I regret giving up so easily, but on the other hand, at that time, I had nowhere to go with my observation. I would take many years for my “little gray cells” (as Agatha Christie's Poirot would say) to dream up a next step.
TEETER-TOTTER: Sometime around 2003 or 2004, I decided to reexamine my 1970s observation. In 2005, I got an idea for an apparatus to test my 1970’s observations. I made two “L” shaped copper electrodes from two 12-inch lengths of solid insulated AWG#12 copper wire (see below). The longer vertical portion of each of the two “L” shaped electrodes comprise a 10-inch length of said solid insulated AWG#12 copper wire. The vertical portion of each of the two “L” shaped electrodes was isolated from the liquid solution by its plastic insulating jacket. The shorter horizontal portion of each of the two “L” shaped electrodes comprise a 2-inch curved length of bare (striped) solid AWG #12 copper wire in direct contact with the liquid solution.
I also built a sort of ‘teeter-totter’ type ‘electrode holder mechanism’ that held the two electrodes motionless within the solution (see below). The ‘teeter-totter’ feature of the holder mechanism allowed me to raise one electrode towards the upper portion of the solution while at the same time lower the other electrode towards the lower portion of the solution, wherein said raising and lowering produced minimal agitation of the solution, and wherein both electrodes remain immersed in the liquid solution at all times. The ‘teeter-totter’ mechanism also featured a ‘compound movement’ that allowed the two electrodes to be moved up and down within the liquid solution more or less in straight lines.
I attached the electrodes to the teeter-totter mechanism and filled 500ml and 10000ml laboratory beakers with various electrolyte solutions (saltwater, CLR, liquid plumber, Brasso™ metal polish, Epsom salt, well you get the idea). WARNING, do not mix household chemicals together; mixing the wrong chemicals together could have disastrous results. I placed a full beaker on the teeter-totter mechanism as shown, attached a voltmeter (not shown) to the two electrodes, and observed that with the two electrodes held motionless at the same depth within the solution, no voltage appeared across the two electrodes.
Figures 1 above depict the basic working principle of a teeter-totter setup. Figure A on the left depicts a top view and Figure B on the right depicts a front view. I built my teeter-totter setup out of ½-inch diameter iron pipe and fittings and I secured it to a wooden base using ½-inch floor flanges. The vertical movement of the two electrodes is more or less in straight lines.
Then I used the ‘teeter-totter’ feature of the electrode holder mechanism to raise one electrode and lower the other and vice versa within the various electrolyte solutions. Each different electrolyte solution yielded a different result. I observed that many times with one electrode held motionless in the upper portion of the solution and the other electrode held motionless in the lower portion of the solution, a cell voltage of a few millivolts appeared across the two electrodes. I also observed that the amount of measured cell voltage across the two electrodes was proportional to the vertical distance separating the two electrodes. The greater the vertical distance between the two electrodes the greater the observed voltage, the lesser the vertical distance between the two electrodes the lesser the observed voltage.
Perhaps a future experiment could study the dependant change in cell voltage as a function of the independent change in vertical separation between the two electrodes for each of the various electrolytes. Additionally, future experiments could study the resulting cell voltage as a function of various electrode metal species such as lead wire, pencil lead graphite, aluminum foil, stainless steel butter knives, brass scrap and zinc coated nails. WARNING, be very careful when immersing metals into solutions; immersing the wrong metal into the wrong solution could have disastrous results. The rule for these experiments is that the two electrodes must always be identical metal species.
Now skipping forward to 2017, the following experiment is the 2017 introductory version adapted from previous versions. This version is relatively easy to construct, mostly out of commonly available items.
§ A liquid container such as an iced tea pitcher to prepare the cupric chloride solution,
§ Measuring cup graduated in milliliters to measure the proper proportion of distilled water,
§ Weigh scale graduated in grams to measure the proper proportion of cupric chloride,
§ 100 grams of ‘Cupric Chloride Dihydrate’ (AKA Copper II chloride Dihydrate), formula CuCl2∙2H2O, Molecular Weight 170.48 g/mole, Source Science Company, CAT NO. -NC-2010, $12.50 at the time of this writing. The quantity of 100 grams of ‘Cupric Chloride Dihydrate’ herein specified is based on one 12oz. capacity iced tea glass. Make alterations as necessary for your circumstances.
§ A tall slender Iced Tea Glass,
§ 2 feet of American Wire Gauge (AWG) number 12 insulated solid copper wire, available at the electrical department of any home improvement store,
§ One gallon of distilled water, available at most any drug store,
§ Mineral oil, available at most any drug store,
§ A Scotch-Brite™ scrubbing pad, for scrubbing the working surfaces of the electrodes (do not use steel wool),
§ Assorted rubber bands or vinyl electrical tape for securing the electrode assemblies to the drinking glass,
§ Digital voltmeter with a DC millivolts range scale, test leads and alligator clip adaptors,
§ One 10,000-ohm 1/8 watt resistor, source Allied Electronics, Vishay Dale RN55D1002FB14, Allied Stock#: 70024004.
§ Other miscellaneous items as needed,
§ Document your experiments and results.
Note, I also used electrolyte solutions of copper II sulfate and copper II nitrate but the results were disappointing. Perhaps I used the wrong molar concentrations or perhaps or something else was not right. Irrespective of the cause, there is plenty of room for further experimentation, research, improvements and discoveries. Perhaps someone else might get better results.
ASSEMBLING A CELL:
Figure 2 depicts the anode assembly ‘52’and the cathode assembly ‘51’ made from lengths of American Wire Gauge (AWG) number 12 insulated solid copper wire, available at the electrical department of any home improvement store. The cathode assembly ‘51’ comprises cathode terminal ‘2’, cathode lead ‘3’ and the cathode working surface ‘4’, and the anode assembly ‘52’ comprises anode terminal ‘5’, anode lead ‘6’ and the anode working surface ‘7’. Shape the two electrode assemblies as shown above. The dimensions of the various elements of the electrode assemblies will depend on the type and size of drinking glass used. Note the red color of cathode lead ‘3’and the black color of anode lead ‘6’ are more ornamental than functional; the colors indicate electrical polarity only. Make cathode working surface ‘4’ and the anode working surface ‘7’ the same diameter.
Figure 3 depicts a top view of the anode assembly ‘52’ comprising anode terminal ‘5’ anode lead ‘6’ and the anode working surface ‘7’, and of the cathode assembly ‘51’comprising cathode terminal ‘2’, cathode lead ‘3’ and the cathode working surface ‘4’. Make the diameters of the electrodes working surfaces ‘7’ and ‘4’ the same diameter and just small enough to fit inside the drinking glass. The exact dimensions of the electrode assemblies are not critical, study the shapes and the functions of the various elements of each cell and apply them to your situation. Some imagination required.
Figure 4 depicts the placement of electrode assemblies 51’ and ‘52’ around the lip of drinking glass ‘1’ and the placement of the anode working surface ‘7’ and the cathode working surface ‘4’ within drinking glass ‘1’.
Figure 5 depicts partial test cell ‘98’ comprising drinking glass ‘1’, electrode assemblies ‘51’ and ‘52’ held in place by a first rubber band ‘13’. The first rubber band ‘13’ alone is not enough to stabilize electrode assemblies ‘51’ and ‘52’ see Figure 6. Alternatively, vinyl electrical tape could be used instead of the rubber band.
Figure 6 depicts a complete but dry test cell assembly ‘99’ comprising drinking glass ‘1’ and electrode assemblies ‘51’ and ‘52’ held in place by a first rubber band ‘13’ and a second rubber band ‘16’. Alternatively, you could use vinyl electrical tape instead of rubber bands ‘13’ and ‘16’.
PREPARING A CELL FOR TESTING
While this is a science, there is also an element of art to achieving good performance. The following preparation instructions also include the art, such as using only distilled water to make the cupric chloride solution and, prior to use, washing all items with warm soapy tap water, then ‘first-rinse’ with fresh clean cold tap water, and then ‘final-rinse’ with fresh clean distilled water.
When preparing the cupric chloride solution wear appropriate ‘Personal Protection Equipment’ (PPE) such as a full-face shield, chemical resistant gloves, a chemical resistant apron, and a well ventilated work area. WARNING the chloride fumes given off by the cupric chloride crystals are a respiratory system irritant at low levels and toxic at high levels.
PREPARING THE ELECTROLYTE SOLUTION -Use ‘cupric chloride dihydrate’ (AKA copper II chloride dihydrate), formula CuCl2∙2H2O, Molecular Weight 170.48 g/mole. Do not use the anhydrous cupric chloride because combining with water may cause heat from the exothermic reaction. Always remember, safety first and fun second. First, wash all items used to prepare and store the electrolyte solution with warm soapy tap water, then ‘first-rinse’ all items with fresh clean cold tap water, then ‘final-rinse’ all items with fresh clean distilled water.
In a properly prepared container dissolve 100 grams of cupric chloride dihydrate into 400 milliliters of room temperature distilled water, do not use tap water. After the cupric chloride has completely dissolved into the distilled water, further add enough distilled water to make a total volume of 586.57 milliliters. This makes enough solution to fill one 12 Oz. (354.882 milliliter) drinking glass, with a little spare solution left over if needed.
PREPARING THE DRINKING GLASS: Wash the inside of the chosen iced tea glass with warm soapy tap water, then ‘first-rinse’ with fresh clean cold tap water, and then ‘final-rinse’ with fresh clean distilled water.
PREPARING THE ELECTRODES: Now referring to figures 2 and 3, to prepare the electrode assemblies ‘51’ and ‘52’ for use, scrub the working surfaces ‘4’ and ‘7’ of electrode assemblies ‘51’ and ‘52’ with a Scotch-Brite™ scrubbing pad until shiny. Do not use steel wool due to the risk of contaminating the working surface with particles of the steel wool. Then wash both electrode assemblies with warm soapy tap water and then ‘first-rinse’ both electrode assemblies with fresh clean cold tap water, and then ‘final-rinse’ both electrodes assemblies with fresh clean distilled water.
Figure 7A depicts a test cell subassembly ‘100’ comprising iced tea glass ‘1’ filled with a 1–mole concentration of cupric chloride ‘30’. Very important, to prevent the evaporation of the cupric chloride solution ‘30’ and the creeping of the cupric chloride salt, float a thin layer of mineral oil ‘14’ on top of the cupric chloride solution ‘30’. The thin layer of mineral oil ‘14’ can be see through the cutaway view of rubber band ‘16’. The old mineral oil trick has been used for almost two hundred years. Caution, mineral oil can be messy so use care and be sure to add the mineral oil after the completion of all the steps so far.
Figure 7B depicts a complete test cell assembly ‘101’comprising iced tea glass ‘1’ filled with electrolyte ‘30’and a piece of plastic cling type food wrap (Saran Wrap™ or equivalent) ‘31’ gently wrapped around the top as shown. Very important, in order to provide a protective barrier to prevent dust, dirt, debris and other foreign objects from falling into the glass, gently place a piece of plastic cling type food wrap (Saran Wrap™ or equivalent) tightly around the top of the glass. Use a 4 cup paper coffee filter as a pattern or template to help you cut a circular piece of plastic cling wrap, then, discard the paper coffee filter, and gently form the circular piece of plastic cling wrap over and around the top of the cell as shown. Be careful not to shake the iced tea glass excessively, or excessively disturb the electrode assemblies.
Figure 8 depicts a test assembly ‘201’ comprising cell under test ‘101’, digital voltmeter ‘20’, and load resistor ‘21’connected to cell under test ‘101’. Digital voltmeter positive (red) lead ‘18’ is connected to the cathode terminal ‘2’ by way of alligator clip ‘24’ and digital voltmeter negative (black) lead ‘19’ is connected to anode terminal ‘5’ by way of alligator clip ‘25’. Test assembly ‘201’ further comprising first load resistor lead ‘20’ of 10,000 ohm load resistor ‘21’ is connected to the cathode terminal ‘2’ by way of alligator clip ‘26’ and second load resistor lead ‘22’ of 10,000 ohm load resistor ‘21’ is connected to the anode terminal ‘5’ by way of alligator clip ‘27’.
CELL TESTING PROCEDURE:
Assemble test assembly ‘201’ as depicted in Figure 8. Turn on digital voltmeter ‘20’ and select the millivolts range. Measure and document the cell voltage and then turn off and disconnect the digital voltmeter ‘20’. Continue to run the experiment for as long as the cell generates cell voltage. Measure and document the cell voltage at least once every week. After your experiment is over, dissemble the cell, observe and document the appearance of the electrodes, and document any differences in the appearance between the two electrodes.
HOW IT WORKS:
First consider the “Concentration Cell: a galvanic cell in which the chemical energy converted into electrical energy is arising from the concentration difference of a species at the two electrodes of the cell. Source Corrosion Doctors, please refer to this short tutorial for more information.
Now consider Electrolyte Stratification, “As you know, lead acid battery electrolyte is a mixture of water and sulfuric acid. Sulfuric acid is heavier than water. So, when the battery is not in use, the acid tends to settle down at the bottom of the cell,” Source UPS Battery Center.
Next, consider Electrolyte stratification in a tall iced tea glass filled with a cupric chloride solution and using two copper electrodes. The electrolyte stratification comprises a greater than average concentration of heavier than water dissolved copper ions displaced by weight downward towards the lower portion of the solution, leaving a lesser than average concentration of heavy dissolved copper ions displaced upward towards the upper portion of the solution. This downward displacement of heavy dissolved copper ions creates a vertical concentration gradient of copper ions comprising a greater than average concentration of heavier than water dissolved copper ions in contact with the lower electrode and a lesser than average concentration of heavier that water dissolved copper ions in contact with the upper electrode.
From the point of view of the second law of thermodynamics, having two different concentrations of dissolved copper ions in the same container is a highly non-random situation. Consequently, the system will attempt to restore randomness by diffusing the lower greater than average concentration of heavier than water dissolved copper ions upward into the upper lesser than average concentration of copper ions, thus forming a uniform concentration of dissolved copper ions throughout the bulk volume of the solution. However, the weight of the heavier that water copper ions acts to prevent this upward diffusion, thus preventing the formation of a uniform concentration, and acting instead to maintain the electrolyte-stratification over time.
The two copper electrodes immersed in the copper II chloride solution offer an alternative method of achieving the same end. On the surface of the upper electrode, oxidation reactions release heavier than water copper ions into the upper solution, thus increasing the local copper ion concentration within the copper ion poor upper portion of the electrolyte solution. On the surface of the lower electrode, reduction reactions plate out heavier than water copper ions out of the lower solution onto the surface of the lower electrode, thus decreasing the local copper ion concentration within the copper ion rich lower portion of the electrolyte solution.
By increasing the copper ion concentration of the copper ion poor solution while at the same time decreasing the copper ion concentration of the copper ion rich solution, the system seems to be achieving its goal of forming a uniform concentration of dissolved copper ions throughout the bulk volume of the electrolyte solution. However, the continuous downward settling of newly released heavier than water copper ions, released from the upper copper electrode due to oxidation reactions, continuously restores the electrolyte stratification. This action continuously decreases the local copper ion concentration in the upper copper ion poor portion of the electrolyte solution. While at the same time continuously increases the local copper ion concentration in the lower ion rich portion of the electrolyte solution, thus continually restoring (or recharging) the cell back to its original electrolyte stratification.
The electrons produced by oxidation reactions at the upper copper electrode flow out from the cell and through the external load resistor and return to the cell at the lower copper electrode to be used in the reduction reactions. The system wants to achieve randomness (uniform distribution of copper ions) strongly enough that it will give the electrons sufficient push (the cell voltage) to push electrons (do electrical work) through the external electrical load resistance.
The system transports copper mass from the upper electrode to the lower electrode. At the upper electrode, copper material dissolves into solution, and at the lower electrode dissolved copper material plates out of solution. As copper material dissolves into solution, the upper electrode looses mass. As copper material plates out onto the lower electrode, the lower electrode gains mass. Theoretically, when the upper electrode looses enough mass that its performance begins to decline, inverting the entire cell restores the electrodes to roughly their original state.