Hi Magori,

Do you know that you cannot maintain both potential cell and intensity constant? In an electrochemical system you can only fix one of the variables, which make you to work in galvanostatic (constant current) or potentiostatic (constant voltage). Generally, with a DC power supply, nor a potentiostat, it is better to work at galvanostatic mode but maybe it is different in electrocoagulation.

Thanks Jordi, I know that the two can't varied at one time. My design is to vary current and fix voltage (CV mode), but problem comes when I use the 2.4A and 7V, the current will start at a lesser value of around 1.7A it will then increase slowly to 2.3A (because of increase in Aluminium electrode dissolution and ions buildup in the electrolytic reactor that tend to increase conductivity of electrolyte).

So my thought was no matter how long it can take to run the experiment the voltage shall remain at 7V because its in a fixed mode, surprisingly as the current reaches 2.3A the voltage started to decline to about 5.8V then it stabilizes at this point. Any explanation to this behavior??

Dear Magori Nyangi,

your cell is a much heavier (transient) power load[1] for your (aged ?) DC power supply (having special specs for "Electrocoagulation experiments"), due to (high noise, Z-noisy) bubbles appearing near the Al electrode (WE ). We can propose some, possible, solutions:

1) modify the size of your cell: reduce a) the area size of the WE, or reduce b) the height level of the electrolyte your cell;

2) modify the (heavy load) impedance-Z and/or use a (transient) power buffer:

a) use an isolation, series, Resistor (Ri: 1 to 5 Ohm) between the power and your cell;

b) In addition, a big[1] value (isolation, buffer) Capacitor Cib (in parallel with your cell) might help, also.

A wise combination of the above proposal solutions, might be helpful, also, to avoid, or, at least, to minimize, such "abnormalities".

1. This (external) time constant (Tb=Ri*Cib) should be much far away (the high the better) from the power supply (PS), time constant (Tfeedback < < Tb) regime pre-existing in the (High DC regulated) PS, commonly, helpful as a (V-stabilizing) feedback, internal regulation(s), circuit for simpler (quasi-resistive) loads.

Thanks Ioannis Samaras & Jordi Carrillo Abad .

The active area of Al plates am using for each electrode is 20cm2 (7.5*2.85) and I have connected them in Monopolar parallel (MP-P), two as anode and two as cathode. Thus the total surface area (A) becomes 80cm2 (20*4), their are four surfaces from the two anodes??.

The current density J that am using is in the range 2-60 mA/cm2 which means the current to apply is 2*80=160mA, 60*80=4800mA i.e (160-4800mA). Before I make any change regarding the size of the electrodes!!

a. Is this the correct way of getting the total surface area where current is applied upon? or we just consider the surface area of a single electrode surface ie 20cm? for current calculation from J=I/A

b. The total height of Al electrode is 15cm though only 7.5cm is dipped into the water sample can this have significance to a potential drop?

Thanks in advance

1 step back: at any time you can control either the voltage or the current - not both. Provided you've set the current of your power supply to 2.4 A, this would explain the later voltage drop: when conductivity becomes high enough, the set current (= current limit) overrules the voltage (= voltage limit) setting.

The other way: when conductivity is bad, you won't get the 2.4 A your power supply seems to be set to.

Reading the prior posts, it seems you already know that your conductivity is increasing over time due to Al ions dissolving. Which - together with the power supply's characteristics - suffices to explain your observations regarding voltage and current trends over time.

Re: varying the current density other than by varying the voltage is possible by 2 means:

# varying the conductivity of your medium or

# varying the voltage

Via Ohm's law V = R * I (R being the inverse of the conductivity), voltage and current are coupled. Varying the active area of the plates (by raising or lowering them) would change the current, but not the current density (= current per unit area), as only the submerged area of the plates counts. (Air is an isolator.)

Dear Dreher.

Thanks for a nice explanation.. I have two confusing scenarios

1. At 7V and 2.4A 1.5g/L NaCl and 25mg/L F water sample the output current with a load on, started to read at 1.7A, this moved and remains constant at 2.3A where the voltage started to drop. For this case should i lower the voltage or incraese the conductivity??

2. At 7V, 4.8A. with the same conductivity as above, when the output load is set on, current start reading at 1.9A it then went up to a maximum of 2.9A and stack, in this case Voltage remain unchanged no any drop. For this case should i add conductivity??

The issue now is i want to conduct experiments at the same potential but variable current i.e 1A, 2.4A, 4.8A. And the added NaCl for conductivity enhancement be the same in all experiments. Any idea

7 V/2.4 A resp. 7 V/4.8 A reflect the settings of your power supply?

If so, in #1 the current limit is reached due to the ever-increasing conductivity. Thus the current limitation comes into effect, resulting in the decreasing voltage.

In #2, the current limit is never reached. Thus the voltage remains constant while the current is increasing with increasing conductivity. (I interpret your statement "with the same conductivity as above" as "with the (about) same starting conditions".)

And no - what you want to do is hardly feasible. Either you have to allow the voltage to deviate from your current setpoint of 7 V, or the current is a function of voltage and conductivity and cannot be controlled. And unless you vary the voltage (or the conductivity), current density will be constant (a function of voltage and conductivity).

What should work: varying the distance of your plates: larger distance for lower currents, smaller distance for higher currents. It will still suffice to set the voltage to 7 V - and the current to the maximum value your supply allows. (This is the only way to mostly prevent the voltage from sagging at some point.)

The product of distance and current is expected to be about constant (not exact, just an approximation), but this is the only way I can imagine a successful variation of the current density. And maybe you should substitute the Al plates by something thatvdoesn't dissolve during the experiment - e.g. carbon plates. (Platinum, gold and some other materials are expected to work as well but are somewhat expensive.)

You can, easily, modify the "active area" (size) of your cell, by reducing the number[1] of the WEs, from the (initial) number 2 (of the WEs) to be, only, (your) number 1 (WE).

So, the current (imax) will be divided by two; this minor modification might help to overcome the PS instabilities' issues.

1. You noticed (10h, ago): "The active area of Al plates am using for each electrode is 20cm2 (7.5*2.85) and I have connected them in Monopolar parallel (MP-P), two as anode and two as cathode."

Hi Magori,

I agree with Jordi Carrillo Abad opinion.

In electrocoagulation process, cannot maintain both voltage and current constant? In an electrochemical system you can only fix one of the variables.

When you set the current and voltage for doing experiment, if you at first encounter voltage is higher than that the current, it indicates the electrical conductivity is high, which means the reactor is working well.

In all electrical (... and electrochemical) measurements (and systems) you can, preset, fix, only, one of the ideal sources' function(ing) mode.

Only an ideal resistor and some non-electrochemical devices keep the other electrical parameters stable, at steady state. This case study, in Magori Nyangi question, is a very complex situation, mainly, because there is not, even a (temporal) steady state condition.

There is an analogy between Voltage (Volts) and Electric Current Intensity (A (Coulomb/s) and Pressure (Pa) and Flow Rate (kg/s).

If you maintain Voltage constant as you maintain Pressure constant.

If you maintain Electric Current Intensity constant as yo maintain Flow Rate constant.

You may maintain constant your Voltage and note the variation of the Electric Current Intensity during your test.

The Electric Current Intensity varies following the electric conductivity of the solution and the physical state of the electrodes used in Electrocoagulation process (as well as the distance between them).

Best wishes,

Djamel