chemical principles exemplified V
Sparkling Pure Water-in
a3wimming Pool
illustratingprinciples of enzyme inhibition
Most of us are familiar with swimming pool water that is loaded with chlorine-it kills the hacteria but burns the eyes. An inadvertant mouthful or noseful can he distressing. NASA has developed methods for making waste water in a spacecraft potable using an alternative to chlorination, and commercial firms are now developing a related technoloev for keeoinz . the water in swimmine-.nools of drinkahle quality. This new technoloev uses silver ion as a bactericide. Drinking water may c&in up to 50 pph of silver with no ill effects and without being detectable to the taste-yet I S 3 0 pph of silver is sufficient to prevent the growth of hacteria, algae, and other microorganisms! Silver ion is so effective because it acts right where the action will do the most good, at the enzme level in microorganisms, disrupting their life processes. The process by which silver acts on the enzymes is an example of noncompetitive inhibition, The enzymes contain cysteine residues which have thiol groups. Silver can combine reversibly with the thiol groups to form silver mercaptides. E-SH + Agt = E-S-Ag + Hi
These mercaptides inhibit the normal function of the enzvme: - ,thev .mav. he located a t the active catalvtic site itself or may he some distance away, interfering with the threedimensional conformation of the enzvme molecule. Some other heavy metal ions, notahly cu2+-and Hg2+ can function in the same way; Cu2+ finds use in controlling growths of algae in lakes but neither Cu2+ or Hg2+ are very desirable comoonents of water which needs to he fit for drinking. Devices for silver.ion purification of swimming pools are now available. In S e ~ t e m h e r1973. as a ~romotional stunt a t the Denver ~ i l t o ndirectors , of the ~il;er Institute and their guests used swimming pool water as a mixer in their refreshments (silver treated of course, not chlorinated) and claimed it was better than tap water. How can one design a system which will maintain a hactericidal concentration of silver ion in swimming pool water? Two alternatives are considered in the following, both based upon fundamental chemical principles.
viding silver ions a t a hactericidal concentration. The principles of the operation of the device are intriguing hut i t would seem that there mav .he .~racticalobstacles to its successful operation. First the Princinles: The electrolvtic dissolution of silver from one electrode and deposition onto another electrode is a thermodynamically reversible process, not complicated hy overvoltages, so that a small electric potential difference between a silver anode and a silver cathode will cause the dissolution reaction to take place. Water has to he pumped through a swimming pool filter circuit anyway. Suppose one imposes a magnetic field across a segment of the filter circuit; then ions in the flowing water will be deflected producing an electric potential difference from one side of the liquid stream to the other. This electric field will he perpendicular to the direction of the magnetic field. The generation of the voltage is somewhat analogous to the Hall effect of solid state science. Now if one placed a piece of silver metal as an anode in contact with one side of the liquid stream, and another as a cathode on the other side and joined them by an external wire, the electric potential difference in the solution would cause the electrolysis to take place. Although the elementary theoretical picture of this device and its operation appear to he straightforward, there could he some interesting practical problems in its operation. How large is the electric field produced by the flow of reasonably pure water through an achievable magnetic field-and how rapidly could the electrolysis reaction be made to proceed? The device is supposed to supply silver ions to the pool water, not just dissolve silver a t the anode and plate it back on the cathode. Thus, if the silver ion produced at the anode is sweDt awav what alternative reaction occurs a t the cathode? Wouldn't alternative cathodic reactions such as hvdroeen evolution he suhiect to overvoltazes . and polarization, stopping the generation of silver ions?
-
An Elementary Approach Illustrating Principles of Chemical Equifibrium
If one wants to maintain the concentration of silver in a swimming pool a t a fixed value, say 30 pph, why not saturate the water with a sparingly soluble salt of silver? Then as the silver ion is used up in its bactericidal role more of the silver salt will dissolve. Here is an interesting practical K,, prohlem for your students to work on:
An Approach Using Electrolysis without an Applied Voltage lllustrating Principles of Electrochemistry
If a metallic conductor is moved through a magnetic field, a voltage will he generated-as in an electric generator. Recently a patent has heen granted [US Patent 3,522,162) which claims that a solution (a conductor) moved through a magnetic field will cause a voltage to he generated and that this voltage can be used to drive electrolysis reactions. Devices using this principle have been marketed for swimming pool purification. Swimming pool water is pumped through a magnetic field and the voltage produced is claimed to prompt an e:ectrolysis reaction pro-
Ten to thirty ppb (parts per billion) of silver ion (MW 107.9 g mol-') is adequate to keep swimming p o d water pure but the eoncentration of silver ion should not exceed a few hundred ppb for safety to the swimmers. Which of the following sparingly soluble salts of silver will provide the right concentration of silver ion when a pool is saturated with that salt? Salt
AgCb) AgBr(sl AgUs)
K,,>
1.3 X 4.1 x 1 0 - 1 ~ 1.1 x lo-'"
The answer is AgBr, as the reader will no doubt verify; it falls right in the correct range for effective water purificaVolume 53, Number 3. March 1976 / 189
tion. In preliminary prototypes of water purification devices NASA put 1000 ppb of silver ions in the water using silver chloride, then removed the silver with ion exchange resin and reintroduced 50 pph to maintain purity. I t has now been found that 50 ppb, obtained with silver bromide,
190 / Journal of Chemical Education
is adequate for the entire task and the three step process has been reduced to one step. T o saturate a swimming pool with silver bromide one needs a large surface area of silver bromide such as a large charcoal filter in which a small amount of silver bromide has been uniformly precipitated.
Sparkling Pure Water-in
a3wimming Pool
illustratingprinciples of enzyme inhibition
Most of us are familiar with swimming pool water that is loaded with chlorine-it kills the hacteria but burns the eyes. An inadvertant mouthful or noseful can he distressing. NASA has developed methods for making waste water in a spacecraft potable using an alternative to chlorination, and commercial firms are now developing a related technoloev for keeoinz . the water in swimmine-.nools of drinkahle quality. This new technoloev uses silver ion as a bactericide. Drinking water may c&in up to 50 pph of silver with no ill effects and without being detectable to the taste-yet I S 3 0 pph of silver is sufficient to prevent the growth of hacteria, algae, and other microorganisms! Silver ion is so effective because it acts right where the action will do the most good, at the enzme level in microorganisms, disrupting their life processes. The process by which silver acts on the enzymes is an example of noncompetitive inhibition, The enzymes contain cysteine residues which have thiol groups. Silver can combine reversibly with the thiol groups to form silver mercaptides. E-SH + Agt = E-S-Ag + Hi
These mercaptides inhibit the normal function of the enzvme: - ,thev .mav. he located a t the active catalvtic site itself or may he some distance away, interfering with the threedimensional conformation of the enzvme molecule. Some other heavy metal ions, notahly cu2+-and Hg2+ can function in the same way; Cu2+ finds use in controlling growths of algae in lakes but neither Cu2+ or Hg2+ are very desirable comoonents of water which needs to he fit for drinking. Devices for silver.ion purification of swimming pools are now available. In S e ~ t e m h e r1973. as a ~romotional stunt a t the Denver ~ i l t o ndirectors , of the ~il;er Institute and their guests used swimming pool water as a mixer in their refreshments (silver treated of course, not chlorinated) and claimed it was better than tap water. How can one design a system which will maintain a hactericidal concentration of silver ion in swimming pool water? Two alternatives are considered in the following, both based upon fundamental chemical principles.
viding silver ions a t a hactericidal concentration. The principles of the operation of the device are intriguing hut i t would seem that there mav .he .~racticalobstacles to its successful operation. First the Princinles: The electrolvtic dissolution of silver from one electrode and deposition onto another electrode is a thermodynamically reversible process, not complicated hy overvoltages, so that a small electric potential difference between a silver anode and a silver cathode will cause the dissolution reaction to take place. Water has to he pumped through a swimming pool filter circuit anyway. Suppose one imposes a magnetic field across a segment of the filter circuit; then ions in the flowing water will be deflected producing an electric potential difference from one side of the liquid stream to the other. This electric field will he perpendicular to the direction of the magnetic field. The generation of the voltage is somewhat analogous to the Hall effect of solid state science. Now if one placed a piece of silver metal as an anode in contact with one side of the liquid stream, and another as a cathode on the other side and joined them by an external wire, the electric potential difference in the solution would cause the electrolysis to take place. Although the elementary theoretical picture of this device and its operation appear to he straightforward, there could he some interesting practical problems in its operation. How large is the electric field produced by the flow of reasonably pure water through an achievable magnetic field-and how rapidly could the electrolysis reaction be made to proceed? The device is supposed to supply silver ions to the pool water, not just dissolve silver a t the anode and plate it back on the cathode. Thus, if the silver ion produced at the anode is sweDt awav what alternative reaction occurs a t the cathode? Wouldn't alternative cathodic reactions such as hvdroeen evolution he suhiect to overvoltazes . and polarization, stopping the generation of silver ions?
-
An Elementary Approach Illustrating Principles of Chemical Equifibrium
If one wants to maintain the concentration of silver in a swimming pool a t a fixed value, say 30 pph, why not saturate the water with a sparingly soluble salt of silver? Then as the silver ion is used up in its bactericidal role more of the silver salt will dissolve. Here is an interesting practical K,, prohlem for your students to work on:
An Approach Using Electrolysis without an Applied Voltage lllustrating Principles of Electrochemistry
If a metallic conductor is moved through a magnetic field, a voltage will he generated-as in an electric generator. Recently a patent has heen granted [US Patent 3,522,162) which claims that a solution (a conductor) moved through a magnetic field will cause a voltage to he generated and that this voltage can be used to drive electrolysis reactions. Devices using this principle have been marketed for swimming pool purification. Swimming pool water is pumped through a magnetic field and the voltage produced is claimed to prompt an e:ectrolysis reaction pro-
Ten to thirty ppb (parts per billion) of silver ion (MW 107.9 g mol-') is adequate to keep swimming p o d water pure but the eoncentration of silver ion should not exceed a few hundred ppb for safety to the swimmers. Which of the following sparingly soluble salts of silver will provide the right concentration of silver ion when a pool is saturated with that salt? Salt
AgCb) AgBr(sl AgUs)
K,,>
1.3 X 4.1 x 1 0 - 1 ~ 1.1 x lo-'"
The answer is AgBr, as the reader will no doubt verify; it falls right in the correct range for effective water purificaVolume 53, Number 3. March 1976 / 189
tion. In preliminary prototypes of water purification devices NASA put 1000 ppb of silver ions in the water using silver chloride, then removed the silver with ion exchange resin and reintroduced 50 pph to maintain purity. I t has now been found that 50 ppb, obtained with silver bromide,
190 / Journal of Chemical Education
is adequate for the entire task and the three step process has been reduced to one step. T o saturate a swimming pool with silver bromide one needs a large surface area of silver bromide such as a large charcoal filter in which a small amount of silver bromide has been uniformly precipitated.
