US3668102A

US3668102A - System for measuring bod by electrolysis

Info

Publication number: US3668102A
Application number: US849742A
Authority: US
Inventors: James C Young
Original assignee: University of Iowa Research Foundation UIRF
Current assignee: University of Iowa Research Foundation UIRF
Priority date: 1969-08-13
Filing date: 1969-08-13
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06
Also published as: GB1346955A; DE2214963A1

Abstract

A system for measuring BOD by the electrolysis method has a reaction vessel containing the sample. A container of material for absorbing CO2 is received in the mouth of the vessel and is provided with an adaptor. An electrolysis cell storing the electrolyte is received in the adaptor. A cap is placed on the electrolysis cell with an upper circular covering flange spaced from the walls of the cell thus providing an annular space communicating with the electrolyte to permit venting of hydrogen while minimizing evaporation of the electrolyte. When the level of the electrolyte falls below a predetermined minimum, indicating a low 02 pressure in the vessel, a sensing switch energizes a regulated dc current source to start the electrolysis process to replace the 02 consumed by the sample. The system is insensitive to changes in line voltage and electrolyte strength over a design range. Apertures in the cap member and the cell may be selectively aligned by twisting the cap to equalize the pressure in the cell with atmospheric pressure before starting.

Description

[is], v 3,668,102 1 June'6, 197 2- United States Patent Young [54] SYSTEM FOR MEASURING non BY OTHER PUBLICATIONS ELECTROLYSIS Young et a1. Analytical Chem., Vol. 37, May, 1965 p. 784 [72] Inventor:

James C. Young, Ames, Iowa Primary Examiner-T. Tung Attorney-Dawson, Tilton, Fallon & Lungmus [73] Assignee:

low: State University Research Found: t1on,lnc., Ames, Iowa Aug. l3, 1969 21 ApplQNoQ: 849,742

[ ABSTRACT ;A system for measuring BOD by the electrolysis method has a [22] Filed:

reaction vessel containing the sample. A container of material for absorbing CO, is received in the mouth of the vessel and is Us. mam/195 204/] T provided with an adaptor. An electrolysis cell storing the elec- Int Cl trolyte is received in the adaptor. A cap is placed on the electrolysis cell with an upper circular covering flange spaced from the walls of the cell thus providing an annular space com- 3 W1, mm 04 1 1 [51] [58] Field 01 municating with the electrolyte to permit venting of hydrogen References Cited while'minimizing evaporation of the electrolyte. When the level of the electrolyte falls below a predetermined minimum, UNITED STATES PATENTS indicating a low 0 pressure in the vessel, a sensing switch energizes a regulated dc current source to start the electrolysis process to replace the 0 consumed by the sample. The system 204/] is insensitive to changes in line voltage and electrolyte v v strength over a design range. Apertures in the cap member ""204/196 and the cell may be selectively aligned by twisting the cap to 196 4 equalize the pressure in the cell with atmospheric pressure hefore starting.

3,088,905 Poepel et 3,362,900 1/1968 Sabins............

9 Cla ms, 3 Drawing Figures PATENTEDJUH s 1972 SHEET 10F 2 PATENTEDJUH 6 m2 SHEET 2 OF 2 QmBmE 52mm QDU SYSTEM FOR MEASURING BOD BY ELECTROLYSIS BACKGROUND AND SUMlVIARY The present invention relates to improvements in the electrolysis method of detemiining the biochemical oxygen demand (BOD) of polluted water. The electrolysis method (sometimes referred to as the Clark method) for the continuous determination of oxygen uptake of a large inhomogeneous biological sample was first described in an article by J. W. Clark published in 1959 as the Engineering Experimental Station Bulletin No. 11 of New Mexico State University. The principle described was the maintenance of pressure in a closed vessel containing the sample by replenishing the metabolically utilized oxygen through the electrolysis of dilute acid. The metabolically produced CO was absorbed with potassium hydroxide. The internal pressure of the cell was monitored by a device which would sense a pressure decrease to actuate the power supply to the electrolysis cell. A dc current was supplied to the electrolysis cell; and the on time of the current source, measured on an elapsed time meter, was a measure of the oxygen consumed by the sample.

From Faradays law, one Faraday is the number of ampereseconds required to decompose 1 equivalent weight of a substance. In the case of water, for example, 1 Faraday produces 8,000 mg. of at the positive electrode per 96,485 ampereseconds. Thus, for a constant current, time is a direct measure of the 0 production.

Improvements in this technique were described in an article published in ANALYTICAL CHEMISTRY, vol. 37, p. 784, (May, 1965) entitled An Improved Apparatus for Biochemical Oxygen Demand by .l. C. Young, W. Garner, and J. W. Clark.

The electrical design of the power supply used with this earlier model was unsatisfactory. There was no control over the current used for electrolysis; and frequent manual adjustments were necessary. As a result, fluctuations in the rate of oxygen production would develop due to changes in line voltage, changes in the electrolyte concentration due to evaporation from the cell, changes in electrolyte concentration from the electrolytic conversion of water to hydrogen and oxygen, and differences in the makeup of new electrolyte solutions. Thus, frequent adjustment of the units was required to maintain a constant current. However, in spite of these sources of error, the BOD of samples of raw sewage and synthetic wastes of strengths approximating raw sewage could be measured more precisely than was possible with any other method available.

An article reviewing the development of respirometric methods of determining BOD entitled The Determination of Biochemical Oxygen Demand by Respirometric Methods by H. A. C. Montgomery was published in WATER RESEARCH, Pergamon Press, 1967) vol. 1, pp. 631-662.

The present invention provides still further improvements of the electrolysis method of determining BOD first described by Clark and later improved as indicated above. The improvements relate to the design of the electrolysis cell and to the overall system which includes a regulated current source. The cell includes a generally cylindrical outer wall, an intermediate cylindrical wall coaxial with the outer wall and having an aperture so that the electrolyte is free to flow in either side of the intermediate cylindrical wall. A central tube is provided within the intermediate wall; and it extends above the level of the electrolyte. A cap having a frusto-conical body and a circular upper flange is received at the top of the intermediate cylindrical wall of the electrolysis cell; and three electrodes pass through the flange a sensing or level electrode, a common (or negative dc) electrode, and a positive dc electrode.

In the structural aspects of the new system, the diameter of the circular flange of the cell cap is approximately equal to the outer diameter of the exterior cylindrical wall of the cell. When the cap is placed on the intermediate cylindrical wall in I sealing engagement therewith, the circular flange is spaced slightly above the outer cylindrical wall of the cell a clearance of 2-3 mm. to provide an annular opening which communicates with the portion of the electrolyte in which the negative dc electrode is immersed. Thus, the narrow annular opening permits escape of the hydrogen yet minimizes evaporation of the electrolyte.

Further, an aperture is provided in the upper seating portion of the inner cylindrical wall of the cell (above the electrolyte); and a corresponding aperture is provided in the frusto-conical wall of the cell cap. When these two apertures are in register, the interior of the vessel containing the sample is in communi' cation with the atmosphere by means of the central tube of the cell in order to equalize the pressure within the vessel to that of the atmosphere as a reference or starting point.

Still further improvements are made in the overall combination by employing a regulated dc current source to insure that a constant current always flows when the dc electrodes are energized so that an elapsed time meter, actuated only when the regulated dc source is energized, provides a very accurate measurement of the consumed 0 As is well known, the amount of 0 produced is a linear function of the time integral of current. With the new power supply design, the electrolyzing current is held within 1% of the desired value for i 50 percent changes in electrolyte strength and i 10 percent changes in line voltage.

A manually-selectable switch is provided for setting the current level so that the current level times the elapsed time is a measure of the consumed oxygen. The level-sensing electrode in the cell is coupled to a relay. When the level of electrolyte in the space between the intermediate cylindrical wall and the outer wall falls below a predetermined level (indicated by a reduction in the O pressure in the vessel) the relay switches the current-regulated supply to the dc electrodes and also actuates the elapsed time meter. The negative dc electrode serves as a common line for the system. Thus, it is also one of the ac sensing electrodes obviating the need for an additional electrode.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in their various views.

THE DRAWING FIG. 1 is a composite drawing showing the improved electrolysis cell partially broken away together with an illustration of the control cabinet;

FIG. 2 is an exploded perspective view of the improved electrolysis cell with a portion cut away; and

FIG. 3 is a circuit schematic diagram of the control portion of the system.

DETAILED DESCRIPTION Referring then to FIGS. 1 and 2, reference numeral 10 generally designates the reaction vessel, 11 denotes the electrolysis cell, and 12 refers to the cabinet for the power and control module. The reaction vessel 10 is a glass (preferably Pyrex) container having a cylindrical side wall 13 and an upper, open neck 14. The sample generally designated 15 is contained by the vessel 10; and an air space 16 is provided above the surface of the sample 15 within the vessel 10. A magnetic stirrer, diagrammatically shown at S, is located within the vessel 13.

Received in the neck 14 of the vessel 10 is an adaptor generally designated by reference numeral 17 the adaptor 17 is also preferably formed of Pyrex glass.

The adaptor 17 includes an upper, tapered bushing 18, the exterior surface of which sealingly engages the interior tapered surface of the neck 14. Depending from the bottom of the bushing 18 is a tube-shaped container 19 having a closed bottom. The container 19 holds a volume of potassium hydroxide solution 20 for absorbing the CO metabolically produced by the micro-organisms in the sample. A glass wool wick 21 is placed in and extends above the upper level of the solution 20. First and second apertures 23 and 24 (FIG. 2) are formed in the wall of the adaptor 17 toward the upper end of the container portion 19 and in communication with the air space 16 of the vessel 13. The apertures 23 and 24 permit the CO produced from the sample to enter into the container portion 19 of the adaptor l7 and be absorbed by the potassium hydroxide solution contained therein. As will be clear from subsequent explanation, the apertures 23 and 24 also permit the ingress of from the electrolysis cell into the air space 16 where it is consumed by the sample. Both the exterior and interior surfaces of the glass bushing 18 are ground to provide an'air-tight seal respectively with the interior surface of the neck 14 and the electrolysis cell 1 1. The interior surface of the neck 14 is also ground.

Turning now to the electrolysis cell 11, as seen in both FIGS. 1 and 2, it includes an outer cylindrical wall 26, an intermediate cylindrical wall 27 coaxial with the outer wall 26, and a central tube 28 extending coaxially with the

walls

26 and 27. The outer annular space defined by the cylindrical walls 26 and -27 is designated 29a; and the inner annular space between the wall 27 and the tube 28 is designated 29b. The lower portion of the wall 26 is formed inwardly and converges to the intermediate wall 27; and the bottom of the tube 28 diverges to become integral with the interior surface of the wall 27.

Depending from the lower portion of the outer wall 26 is an inwardly-tapered seating member 30 having an exterior ground surface for seating into the bushing 18 of the adaptor 17. The exterior surface of the seating member 30 is correspondingly tapered as seen in FIG. 1. Toward the bottom of the intermediate cylindrical wall 27 there is provided an aperture 31 so that the annular space 29a communicates with the annular space 29b. An electrolyte 33 is located in both of these annular spaces and is free to move through the aperture 31. The preferred electrolyte is a 0.3 to 0.5 N solution of sulfuric acid. The upper portion of theintermediate wall 27 is tapered to a narrowing dimension at the top to define a conical seating surface 27a for receiving a cap, generally designated by reference numeral. 35, and also preferably formed of Pyrex glass. An aperture 34 is formed in the conical portion 27a of the intermediate wall 27.

The cap 35 includes a frusto-conical receptacle 37 for fitting over the correspondingly-tapered section 270 of the intermediate wall 27. The interior surface of the receptacle 37 is ground, as is the exterior-surface of the tapered portion 27a of the intermediate wall 27 to provide an air seal. The cap 35 also ihcludes an upper circular flange 38 integral with the receptacle 37; and it defines three

apertures

39, 40 and 41 for receiving three separate electrodes, as presently to be described. The outer diameter of the circular flange 38 is approximately equal to the outer diameter of the cylindrical side wall 26; and when the cap 35 is fully seated on the intermediate side wall 27, there is an annular spacing designated 44 between the lower surface of the flange 38 and the upper edge of the side wall 26. The annular space 44 is preferably about 2 to 3 mm. This annular aperture permits the escape of hydrogen produced in the electrolysis of the electrolyte 33 yet minimizes'the evaporation of the electrolyte; and it has been found to be of advantage for long-term, accurate use of the cell, as is normally contemplated.

The receptacle 37 of the cap 35 defines an aperture 37a which may be selectively aligned with or sea] the aperture 34 of the seating portion 27a of the intermediate side wall 27 by twisting the cap 35. When the cap 35 is placed on the cell 11 as illustrated in FIG. 1, a chamber designated 47 in FIG. 1 is provided. The chamber 47 is defined by the tapered portion 27a of the intermediate side wall 27 and the cap 35. The chamber 47 communicates by means of the tube 28, the seat- 1 ing portion 30 of the cell 11, and the apertures 23 and 24 of the adaptor 17 with the upper air space 16 of the vessel 13. When the apertures 34 and 37a are aligned, the chamber-47 also communicates with the atmosphere by means of the annular spacing 44 to equalize pressure within that chamber with the atmosphere and bring the level of the electrolyte within the inner annular space 29b to be the same as the level of the electrolyte in the annular space 290. When the cap 35 is then rotated so that the aperture 34 and 37a are not aligned, the chamber 47 and the air space 16 are sealed from the atmosphere. When 0, is then metabolically consumed by the sample and CO, is given off into the chamber 16, the CO, is transmitted through the apertures 23 and 24 into the interior of the tube container 19 wherein it is absorbed by the potassium hydroxide 20, thus generating a slight vacuum. This reduced pressure is transmitted to the chamber 47 to bring the electrolyte in the inner annular space 29b to a higher level thereby lowering the level of the electrolyte in the outer annular space 29a.

A first sensing electrode 50 extends through the aperture 39 in the circular flange 38, and is secured thereto by means of an epoxy cement. The electrode 50 extends to a predetermined level within the outer annular space 29a of the cell 1 l, and it is normally in contact with the electrolyte. When the CO, in the air space 16 is absorbed by the potassium hydroxide solution, as already explained, the electrolyte in the outer annular space reduces its level to break contact with the sensing electrode 50. As will be explained in greater detail within, when this break in contact is made, a dc current is applied to

electrodes

52 and 53 received respectively in the apertures 40 and 41 of the circular flange 38 and sealed thereto with epoxy. The

electrodes

50, 52 and 53 are preferably made of platinum.

The electrode 52 extends through the chamber 47 and into the electrolyte in the inner annular space 29b. The electrode 53 contacts the electrolyte 33 in the outer annular space 29a, as shown. Thus, when the electrolyte 33 breaks contact with the sensing (or level-detecting) electrode 50, indicative of a reduced pressure in the chamber 47, the electrolysis process is initiated; and O is produced at the positive electrode 52 to increase the pressure within the chamber 47 and at the same to replenish the supply of O in the air space 16. Hydrogen is produced at the negative electrode 53 and transmitted to the atmosphere via the annular aperture 44. This process will continue until the pressure within the chamber 47 reduces the level of electrolyte in the inner annular space 2% thereby raising the level of the electrolyte in the outer annular space 29a to contact the electrode 50. When contact occurs, electrolysis is terminated. Thus, the level of electrolyte is maintained within a narrow range, thereby maintaining the O pressure within the air space 16 substantially constant.

Turning now to FIG. 3, power leads 56 and 57 are energized by a conventional 60 Hz. ac source, not shown. A fuse 58 and a main switch 59 are interposed in the line 56. The primary winding of a first transformer 60 is connected to the source by the lines 56 and 57; and a secondary winding of the transformer 60 feeds a full-wave rectifier bridge, generally designated 61, to produce a dc output voltage. A capacitor 62 filters this dc voltage; and it is fed via a manual ON/OFF switch 63 to a current-regulated power supply enclosed within the dashed line 64. Since it is contemplated that a plurality of samples will be tested at one time, it is preferable that there be only one source of dc voltage and that the dc voltage be transmitted to a separate current-regulated power supply for each cell in use. The positive potential of the bridge output is coupled to the collector of an NPN transistor Q1, the base of which is connected to the negative terminal (which is the system common or ground) of the bridge circuit by means of a Zener diode Z. A current path is provided between the collector and base of Q1 by means of a resistor 65 in order to bias Zener Z and provide a path for base current for transistor Q1. The emitter of transistor Q] is coupled through a resistor 66 to the emitter of a PNP transistor Q2. The collector of the transistor Q2 is connected to the movable contact of two-position relay contacts 67a. The normally-open terminal of the contacts 67a is connected to a dummy load resistor 68. The other side of resistor 68 is connected to the system common. The normallyclosed contacts 67a are connected in series with a plug-in jack 69 and a current meter 70 to the positive electrode 52. Electrode 53 is connected to the system common and serves as the negative dc electrode when the electrolyte is below its predetermined level in the annular space 29a (i.e., out of contact with the sensing electrode 50). The electrode 53 also serves as a common line for the ac source, as described below.

The base of transistor O2 is connected to the emitter of a second PNP transistor Q3, the collector of which is connected in common with the collector of transistor Q2. Thus the transistors Q2 and Q3 form a Darlington amplifier. A resistor 70a is connected between the emitter of transistor Q1 and the base of transistor Q3. The resistor 70a acts in combination with a second resistance as a voltage divider network to maintain the voltage at the input of the Darlington amplifier at a selected, constant value. The second branch of the voltage divider network comprises a selected resistance value to determine the current output from the regulator circuit 64. That is to say, there are four separate branches of resistance, selectable by a switch 71 having its movable arm connected directly to the system common and four fixed terminals denoted I-IV. Specifically, a first variable resistor 73 is connected in series with a fixed resistor 74 between the base of transistor Q3 and a fixed terminal I of the switch 71. A second variable resistor 75 is connected in series with a fixed resistor 76 between the base of transistor Q3 and fixed terminal ll of the switch 71. A third variable resistor 77 is connected in series with a fixed resistor 78 to position III of the switch 71; and a fourth variable resistor 79 is connected in series with a fixed resistance 80 to fixed terminal IV of the switch 71. Each of the

variable resistances

73, 75, 77, and 79 permits fine adjustment of the output current to a predetermined value; and once an initial adjustment is made, these settings normally are not changed thereafter.

In operation of the current regulator, the Zener diode Z maintains a constant voltage at the base of transistor Q1. Since transistor Q1 is biased in the active region, its base-emitter junction is forward-biased, so the voltage at the emitter of transistor Q1 is substantially constant for all ranges of current. Thus, the first stage of the current regulator takes the dc output of the bridge circuit 61 which varies with the line voltage and converts it to a lower, but constant value independent of i percent of changes in line voltage. Hence, the voltage across the divider network (including resistor 70a and one of the selected branches connected to the switch 71) maintains a constant but selectable voltage at the base of transistor Q3. As long as the transistors Q2 and Q3 are in the active region, the total collector current is determined by the base current which is set by the voltage across resistor 70a and emitter resistor 66. As the load changes, the voltage change across resistor 66 adjusts the base current of transistors Q2 and Q3 to maintain the load current at a constant value. The total resistance of the branches connected to positions l-IV of switch 71 are progressively smaller to increase the voltage across resistor 70a as the movable contact of switch 71 is moved to a higher position. Thus, the output current from the collectors of transistor Q2 and O3 is substantially independent of the load value within the design range. For example, a constant load current can be achieved for changes of i 50 percent in electrolyte strength and t 10 percent changes in line voltage.

The primary winding of a transformer 84 is connected across the power lines 56 and 57; and the secondary winding of the transformer 84 is connected respectively between the system common and the coil of a relay 67. The other terminal of the coil of relay 67 is connected directly to the sensing electrode 50. The previously-described contacts 67a are actuated by the relay 67 that is, when the coil of relay 67 is not energized. The contacts 670 couple the output of the current regulator directly to the positive electrode 52 (via jack 69 and current meter 70). On the other hand, when the coil of relay 67 is energized (indicating that the level of electrolyte in the annular space 29a is at least as high as the predetermined level) the contacts 67a connect the output of the current regulator circuit directly to a dummy load (resistor 68) so that the current regulator circuit is always operative and operating at a stabilized equilibrium temperature for greater accuracy. The relay 67 is provided with a second set of contacts 67b; and these contacts are connected in series with a switch 85 for coupling the power lines 56 and 57 to an elapsed time meter 86. The contacts 67b are normally closed. Thus, when the electrolyte falls below its predetermined level, thereby de-energizing the coil of relay 67, not only is the current-regulator switched to electrode 52, but the elapsed time meter 86 begins to measure time. An indicator lamp 87 is connected between the power line 57 and the junction of switch 85 and contacts 76b; and when it is lit, it indicates that the current regulator and elapsed time meter circuits are energized. Switch numbers 63 and 85 refer respectively to either side of a single double-pole switch.

Returning to FIG. 1, the dial for current meter 70 (which may simple be a multiplication factor) is designated 90 on the face panel of the module 12; and the scale for the elapsed time meter 86 is shown at 91. The knob for the selector switch 71 is designated 92; and the shaft of ON/OFF switch and elapsed time meter circuit switch 85 is designated 94. The pilot light 87 is also indicated.

Having thus described a preferred embodiment of the improved system for the determination of BOD by electrolysis, it will be appreciated that the operation of the system is rendered more accurate through the use of a regulated and highly accurate dc power supply which is in operative relation between the two electrolyzing

electrodes

52 and 53 only when the electrolyte level is below a predetermined level indicative of the reduced pressure within the vessel containing the sample. The current output of the regulated supply is insensitive tovariations in line voltage and electrolyte strength. Further, one of the electrolyzing electrodes (namely, electrode 53) serves as a system common and is used in combination with the sensing electrode 50 to detect the level of the electrolyte within the cell. When the electrolyte is at least as high as a predetermined level (established by the tip of electrode 50) in the outer annular space 29a, the

electrodes

50 and 53 are energized by an ac source; whereas during the electrolysis stage, the

electrodes

52 and 53 are energized by a regulated dc source. The setting up of the system is facilitated by the provision of the aligned holes 34 and 37a respectively in the intermediate cylindrical wall 27 and the cap 35 to equalize the pressure within the vessel with the atmosphere prior to starting. It will also be appreciated that the circular flange 38 of the cap 35, having substantially the same diameter as the diameter of the exterior wall 26 of the electrolyte cell 11 and being spaced therefrom by a small distance of the order of 2-3 mm., greatly minimizes the evaporation of the electrolyte solution 33 while permitting escape of the hydrogen gas produced at the electrode 53. In addition, by securing the electrodes to the cap 35, access to the interior of the cell 11 for cleaning and maintenance is greatly facilitated.

Persons skilled in the art will be able to modify certain of the structural aspects described and illustrated and to substitute equivalent components while continuing to practice the principles of the invention; and it is, therefore, intended that all such substitutions and modifications be covered as they are embraced within the spirit and scope of the appended claims.

What is claimed is:

1. A system for determining the biochemical oxygen demand of a sample, comprising a vessel for containing the sample and having an upper air space, an electrolysis cell containing an electrolyte in first and second chambers, said chambers communicating with each other at a level below the level of said electrolyte, said second chamber providing a closed space communicating with the air space of said vessel, a sensing electrode extending in said first chamber to a predetermined level, a negative electrode immersed in said electrolyte in said first chamber at a level below said predetermined level, a positive electrode immersed in said electrolyte in said second chamber, circuit means responsive to the falling below said predetermined level in said first chamber by said electrolyte and thereby breaking contact with said sensing electrode, and

current-regulated dc source means for energizing said negative and positive electrodes respectively with a negative and positive dc potential at a constant, pre-selected current rate only when said electrolyte level is below said predetemiined level to produce oxygen at said negative electrode, said oxygen being communicated to said air space of said vessel, and to produce hydrogen at said electrode, said do source means including rectifier means receiving energy from an ac source and generating a dc voltage, isolation means receiving the output voltage of said rectifier means for generating a constant dc voltage independent of fluctuations in said ac source, and regulator circuit means for generating a constant current responsive to the output voltage of said isolation means.

2. The system of claim 1 further comprising circuit means including a relay having a coil connected in circuit with said sensing electrode, ac means to energize said sensing electrode, said coil and said negative electrode with an ac signal, said relay having a normally-closed contact for coupling said do supply between said positive and negative electrodes only when said relay coil is de-energized by said electrolytes breaking contact with said sensing electrode.

3. The apparatus of claim 2 further comprising second normally-closed contacts actuated by said relay, a time mechanism in series with said second relay contacts, and a source for energizing said contacts and said timer whereby when said relay is de-energized, said do source is connected to said positive and negative electrodes and said timer simultaneously.

4. The system of claim 1 wherein said isolation means further comprises selection means for selection of the output voltage thereof to thereby change the output current of said regulator circuit.

5. The system of claim 1 wherein said regulator circuit means includes a circuit-selectable switch and a plurality of resistor networks connected in circuit with said switch for selecting different levels of output current.

6. The system of claim 5 further comprising timing mechanism actuated only when said current-regulator means is energized.

7. The system of claim 6 further comprising a current meter in series with said current-regulator means and said electrolyte to measure the current therein.

8. ln apparatus for measuring BOD by electrolysis, the combination of a vessel containing a sample and provided with a throat and an air space above said sample adjacent said throat, a cell received in said throat and having an outer wall, an intermediate wall and a central tube to provide first and second spaces respectively between said outer wall and said intermediate wall and said intennediate wall and said tube for containing an electrolyte, said intermediate wall defining an aperture below the electrolyte level to communicate said first and second spaces with each other, a sensing electrode normally contacting said electrolyte in said first space, positive and negative electrodes respectively in said first and second spaces and immersed in said electrolyte, dc source means for energizing said positive and negative electrodes responsive to said electrolytes breaking contact with said sensing electrode, a cap member received on said intermediate wall and cooperating therewith to define a chamber above said electrolyte in said second space, said chamber communicating with the air space in said vessel via said tube, said intermediate wall defining a second aperture above the level of said electrolyte, said cap member defining a third aperture for selective registration with said second aperture of said intermediate wall whereby when said second aperture of said intermediate wall and said aperture of said cap are aligned, the air space of said vessel communicates with the atmosphere to equalize the pressure therein.

9. The apparatus of claim 8 wherein said outer and intermediate walls of said cell are cylindrical and said intermediate wall is provided with an inwardly-tapered upper portion, and

wherein said cap member includes a frusto-conical receptacle for fitting over said tapered portion of said intermediate wall in sealing engagement therewith, and a circular flange integral with said receptacle, the periphery of said flange extending substantially to the outer diameter of said outer wall and spaced therefrom of the order of three millimeters to provide a narrow annular opening therebetween for permitting hydrogen generated at said negative electrode in said first space to escape while minimizing the evaporation of said electrolyte from said first space.

Claims

1. A system for determining the biochemical oxygen demand of a sample, comprising a vessel for containing the sample and having an upper air space, an electrolysis cell containing an electrolyte in first and second chambers, said chambers communicating with each other at a level below the level of said electrolyte, said second chamber providing a closed space communicating with the air space of said vessel, a sensing electrode extending in said first chamber to a predetermined level, a negative electrode immersed in said electrolyte in said first chamber at a level below said predetermined level, a positive electrode immersed in said electrolyte in said second chamber, circuit means responsive to the falling below said predetermined level in said first chamber by said electrolyte and thereby breaking contact with said sensing electrode, and current-regulated dc source means for energizing said negative and positive electrodes respectively with a negative and positive dc potential at a constant, pre-selected current rate only when said electrolyte level is below said predetermined level to produce oxygen at said negative electrode, said oxygen being communicated to said air space of said vessel, and to produce hydrogen at said electrode, said dc source means including rectifier means receiving energy from an ac source and generating a dc voltage, isolation means receiving the output voltage of said rectifier means for generating a constant dc voltage independent of fluctuations in said ac source, and regulator circuit means for generating a constant current responsive to the output voltage of said isolation means.

2. The system of claim 1 further comprising circuit means including a relay having a coil connected in circuit with said sensing electrode, ac means to energize said sensing electrode, said coil and said negative electrode with an ac signal, said relay having a normally-closed contact for coupling said dc supply between said positive and negative electrodes only when said relay coil is de-energized by said electrolyte''s breaking contact with said sensing electrode.

3. The apparatus of claim 2 further comprising second normally-closed contacts actuated by said relay, a time mechanism in series with said second relay contacts, and a source for energizing said contacts and said timer whereby when said relay is de-energized, said dc source is connected to said positive and negative electrodes and said timer simultaneously.

8. In apparatus for measuring BOD by electrolysis, the combination of a vessel containing a sample and provided with a throat and an air space above said sample adjacent said throat, a cell received in said throat and having an outer wall, an intermediate wall and a central tube to provide first and second spaces respectively between said outer wall and said intermediate wall and said intermediate wall and said tube for containing an electrolyte, said intermediate wall defining an aperture below the electrolyte level to communicate said first and second spaces with each other, a sensing electrode normally contacting said electrolyte in said first space, positive and negative electrodes respectively in said first and second spaces and immersed in said electrolyte, dc source means for energizing said positive and negative electrodes responsive to said electrolyte''s breaking contact with said sensing electrode, a cap member received on said intermediate wall and cooperating therewith to define a chamber above said electrolyte in said second space, said chamber communicating with the air space in said vessel via said tube, said intermediate wall defining a second aperture above the level of said electrolyte, said cap member defining a third aperture for selective registration with said second aperture of said intermediate wall whereby when said second aperture of said intermediate wall and said aperture of said cap are aligned, the air space of said vessel communicates with the atmosphere to equalize the pressure therein.

9. The apparatus of claim 8 wherein said outer and intermediate walls of said cell are cylindrical and said intermeDiate wall is provided with an inwardly-tapered upper portion, and wherein said cap member includes a frusto-conical receptacle for fitting over said tapered portion of said intermediate wall in sealing engagement therewith, and a circular flange integral with said receptacle, the periphery of said flange extending substantially to the outer diameter of said outer wall and spaced therefrom of the order of three millimeters to provide a narrow annular opening therebetween for permitting hydrogen generated at said negative electrode in said first space to escape while minimizing the evaporation of said electrolyte from said first space.