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A Novel optical display device
Journal ofMembmne Sczence, 1(1976) 311-312 Q Elsevier Scientific Pubhshing Company, Amsterdam - Prmted m The Netherlands
311
Short Communication A NOVEL OPTJCALDISPLAY DEVICE M.E. HEYDE and J.E. ANDERSON
Sczentsfic Research Staff, Ford Motor Company, Dearborn, Mzchagan 48121 (US A ) (Received February l&1976)
Wehaveexamined* a novel optical displaybased on a membrane,a pH indicator, and the passageof electric current.The device, shown schematicallyin Fig. 1, consists of two chambersdivided by a membrane. Each chamber contains an electrode and is filled with an aqueous solution containing both an electrolyte and the pH indicator. Passageof (It) coulombs increasesthe hydrogen ion concentration, [H'] , of one chamber by (It/FVi). It lowers [H+] in the other chamber by (WFV,). I is the current (amperes), t is time (s), V, and V, are the volumes of the two chambers, and F is Faraday’sconstant. The pH indicator changescolor when [H+] exceeds a well-definedvalue. Any further [H’] increasehas no effect on the color or its intensity.
TO EXTERNAL
O.C. SOURCE
=7zzJ
MEMSRAl’iE
CLAMPED
Of BALL JOINT
BETWEEN
HALVES
( CLAMP NUT StlOWN
)
Fig 1. Schematic representation of the optlcal display device. Solutions marked “A” were 1.0 N KNO, Baturated with phenolphthalein.
typically
* J.R. Anderson, U.S. Patent No. 3.303.488 describes a display that is essenmy the same aa the one deacnbed here. Although he clearly understands the chermcal principles underlying the membrane cell, they are not set forth quantitatively in the patent. We became aware of thii patent only recently, and made our experiments independently of the earlier results.
312
Usingphenolphthaleinmdicator together with two 175 ml chambers and platinum electrodes, we produced substantialcolor changes(colorless to red) upon passageof 0.38 coulombs at 50 ma. The color disappearswhen current is passed with opposite polarity. The cell seems fully reversible(40-50 cycles of testing). Switchingtimes are very slow relativeto solid state devices, typically 10 s with our test cell. However, once the device is activated,the color change persistsfor hours without drawmgadditional current. One could anticipate switchingtimes of 0.1-l s if the chamber volumes and inter-electrodedistance were reduced and d the electrode surface area were increased.Note that reducmg the chamber volume will also reduce (It/F), the net charge that must be transportedto produce a color change. Kxperimentxverify that color intensityis directly proportional to indicator concentration in solution. We carried out a limited number of experiments with other pH indicatorsand found similarresults.Of course, the colors involved in the activation/deactivationprocess depend upon the particularindicator used. The gradientof [H’] tends to disappear,via slow diffusion, on standing. Consequently,it is useful to keep the pH of the cell near the pKa of the indicator. Not only does this minimize the net chargethat must be transported, (It/F), but it also reduces the drivingforces responsiblefor H’ diffusion after activation. Shifts in the equilibrium [H+] are made by adding acid or base to the system. The nature of the membraneand the electrolyte are not crucial. Both must be chosen to reduce electricalresistancebetween the electrodes. We used 0.1 and 1.0 N KNOBin our experiments: the cation and anion transferencenumbers of this salt in water are approximately equal. Dialysistubing was used as a membraneafter largeelectricalresistances were encountered with nonionic desalinationmembranes. The principal deficiency with the present apparatus18its use of non-reversible electrodes. Chemicalreactionsproduce gaseousO2 at one electrode and gaseous H2 at the other. This would preclude its use in most applications. However, it seems naturalto extend the basic concept to reversibleelectrodes and oxidation-reductionindicators. Preliminaryexperimentsalong these lines met with only limited success owing to (a) weak optical contrast between oxidized and reduced forms of common indicators; (b) irreversiblereactions and precipitation; (c) the relativeinsensitivityof redox indicatorsto ionic concentrations (with respect to pH indicators). A device based on this principle could be inexpensivelyconstructed. An arrayof such devices could provide, for example, long-lastingvisualmessageswithout consumingenergy.
311
Short Communication A NOVEL OPTJCALDISPLAY DEVICE M.E. HEYDE and J.E. ANDERSON
Sczentsfic Research Staff, Ford Motor Company, Dearborn, Mzchagan 48121 (US A ) (Received February l&1976)
Wehaveexamined* a novel optical displaybased on a membrane,a pH indicator, and the passageof electric current.The device, shown schematicallyin Fig. 1, consists of two chambersdivided by a membrane. Each chamber contains an electrode and is filled with an aqueous solution containing both an electrolyte and the pH indicator. Passageof (It) coulombs increasesthe hydrogen ion concentration, [H'] , of one chamber by (It/FVi). It lowers [H+] in the other chamber by (WFV,). I is the current (amperes), t is time (s), V, and V, are the volumes of the two chambers, and F is Faraday’sconstant. The pH indicator changescolor when [H+] exceeds a well-definedvalue. Any further [H’] increasehas no effect on the color or its intensity.
TO EXTERNAL
O.C. SOURCE
=7zzJ
MEMSRAl’iE
CLAMPED
Of BALL JOINT
BETWEEN
HALVES
( CLAMP NUT StlOWN
)
Fig 1. Schematic representation of the optlcal display device. Solutions marked “A” were 1.0 N KNO, Baturated with phenolphthalein.
typically
* J.R. Anderson, U.S. Patent No. 3.303.488 describes a display that is essenmy the same aa the one deacnbed here. Although he clearly understands the chermcal principles underlying the membrane cell, they are not set forth quantitatively in the patent. We became aware of thii patent only recently, and made our experiments independently of the earlier results.
312
Usingphenolphthaleinmdicator together with two 175 ml chambers and platinum electrodes, we produced substantialcolor changes(colorless to red) upon passageof 0.38 coulombs at 50 ma. The color disappearswhen current is passed with opposite polarity. The cell seems fully reversible(40-50 cycles of testing). Switchingtimes are very slow relativeto solid state devices, typically 10 s with our test cell. However, once the device is activated,the color change persistsfor hours without drawmgadditional current. One could anticipate switchingtimes of 0.1-l s if the chamber volumes and inter-electrodedistance were reduced and d the electrode surface area were increased.Note that reducmg the chamber volume will also reduce (It/F), the net charge that must be transportedto produce a color change. Kxperimentxverify that color intensityis directly proportional to indicator concentration in solution. We carried out a limited number of experiments with other pH indicatorsand found similarresults.Of course, the colors involved in the activation/deactivationprocess depend upon the particularindicator used. The gradientof [H’] tends to disappear,via slow diffusion, on standing. Consequently,it is useful to keep the pH of the cell near the pKa of the indicator. Not only does this minimize the net chargethat must be transported, (It/F), but it also reduces the drivingforces responsiblefor H’ diffusion after activation. Shifts in the equilibrium [H+] are made by adding acid or base to the system. The nature of the membraneand the electrolyte are not crucial. Both must be chosen to reduce electricalresistancebetween the electrodes. We used 0.1 and 1.0 N KNOBin our experiments: the cation and anion transferencenumbers of this salt in water are approximately equal. Dialysistubing was used as a membraneafter largeelectricalresistances were encountered with nonionic desalinationmembranes. The principal deficiency with the present apparatus18its use of non-reversible electrodes. Chemicalreactionsproduce gaseousO2 at one electrode and gaseous H2 at the other. This would preclude its use in most applications. However, it seems naturalto extend the basic concept to reversibleelectrodes and oxidation-reductionindicators. Preliminaryexperimentsalong these lines met with only limited success owing to (a) weak optical contrast between oxidized and reduced forms of common indicators; (b) irreversiblereactions and precipitation; (c) the relativeinsensitivityof redox indicatorsto ionic concentrations (with respect to pH indicators). A device based on this principle could be inexpensivelyconstructed. An arrayof such devices could provide, for example, long-lastingvisualmessageswithout consumingenergy.