Apparatus for controlling pH of culture solution for a living organism

Electrolysis is employed for raising or reducing pH of culture solution. Electrolyte contained in a vessel having at least one ion exchange membrane as a wall is provided so as to confront the culture solution, with the ion exchange membrane lying therebetween. Electrodes are provided both in the electrolyte and the culture solution. With these electrodes the electrolysis is carried out through the ion exchange membrane. Thus formed alkali or acid changes the pH of the culture solution. By controlling the time or supplied current for the electrolysis, required pH can be accurately obtained.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of and an apparatus for 
controlling the pH of culture solution for a living organism. 
It is quite important to control pH in the culture of a living organism 
such as in hydroponics. Such method has hitherto been used as neutralizing 
by adding alkali or acid into a culture solution tank from storage tanks. 
The amount of alkali or acid to be added is based on the measurement with 
a pH meter. When automatic control is required, the storage tanks for 
alkali and acid are provided with electrovalves which control the release 
of the contents, and the electrovalves are driven by a controller which 
works responsively to the signal from the pH meter having an electrode 
inserted into the culture solution. 
Such method, however, has not been satisfactory for reasons as follows: 
A desirable value of a pH of the culture solution is in a narrow range near 
neutrality. Therefore when concentrated alkali and acid is used, it is 
difficult to maintain pH in the desirable range because pouring thereof 
easily causes large change of pH. To accurately control a pH, it is 
desirable to use dilute alkali and acid. The use of dilute alkali and acid 
requires large tanks for storing them. Further it is necessary to 
supplement alkali or acid in the storage tanks frequently. Further the 
electrovalves for the tanks must be corrosion-resistant because alkali or 
acid flows therethrough. Further, the case possibly occurs that alkali 
solution reacts with CO.sub.2 gas in air to make carbonate, and it enters 
into the culture solution through neutralization, resulting in bad effect 
on plants, or it chokes up the gateway by solidifying. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of controlling the pH 
of culture solution whereby such control can be performed at high 
accuracy. 
It is a further object of the invention to provide an apparatus for such 
pH-control having a simple and compact construction. 
According to the present invention, electrolysis through an ion exchange 
membrane is employed. A vessel having a partition wall of an ion exchange 
membrane is provided in the manner such that the culture solution abuts on 
the ion exchange membrane from outside of the vessel. Within the vessel is 
held electrolyte of inorganic ions. Electrodes are provided on both sides 
of the ion exchange membrane, i.e. both in the culture solution and the 
electrolyte. With these electrodes electrolysis is performed through the 
ion exchange membrane. 
As the result of the electrolysis, formed alkali or acid changes a pH of 
the culture solution. By controlling the time and supplied current of the 
electrolysis, required pH can easily be obtained.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a pH controlling apparatus which is used in a 
hydroponics system. A culture solution tank 1 is provided for storing 
culture solution 2. The culture solution 2 is conveyed to a growing vessel 
3 by a pump 4. It is pumped up through a supply pipe 5 and returns from 
the growing vessel 3 to the culture solution tank 1 through a drain pipe 
6. The operation of the pump 4 is controlled by a timer 7 so as to be 
driven intermittently. As the case may be the pump 4 is driven 
continuously. The level of the culture solution is maintained constant by 
a constant-level regulator 8. The pH of the culture solution 2 is measured 
by a pH electrode 9 and obtained data signal is transmitted to a 
controller 10. The controller 10 is connected with electrodes 11, 12 and 
13 for electrolysis and a DC power source 14, and controls the supplying 
of current from the DC power source 14 to the electrodes 11, 12 and 13. 
Such control is made in accordance with the result of collation between 
the pH measured by the pH electrode 9 and a required range of pH preset in 
the controller 10. When the measured value is out of the preset range, the 
DC current from the DC power source 14 is supplied between the electrodes 
11 and 12 or the electrodes 12 and 13, whereby the electrolysis occurs so 
as to maintain the pH of culture solution 2 in the preset range. The 
electrode 12 is placed within an electrolyzing vessel 15 in which an 
electrolyte 16 is held. 
As illustrated in FIG. 2, the electrolyzing vessel 15 is box-shaped and two 
opposed walls thereof are partially constructed by ion exchange membranes 
17 and 18. In this embodiment, the membrane 17 is a cation exchange 
membrane and the membrane 18 an anion exchange membrane. The electrodes 11 
and 12 confront each other, with the cation exchange membrane 17 lying 
therebetween. And the electrodes 12 and 13 confront each other, with the 
anion exchange membrane 18 lying therebetween. As the electrolyte 16, for 
example, K.sub.2 SO.sub.4 aqueous solution is used. The electrodes 11, 12 
and 13 may be of platinum. 
The controller is, for example, constructed as illustrated in FIG. 3. A pH 
signal voltage from the pH electrode 9 is supplied to the comparators 19 
and 20. Reference voltages for the comparators 19 and 20 are supplied from 
variable voltage sources 21 and 22, respectively. The variable voltage 
source 21 is preset correspondingly to the lower limit of the required pH 
range and the variable voltage source 22 to the upper limit. The coupling 
between the comparators 19 and 20 and the input signals are in such 
relation that the comparator 19 generates an output signal if the pH 
signal voltage is lower than the reference voltage of the variable voltage 
source 21 and the comparator 20 generates an output signal if the pH 
signal voltage is higher than the reference voltage of the variable 
voltage source 22. The outputs of the comparators 19 and 20 are coupled to 
relays CR.sub.1 23 and CR.sub.2 24, respectively. The electrode 11 is 
connected with the negative terminal of the DC power source 14 through a 
contact SW.sub.11 which is one of the contacts of the relay CR.sub.1 23. 
The electrode 13 is connected with the positive terminal of the DC power 
source 14 through a contact SW.sub.21 which is one of the contacts of the 
relay CR.sub.2 24. The electrode 12 is connected with the positive 
terminal of the DC power source 14 through another contact SW.sub.12 of 
the relay CR.sub.1 23, and with the negative terminal of the DC power 
source 14 through another contact SW.sub.22 of the relay CR.sub.2 24. 
Thus when the pH signal voltage is lower than the preset range, i.e. lower 
than the reference voltage of the variable voltage source 21, the relay 
CR.sub.1 23 is driven by the output of the comparator 19 and the contacts 
SW.sub.11 and SW.sub.12 are made. Thereby electrolysis through the cation 
exchange membrane 17 is carried out by the current supplied from the DC 
power source 14, with the electrode 12 as the anode and the electrode 11 
as the cathode. 
The reaction in the electrolyzing vessel 15 is represented as follows: 
EQU 2K.sup.+ +SO.sub.4.sup.2- +2H.sub.2 O.fwdarw.1/20.sub.2 .uparw.+2H.sup.+ 
+SO.sub.4.sup.2- +2e 
The K.sup.+ ions are transferred by migration to the side of the electrode 
11 through the cation exchange membrane 17. 
On the other hand, the reaction in the culture solution 2 is as follows: 
EQU 2H.sub.2 O+2e.fwdarw.H.sub.2 .uparw.+2OH.sup.- +2K.sup.+ 
the OH.sup.- ions thus generated on the electrode 11 raise the pH of the 
culture solution 2. The electro neutrality in the culture solution 2 is 
maintained by the K.sup.+ ions transferred from the electrolyte 16 by the 
amount corresponding to electrical quantity for the electrolysis. 
Thus the pH of the culture solution is raised to within the required range. 
When the pH signal voltage is higher than the preset range, i.e. higher 
than the reference voltage of the variable voltage source 22, the relay 
CR.sub.2 24 is driven by the output of the comparator 20 and the contacts 
SW.sub.21 and SW.sub.22 are made. Thereby electrolysis through the anion 
exchange membrane 18 is carried out, with the electrode 13 as the anode 
and the electrode 12 as the cathode. 
The reactions thereof are as follows: 
EQU Anode: 2H.sub.2 O.fwdarw.2H.sup.+ +SO.sub.4.sup.2- +1/20.sub.2 .uparw.+2e 
EQU Cathode: 2H.sub.2 O+2K.sup.+ +SO.sub.4.sup.2- +2e.fwdarw.H.sub.2 
.uparw.+2OH.sup.- +2K.sup.+ 
thus in the culture solution 2 wherein the anodic reaction occurs, the 
concentration of H.sup.+ ion increases accordingly as the electrolysis 
progresses, and the pH falls. 
Although K.sub.2 SO.sub.4 aqueous solution is used as the electrolyte 16 in 
the above-mentioned embodiment, other material may be used. It is 
preferable that the electrolyte 16 not have a bad effect on plants because 
the ions within the electrolyzing vessel 15 are transferred to the culture 
solution 2 through the ion exchange membranes 17 and 18. Examples of 
satisfactory materials result from combination between K.sup.+, 
NH.sub.4.sup.+, Ca.sup.++, Mg.sup.++, NO.sub.3.sup.-, SO.sub.4.sup.2- and 
PO.sub.4.sup.3- which are the ions contained in the culture solution 2 in 
substantial amounts. 
The electrolysis may be carried out by the use of two electrodes and one 
ion exchange membrane. As illustrated in FIG. 4, for example, an anion 
exchange membrane 25 is employed and formed as one partition wall of an 
electrolyzing vessel 26. Electrodes 27 and 28 confront each other, with 
the anion exchange membrane 25 lying therebetween. 
If the pH of the culture solution should be raised, the electrode 28 in the 
vessel 26 is used as an anode and the electrode 27 as an cathode. The 
reactions in such case are as follows: 
##STR1## 
where the X.sup.- and M.sup.+ represent ions contained in the culture 
solution 2, as fertilizer such as K.sup.+, NH.sub.4.sup.+, NO.sub.3.sup.-, 
etc. These ions may be a multi valent ion such as Ca.sup.++, Mg.sup.++, 
PO.sub.4.sup.3-, SO.sub.4.sup.2-, etc.. 
If the pH of the culture solution should be lowered, the electrodes 27 and 
28 are used with polarities the reverse of the above case. The reactions 
are as follows: 
EQU Cathode: 2H.sub.2 O+K.sup.+ +SO.sub.4.sup.2- +2e.fwdarw.H.sub.2 +2OH.sup.- 
+K.sup.+ 
EQU anode: H.sub.2 O+2M.sup.+ +2X.sup.- .fwdarw.1/2O.sub.2 +2H.sup.+ +2X.sup.- 
+SO.sub.4.sup.2- 
instead of the anion exchange membrane 25, a cation exchange membrane can 
be used for constructing the system in the similar manner. 
Furthermore, pH of the culture solution may be also controlled by an 
apparatus as shown in FIG. 5. This apparatus has two electrolyzing vessels 
29 and 30 which respectively contain electrolyte 31 and electrolyte 32, 
two electrodes 33 and 34 which are respectively immersed in the 
electrolyte 31 and electrolyte 32, and two electrodes 35 and 36 immersed 
in the culture solution 2. This apparatus is particularly available for 
the case where ion-species of cations and anions, which respectively 
migrate from electrolyte 31 and electrolyte 32 to the culture solution 2, 
have to be restricted for some reason and the compounds composed of these 
ions show poor solubility. For example, if cations are to be restricted to 
Ca.sup.++ and anions to SO.sub.4.sup.2-, CaSO.sub.4 is produced as a 
compound of these ions. This compound shows poor solubility of the order 
of 2g/l and thereby it becomes difficult to allow the electrolyte to have 
a sufficient conductivity for causing satisfactory electrolysis. In this 
case, where the construction as shown in FIG. 5 is employed, an exchange 
membrane 37 provided at a portion of the electrolyzing vessel 29 is 
composed of a cation exchange membrane, the electrolyte 31 contains 
Ca(NO.sub.3).sub.2 4H.sub.2 O solution with a high solubility, and where 
the electrodes 33 and 35 provided on either side of the exchange membrane 
37 are respectively used as an anode and a cathode, it is possible to 
permit Ca.sup.++ cations in the electrolyte 31 to migrate into the 
culture solution 2 by electrolysis. On the other hand, where an exchange 
membrane 38 provided at a portion of the electrolyzing vessel 30 is 
composed of anion exchange membrane, the electrolyte 32 contains K.sub.2 
SO.sub.4 solution, and where the electrodes 34 and 36 provided on either 
side of the exchange membrane 38 are respectively used as a cathode and an 
anode, it is possible to permit SO.sub.4.sup.2- anions in the electrolyte 
32 to migrate into the culture solution by electrolysis. 
In order to make the culture solution pH controlling apparatus miniaturize 
in size and light in weight, it becomes necessary to make the amount of 
the electrolyte 16 in the electrolyzing vessel 15 less as compared with 
that of the culture solution. However, when pH of the culture solution 2 
is rectified by electrolysis, pH of the electrolyte 16 changes to the 
adverse direction with respect to the pH rectifying direction of the 
culture solution and this pH change of the electrolyte becomes larger with 
the decreasing of the amount of the electrolyte. Therefore, in order to 
restrict the pH change of the electrolyte 16 to as small as possible, it 
is preferable that the electrolyte 33 includes, as components, weak acid 
or weak base ion with a large buffer capacity, for example H.sub.2 
PO.sub.4.sup.-, HPO.sub.4.sup.2-, Ca.sup.++, Mg.sup.++ or mixture 
thereof, among the combinations of a large number of ions included as 
fertilizers in the culture solution 2. 
Furthermore, in FIGS. 2 and 3, if the ion exchange membrane 17 is composed 
of an anion exchange membrane, the ion exchange membrane 18 a cation 
exchange membrane, and like operation as described above is performed, 
namely, if electrolysis is caused by arranging, as an anode, the electrode 
12 in the electrolyzing vessel 15 and, as a cathode, the electrode 11 in 
the culture solution 2 in a manner that they are positioned on either side 
of anion exchange membrane 17 or by arranging, as an anode, the electrode 
13 in the culture solution 2 and, as a cathode, the electrode 12 in the 
electrolyzing vessel 15 in a manner that they are positioned on either 
side of the cation exchange membrane 18, ions in the culture solution 2 
migrate into the electrolyzing vessel 15 through the ion exchange 
membranes 17 and 18, and thereby pH of the culture solution 2 may be 
controlled with maintenance of electro neutrality of the culture solution 
2 and the electrolyte 16. In this case, because components included in the 
electrolyte 16 are never carried into the culture solution 2 by the 
electro-migration, it is unnecessary to pay attention to whether the 
components to be added in the electrolyte 16 adversely affect living 
organisms, and therefore they may be optionally selected. Thus, it becomes 
possible to add to the electrolyte 16 neutral salts with a high equivalent 
conductivity such as KClO.sub.4, K.sub.4 Fe(CN).sub.6, K.sub.3 
Fe(CN).sub.6, KI or the like, other than the combination of ions included 
as fertilizer components in the culture solution 2, and moreover it 
becomes also possible to add to the electrolyte 16 materials which give a 
large pH-buffer-capacity to the electrolyte, for example the material such 
as MH.sub.3 (C.sub.2 O.sub.4).sub.2 2H.sub.2 O, C.sub.6 H.sub.4 
(COOM)COOH), MH.sub.2 PO.sub.4, M.sub.2 HPO.sub.4, M.sub.2 HPO.sub.4, 
M.sub.2 B.sub.4 O.sub.7, MHCO.sub.3, M.sub.2 CO.sub.3 or the mixture 
thereof wherein M denotes an alkaline metal such as K, Na, Li, or the 
like. Furthermore, if there is employed such a construction that ions 
emigrate only from the culture solution 2 into the electrolyzing vessel 15 
by electrolysis, it becomes unnecessary to restrict the electrode reaction 
on the electrode 12 to water decomposition which generates H.sup.- and 
OH.sup.-. Rather, in order to avoid a large pH-variation of the 
electrolyte 16, the electrolyte 16 and the electrode 12 may be composed of 
a material which permits the reaction which occurs prior to an O.sub.2 
evolution reaction as an anode reaction and prior to H.sub.2 evolution as 
a cathode reaction; that is a dissolution-decomposition-reaction of Cu and 
Ag or a dissolution-deposition-reaction of Pb, Ni or Sn which occurs, due 
to a hydrogen overvoltage, prior to charge-discharge of H.sub.2 ; or an 
oxidation-reduction reaction of ions such as 
EQU Sn.sup.4+ .revreaction.Sn, Fe(CN).sub.6.sup.3- 
.revreaction.Fe(CN).sub.6.sup.4-, Fe.sup.3+ .revreaction.Fe.sup.2+ or 
the like. For example, in the event that Cu and CuSO.sub.4 solution are 
respectively employed as the electrode 13 and the electrolyte 16, if the 
electrode 13 is used as an anode, an anode process becomes as follows: 
EQU Cu.fwdarw.Cu.sup.2+ +2e+2X.sup.- 
wherein 2X.sup.- emigrates from the culture solution, and if the electrode 
13 is used as a cathode, a cathode process becomes as follows: 
EQU Cu.sup.2+ +SO.sub.4.sup.2+ +2e.fwdarw.Cu.uparw.+SO.sub.4.sup.2- +2M.sup.+ 
wherein 2M.sup.+ emigrates from the culture solution, whereby pH of the 
electrolyte in the electrolyzing vessel 15 substantially does not change. 
In such arrangement, the material of the electrode 12 in the electrolyzing 
vessel 15 should be selected by considering the composition of the 
electrolyte 16. Namely, if it is desired to produce a 
dissolution-deposition reaction by employing the electrolyte including a 
salt such as CuSO.sub.4, AgNO.sub.3, NiCl.sub.2 or (CH.sub.3 COO).sub.2 
Pb, the electrode material should be composed of Cu, Ag, Ni or Pb and if 
it is desired to produce an oxidation-reduction reaction of ions or a 
water decomposition by employing the electrolyte which includes mixed 
solution of SnCl.sub.2 -SnCl.sub.4, FeSO.sub.4 -Fe(SO.sub.4).sub.3 or 
K.sub.4 Fe(CN).sub.6 -K.sub.3 Fe(CN).sub.6, the electrode material should 
be composed of a material being very or relatively stable in a range of 
3-10 pH, for example Pt, Au, Carbon, stainless steel or Ni. 
Furthermore, the electrolyzing apparatus for controlling pH of the culture 
solution may be set not only in the culture solution tank 1 as shown in 
FIG. 1, but also in the culture supplying tube 5, the growing vessel 3 or 
the draining tube 6. If the culture solution 2 is circulated, it may be 
set in any portion other than the portions as described above. 
In FIGS. 6 and 7, there is shown a pH controlling system for controlling pH 
of the culture solution wherein the electrolyzing device is placed at the 
end of a draining tube 39 with the culture solution being circulated. The 
electrolyzing vessel 40 of the electrolyzing device has therein an ion 
exchange membrane 41 and a pair of electrodes 42 and 43. In this system, 
when the depth of the culture solution 2 exceeds the height (H) of a water 
level controlling tube 44 for controlling the level of the culture 
solution to a constant level, the culture solution overflows into the 
electrolyzing vessel 40 through the inside of the level controlling tube 
44, the draining tube 39 and an inlet 45 of the electrolyzing vessel 40, 
which inlet 45 is below the lower edge of the electrode 42, and thereafter 
it flows into a tube 46 through a space between the electrode 42 and the 
ion exchange membrane 41 and through an outlet 47 of the electrolyzing 
vessel 40, which outlet 47 is above the upper edge of the electrode 42. If 
the outlet 47 is below the upper level of the water level controlling tube 
44, the culture solution in the electrolyzing vessel 40 keeps its level 
equal to the upper level of the water level controlling tube 44, thereby 
preventing the ion exchange membrane 41 from being damaged due to its 
drying. Furthermore, this system is effective in that an eduction of 
insoluble matter such as Ca(OH).sub.2, Mg(OH).sub.2 or the like, on the 
electrode 42 or on the exchange membrane 41 may be avoided because the 
culture solution is churned at the vicinity of the electrode 42 and the 
exchange membrane 41 by the circulation of the culture solution, which 
Ca(OH).sub.2 and Mg(OH).sub.2 is formed when Ca.sup.++ and Mg.sup.+ + in 
the culture solution combine with OH.sup.- produced by electrolysis. 
In the system as described above, it has a capacity to form in the culture 
solution by the electrolysis for 1 Hr, for example, an amount of an acid 
which is nearly equal to that in the case where 36 N-H.sub.2 SO.sub.4 of 
about 1 ml is added to the culture solution. Since it is possible to 
decrease an electric current and to shorten a period of electrolysis, the 
electrolyzing apparatus of the present invention is available for a very 
accurate pH control and for a pH control which requires a slow pH change 
such as for a living organism which should not be subjected to an abrupt 
change of pH in a neutralization range of solution.