US 3208926 A
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3 Sheets-Sheet 2 m E w V. S C M m M m U o C Filed Aug. 25, 1960 P 1965 E. 'L. ECKFELDT COULOME'PEIC SYSTEMS 3 Sheets-Sheet 3 Fig. 37c
Filed Aug. 25, 1960 3,208,926 COULOMETRIC SYSTEMS Edgar L. Eckfeldt, Ambler, Pa., assignor to Leeds and Northrup Company, Philadelphia, Pa., a corporation of Pennsylvani Filed Aug. 25, 1960, Ser. No.
19 Claims. (Cl. 204-195) This invention relates to coulometric systems of the type W erein a constituent in a sample solution is retained on the working electrode of the system.
In my Patent No. 2,621,671 there claimed coulometric systems in which the concentration of a constituent in a solution was varied by the passage mined potentiometrically.
In accordance with the present invention, the potentiometric detecting instrument of my said patent need not not be utilized. More particularly, use is made of the fact that electro-oxidizable or reducible substances pres- Thus, a given substance may be removed or its state changed by establishing between a reference electrode and a working electrode a potential difference which will produce at the working electrode the characteristic potential for that given substances. By means of this transformation, its concentration in the sample solution can be determined in terms duce the transformation.
An explanation of the reaction mechanism will bring is based and will make clear the uniqueness of the cell design features. When an electrode reaction takes place, there is a solution zone directly adjacent an electrode in which the concentration of the reacting substance becomes partially depleted as a result of electrolysis. The thickness of this solution zone is of the order of 0.010 to 0.015 inch or less. Reactive materials that lie in this zone are under the direct influence of the electrode by reason of a concentration gradient which causes diffusion of the material toward the electrode. Hence, materials in this zone will tend to react quantitatively with the electrode. Materials that are more remotely located in the the solution. Forced agitation can variety of ways, as it is a purpose of to make clear.
Another important design consideration that follows m the of the current required to pro the materials to the close vicinity 2 solution thickness throughout will about 0. 1 inch, the thickness of nated altogether, .and thus the restrictive step of bringing by avoided.
ture having one face disposed for the reference solution and the other extending lengthwise of the channel.
G. 2 1s a broadside view of the flow channel and Working electrode of the cell of FIG. 1;
FIG. 3 is an enlarged sectional view of a fractional part of the flow channel;
FIG. 4 is a fractional view of a part of the cell of FIG. 1 and illustrates a solution-agitating means for the flow channel;
FIG. 5 is a graph useful in explaining the operation of the invention; and
FIG. 6 illustrates a further embodiment of the invention, in particular a cell including two flow channels and Patented Sept. 28, 1965 I Thus, the reference'solution provides a 'line to= the flow channel, 11 forming a line 10 flow-s throug 24 within the space 23: and V the modification shown in. FIG- I comprises the conduc-- 'tion in contact with it.
'within the tube.
a'common reference electrode and a common source of reference solution.
Referring now to FIG. 1', a regulated flow of a sample solution is supplied through a line 10 to a flow channel 11. As shown, the sample solution maybe that of a liquid within a constant-head device type which, through a tuber'l'3, provides a constant head on the liquid delivered to a valve '14. A capillary tube 15 in conjunction with the device 12. produces a regulated constant flow of the sample solution through the supply part ofthe cell 18. i
As best shown in FIG. 2, the flow channel 11 has a tortuous path, that is to say; fluid entering from the inlet an outer circular path 11a, thence to a second inwardly spaced circular path 11b, and eventually through. the inmost circular 11c to an outlet line The flow channel'is'cliosed by meanstof porous wall structure 21; This wall structure forms not only a part of the wall structure of the reference-solution chamber but also part of the wall structure'of channel 11. One face of the porous wall 21 is in communication with a reference solution. 22 within the liquid space 23 and provides for transfer of that solution through There is thus 1 to the channel 11.. path between a reference electrode working electrode which in the porous structure formed a conductive tive plate 2'5 within whose face there is formed the dew channel 11. More particularly, the flow channel It. is milled into a metallic plate 25,. preferably of gold.- The metal electrode 25 is relatively: thin and, provided with a baclc'up member 26 which, in turn, clamped-against the porous member 21 by clampingv member 27 having a. plurality orifastening bolts 28 disposed about its periphery. The porous wall 21may be of fritted glass or unglazed porous ceramic material.
The cell 18 is provided with a elean-outplug 29. Through a supply line 30*, the reference solution can be supplied through a valve 31. to-the liquid space As shown, a stopper 321 for the cell? 18 has an air vent 33'. The reference electrode 24 may be of silver with. a coating of silver chloride.
accordingly, it is- 12 of the Mariette arise from the flow of cell currents. The seepage of reference solution into the porous structure 21 will keep the electrolytic conductivity of that member at a satisfactory level to avoid. objectionable potential losses in the form of IR drops in that member. It will be noted thatpipe 30.
and valve 3E are provided so that the reference solution can be added to: keep the solutionat its proper level.
The reference electrode is chosen to maintain an essentially' constant potential with respect to'thenreference As has been mentionechan objective of the present invention is a cell construction which will provide a working electrode that can" be set at a predetermined desired value of potential with respect to a flowing sample sol'u' It is desired that this objective be met even when. measurement currents are flowing through the cell. The design must also take into account the dilutivei effect. causediby the flow of the sample solution through the cell. This can be achieved as in cell 18 by bringing a reference solution into: close proximity with the region of the electrode reaction, throughout "the full extent of the electrode solution interface. Since it is nec-- essary to separatethe sample solution from the reference solution to prevent intermixing; a porous material is interposed between them. This prevents mixing of the two solutions but provid s an electrolytic" conductive path between them. The porous ceramic plate .21 achieves this function in. cell 18", but other formsof porous separators and electrode-solutionarrangements can be used. It will be noted that in cell 18 the ceramic member forms a portion of the wall of the solution channel. In some cell designs the porous member may take the form of an extended tube and through which tion. passes in contact" with a working electrode disposed The reference solution is able to maintain a constant potential when the: cell is operated because. of two important factors, as follows: (I) a reference electrode system 24 of constant potential is employed ence solution 22 and (2) the reference solution 22- ,is
tube the sample solu in the rcfer- I chosen to have good electrical conductivity thereby av0id- I 7 ing objectionable IR drop potentials that could otherwise to drain as by way of line 34. It
. above aboutv solution, even when the current flows as a result of normal operation of the cell. The electrode is of the thermodynamically reversible type such as is used for reference electrodes in making measurements. of pH. 7 In the PICS-1 ent case, however, suflicient area must be provided to avoid polarization effects arising from cell currents. The electrode 24 may be'a sheet of silver metal 1 x 3 inches covered with a coating of silver chloride. The reference electrode potential, influenced by the changes in the composition of the reference solution,
supply of this'solutionbut also by virtue of the shielding function of the porous separator plate protects against gross changes in reference solution composition.
The potential between the electrode 24 and. the reference solution 22', as has been indicated, remains constant even with current flow through the cell. Accordingly, if the voltage: as: read on: the? voltmeter 41 is varied, there willbe a corresponding change in the potential on electrode 25'. Thus, the potential of ,theworkingel'ectrode 25 may beset atany selected value co-responding with the reaction-voltage needed for removal or change of state: of the constituent to be measured. 7 j
. For convenience, the potentials hereinafter given for the working electrode 25- are thosewhich. may be readily set on the scale: of voltmeter 41.. Where positive volta ges are'given, the working electrode 25 will be made positive and where negative voltagesl are given the working electrode 25 will be made negative.
In the operation of the system, the liquid space 23 will to admit the flow of the sample line Ill-to the flow channelll. After coursing through the flow channel of extended length, the sample solution will exit'by way of the: outlet line; 20 and be discharged .will be. noted that the endof line 20 has an elevation approximating that midway of-the cell- This contributes to the uniformity of flow. A switch 36 is then closed to apply between the reference electrode 24 and. the working electrode 25 a potential of a predetermined selected magnitude This potential is. determined by the positions of contacts 37a and 380 relative to their associated resistors. ,37 and 38. These resistors and the associated contacts form a source of direct current. supply form a battery 39 connected to them byway of a rheostat 40. The resistors 37 and 38 have resistances of a relatively low order, to 200 ohmseach, for flow of current through each ofthem of a mag'nitudemany times the current flow between the electodes 2i4and 25. Thus, the resistors. 37 and $8 with their adjustable taps provide means for the adjustment of the voltage applied to the cell terminals and hence the potential of the'workingelectrode 25. p
If the sample solution from vessel 12 includes iodide, such, for example, as might be present in salt solutions, the reference solution in cell 18 will be 2 Normal potassium chloride solution- For determination of the iodide present in the sample solution, the contacts 371: and 38a will be set to positions which will produce on a voltmeter 41 a reading of 067 volt.
' 0.6 and below about 0.7 will be satisfactory, since 'the voltages within this. range singularly represent the characteristic value of the potential ,at which the iodide will react at the working cl'cctrodc 2'5.' The foregoing potential is in terms of the use of a silver, silver chloride electrode for the reference electrode 24.. Tough the workstream by way of inlet is maintained relatively 1 constant. .The cell design not only ensures an adequate A voltage within the range ofing electrode has been described as of gold, it may be of other materials inert to the sample solution and to the iodide present therein. It should also be inert to the electro-oxidizing effects due to the applied potential differences.
With a sample solution flowing through the flow channel at the rate of one milliliter per minute and the sample solution comprising 1X 10* N potassium iodide disolved in 2 molar sulphuric acid, the cell current was approximately 0.16 milliamperes. This current flowing through voltmeter 41 of +0.06, though the permissible range here is relatively wide, as
In connection with the foregoing, the positive and negative voltages are in reference to the polarity on the conductor connected to the working electrode 25. Inasmuch as the foregoing range clearly shows the cell will be operative with zero voltage existing between the electrodes 24 and More particularly, the reaction occurring at the reference electrode will be as follows:
Ag+Cl- AgCl+e* (1) The reaction at the working electrode is:
From these reactions, it will be seen that electrons will be released or given off at the reference electrode 24,
From the foregoing equations, it will be seen that the calibration may be made in terms of the magnitude of the electrolysis current and in accordance with the following equation based upon Faradays law:
I -m where N is the sample solution normality,
I is the electrolysis current, and R is the flow rate of the sample solution in liters per second.
Now that the invention has been described in connection with two constituents, it is to be electrode or a saturated calomel electrode.
characteristic values of potentials for the working electrode.
Table I.Typical cell voltages for coulometric reduction of various substances Substance Voltage a 32: u++ Pb++ -O.72 Cd" -0.94 Zn++ 1.30
Table I gives a listing of typical cell voltages for coulometric reduction of a number of substances contained in chloride solutions. The values are predicated on using as the reference electrode 24 either a 2 N silver chloride should be realized that the mate only, since the potential at which a substance reacts will be determined by particular conditions especially by the anion content of the solution. Furthermore, measuring a given substance good operation will occur over a moderate range of voltage. In the case of reductions, it is only necessary to be sure the voltage is more negative than the critical value for the substance to be at higher potentials.
The reference electrode 24, electrode, may take the form of a saturated calomel electrode.
calomel. potassium chloride, but its concentration would be increased to about 4 N.
In the preferred form of the invention, the surface area of the working electrode 25 is increased or extended as by providing a roughened surface channel 11.
tortuous passage with finely divided particles of uniform size and which may be in the pellets of the same material as the electrode.
its mass, has a ratio. correspondwool and finely divided metallic 45 operates through with'the arrangement described above,
ing the operation of the cell 18. This has been accomplished by applying a vibrator to the outlet tube 20,"shown in FIG. 1 as a flexible tube: A vibrator acting on tube 20 causes small pulses of back pressure through the tube 20 into the channel-11, thus producing agitation of the liquid within that channel. There has been shown in FIG. 4 a preferred arrangement for producing agitation of the liquid within the flow channelll.
a rod 46, a diaphragm'47t one side flexible wall of a channel passage 48 connecting with the flow channel '11. The to-and-fro movement of the diaphragm 47 produces throughoutthe flow channel 11 a to-and-fro movement of the liquid therein, thus increasing the efficiency of the operation of the cell.
Though in general it is preferred to utilize a flowing stream of the sample solution through the flow channel 11, nevertheless, the arrangement permits a batch type of operation. Thus, after the opening of the valve l4 and the filling of the flow channel 11,'that valve may be closed and the operation initiated as before by closing the switch 36. For the batch type of 'operation, the recorder 44 will apply its output to an integrator 50. The integrator will provide a reading on its scale" 50a corresponding with the integral with of the current flowing through the series resistor 42. Thus, the total current flowing between electrodes 24 and25 will on the integrator produce an indication representative of the quantity of the constituent in the sample solution which has been removed, v
There has also been emphasized the fact that the reaction zone formed by the'tortuous path of the solution adjacent to electrode25 contributes to the efficiency of the reactions which take place. Though the actual dimensions of the fiow channel 11 are not critical, neverthelesspthe thickness of the solution layer adjacent to the electrode should ample, in one embodiment of the invention, the groove formingthe flow channel 11 had the following approximate dimensions: width 0.25 inchand depth-0.015 inch. For width values above and below these dimensions there may bemore or less metallic wool disposed throughout the flow channel. The tortuous 'path may'range in total length from one foot'to several feet. It will be desirable to maintain the area of the flow channel of the order from five square inches to tenor more, based on the assumptionit has a smooth surface contour. The inclusion of metallic wool, helices of finely divided-wire and metallic particles not only increase the electrode area but also prevent the formation of alaminar flow pattern of the solution. The, introduction of eddies in addition to the agitation provided by the agitating means 45 promotes of which forms a solution mixing and interchange of the solution at the electrode surface to maintain high the efficiency of the reactions. I I
As a specific example of the determination of iodine and the flow rate was set at 2.1 milliliters per minute.- 'The voltage on-t-he voltmeter 41 was .set to +0.06 voltage. The sample stream contained potassium iodide andiodine. The current was found to be 0.063 milliampere. Applying Equation 3 la-= l" =0.00o019. (4).
This means the presence of iodine 19x10" Normal.
Referring now to FIG. 5, there has been illustrated a graph 60-plotted with voltage as abs'cissae and current as ordinates. In the absence of any constituent in the material there may be attained a" fiow of" current with change of applied voltage to the working. electrode 25 For example, a vibrator.v
be maintained ofa low order. For exas represented by the broken line graph 61. This curve 61 shows substantially no change of current with voltage over a substantial range, a result which may be anticipated because no reaction is taking place at electrode 25. The
later steep upswing ofthe curve 61 in indicative of a reaction at the'higher voltages which takes place be tween the electrode 25 and the liquid in channel 11.
1 If it now be assumed that the sample solution in vessel 12 has two constituents'present, then as the potential on electrode. 25 is increased,'the current flow will be that indicated by the curve60. The constituent which reacts at-"the lower voltage is first removed from the sample stream.. This occurs at a region ofsubstantially constant current flow corresponding with a reactionvoltage V representing the preferred voltage for establishing the reaction potential forconstituent A. The current required for the removable of substance A from the ,solution will be the difference between the total current flow C at the voltage V and the C To remove the residual current, that current'fiowing in the absence of constituent A but with flow of liquid through the channel 11, the zero-adjusting knob 440 of recorder 44 is rotated to provide a zero reading on the recorder with flow of current of magnitude corresponding with C In this manner there is taken into account the presenceof the residual current and the manher in which that current is removed from the measurement.
If it is now desired to determine the concentration of substance B in the solution, the reaction potential V is set by contacts 37a and 38a ofFIG. 1 on the working electrode 25. To remove from indication on recorder 44 of the residual current, now of magnitude C knob 440 of recorder 44is rotated until the-recorder reads zero with flow of current corresponding to the value C Thus, the recorder responds to the current (C -C to provide measurement of the concentration of constituent B in the sample solution. While the foregoing operation is feasible in instances where the concentration of substance A remains constant, problems arise when there is variation in the concentration of substance A. To remove this variable there may be utilized the modification of FIG. 6.
In FIG. 6 there is a reference electrode 24 common to two working electrodes 25a and 25b, each of which is provided with flow channels 11d and 11e closed by porous wall structures 21a and 21b. The sample solution enters through supply line 10 from suitable means for producing a regulated constant flow of sample solution in supply line 10. Thecontacts 37a and 38a are set to establish on electrode 25a the voltage V of FIG. 5. The current tlowingbetween electrodes 24 and' 25a will then be indicated on a measuring instrument such as an ammeter 65, the zero of which is adjusted to remove the residual current C from its indication. Thus, the meter. 65 will indicate directly the concentration of constituent A in the solution.
Since constituent A is entirely removed or its state changed in flow channel 11 and regardless of whether or not its concentration may vary, it will beseen that while theimeter 65 will respond to theforegoing changes, substance A in outlet line 20 will not be present in a form which will affect the measurement in the second reaction zoneformed by the flow channel 11b. Thus, the sample solution with substance A removed or in a changed state will pass through reaction zone 11b for determination of the concentration of constituent B. The reaction potential V of FIG. 5 will be established for the working electrode 25b by suitable positioning of the contacts 37c and 37 of resistors 37d and 370, these resistors being supplied by way of rheostat 40a and the battery 39a. An ammeter 66 set for a zero reading with a current flow corresponding to the residual current C FIG. 5, will then indicate the concentration of constituent B.
The residual current C is of a lower order than the residual current flow residual current G for the reason that in measuring constituent B in the same reaction zone with constituent A present the current flow will be high enough for re- A and B in the sample streams. The zero reading of ammeter 67 is set to take into account the sum of the residual currents C and C In some cases it addition of line and into transfer tube 20.
The added material is useful, for example, when it is desired to convert combined: chlorine to free iodine. This can be readily done by adding a small amount of potassium iodide solution. to the sample stream.
In general, the choice of the solvent electrolyte forming the substrate, that is, the liquid comprising the sample solution in the absence of the constituent to be mony, will be reversed. The lead can then be measured with the potential of electrode a at around 1.1 volts, at which potential the antimony'will not react. To measure the antimony, the potential at the electrode 25b will be made, more negative, as for example, it can be It is to be understood that the two examples of lead and antimony may be sequentially a cell having a first liquid space for a reference solution, a reference electrode measure of the constituent content of said sample solution.
2. The coulometric system of claim 1 in which solution-agitating means are provided, means for coupling said agitating means to said flow channel for producing agitation of said sample solution during operation of the cell.
3. The coulometric system of claim 1 in which said flow channel is in the form of a tortuous passage.
4. The coulometric system of claim 1 in which said flow channel takes the form of a plurality of arcuate paths of differing degrees of curvature.
5. The coulometric system of claim 1 in which said can range from positive to the potential of said reference elec- 10. The coulometric system of claim 1 means including said electrolytlcally conductive member is an extended tube through which tube said sample solution passes in contact with said working electrode.
ing finely divided particles.
13. The coulometric system of claim 1 wherein said fiow channel is filled with finely divided particles of the same material as said working electrode.
14. The coulometric system of claim 1 in which said electrolytically conductive member comprises porous insulating material.
15. The coulometric system of claim 14 in which said flow channel extends inwardly from a fiat face of a' metal plate forming said working electrode, and in which that other face of said electrolytically conductive member is flat and is disposed opposite to said flat face of said metal plate.
16. The coulometric system of claim surface area of said the surface area of contour.
15 in which the working electrode materially exceeds a like flow channel of smooth-surface liquid space for a reference solution, disposed within said liquid space, an electrolytically c0nductive member having one face only exposed to said of extended area having a working open side closed by said working application thereto of said reference solution to establish an electrically conductive path to the opposite working face of said member, aworking electrode face in juxtaposition with said working face of said member, said working face of said working electrode having formed therein an elongated, tortuous, shallow, open-sided flow passage with its face ofsaid member to form an elongated flow channel, said flow channel having a maximum solution iiquid space for thousandths of an inch,,flow connections for filling said to said electrodes andincluding means for producing at said working electrode a predetermined potential relative to. said reference electrode for the establishment of a reaction potential at said working electrode of magnitude for the removal from said solution of said constituent to the exclusion of reactions of other constituents in the sample solution having higher potentials of reaction, and means operable in response to the current required for said removal of saidconstituent for indicating the concentration of said, constituent in said sample solution.
18. A coulometric system comprising .a cell having a first liquid space for a reference solution,
a reference electrode disposed within said liquid space, means including electrolytically conductive structure forming a second liquid space and a third liquid space inflow communication with each other and respectively forming a portion of a fiow channel for a solution having constituents therein transformable by, electrolysis from one state to a second state, said electrolytically conductive structure in the region of said second liquid space and in the region of said third liquid space having a face portion exposed to said first liquid space and having in each of said second liquid space and said third liquid space an opposing face portion respectively exposed to said second liquid space and to said'third liquid space, working electrodes of large surface area, one disposed within said second liquid space'and one within said third liquid space, Y i
' said portion of said flow chann'el including said second liquid space and said third liquid space having maximum solution thicknesses with respect to said sur-, face areas of said working electrodes of about fifteenthousandths of an inch,
external circuits-extending between each working electrode and said reference electrode, and
means for relatively adjusting the potentials of said working electrodes for'removal in said second liquid space of one constituent and for removal in said third liquid space of a second constituent having a thickness with respect to said, working face of-said working electrode of about fifteen- COB- 191A COlllOl'llBtllO system comprising a cell having a first liquid space for a reference solution,
a reference electrode disposed within said liquid space,
means including electrolytically conductive structure forming a second liquid space and a third liquid space in flow communication with each other and respectively forming a portion of a flow channel for a solution having constituents therein transformable by electrolysis from one state to a second state, said electrolytically conductive structure in the region of, said second liquid space and in the region of said third liquid space having a face portion exposed to said first liquid space and having in each of said second liquid space and saidthird liquid space an opposing face portion'respectively exposed to said second liquid space and to said third liquid spac'e, flow connections for producing a flow of said solution first through said second liquid space and then through said third liquid space, working electrodes of large surface area, one disposed within said second liquid space and one within said third liquid space, said portion of said flow channel including said second liquid space and said third liquid space having maximum solution thicknesses with respect to said surface areas of said working electrodes of about fifteen-thousandths of an inch, external circuits extending between each trode and said reference electrode,. current responsive means included in each of said external circuits, and means for relatively adjusting the potentials of said working electrodes for removal in said, second liquid space of one constituent and for removal in said third liquid space of a second constituents having a higher characteristic potential than said one constituent.
working elec- References Cited by the Examiner UNITED STATES PATENTS WINSTON A.; DOUGLAS, Primary Examiner. JOSEPH REBOLD, JOHN H. MACK, Examiners.