OH- spread more than H+ ions?

In message , WAYNEL writes

What is the effect of the charge gradient on the constituents of the indicator itself ?

Cheers, J/.

I suspect you've got some dissolved copper in there, in some form. What happens if you use gold-plated electrodes, like wire-wrap pins?

Thanks, Rich

Do you mean besides red cabbage juice? That's almost free, and I saw a demo on some TV show - either the science segment of some variety show, or something like that Newton's Apple. Anyway, they juiced a cabbage, and had about seven containers, where they put stuff like vinegar, and drain cleaner, and ammonia, and various things, with different pH, and that cabbage juice turned about seven different colors!

But then you have the same problem with all kinds of chemical compounds - I think the indicator is called a "confounder".

Is there any such thing as a noninvasive pH meter?

Thanks, Rich

If they are charged, they will migrate in a direction and at a rate depending on the gradient.

on the gradient.

Would viscosity have anything to do in this equation?

Thanks, Rich

I think these points have already been mentioned but

1. The sensitivity of the universal indicator is probably different for the H+ (H3O) and OH- ions. So the fact that one extends over a larger range might be do to this.

1. How does the indicator react to electric fields and ionic currents? It might be attracted to one electrode and repelled from the other.

2. The mobility and recombination rate of the H+ and OH- may be different.

Best regards mark

WAYNEL wrote:

Just an idea about indicator in the liquid.

It is possible to remove small samples of the liquid to test it, so the indicator does not compromise the experiment.

One way of doing it is to put the liquid in long tubes or ditches, so any change has to move along a certain path. Then we can take up a few milli-liters at certain locations at certain points in time and add a pH indicator to each test sample.

You might see some sort of an effect due to the seventeen fold difference in mass of the two ions.

Michael

Good points. Here is another scenario: At the anode, you are dissolving copper to make Cu++ (more likely Cu+ but that's a small detail). These hydrolyse to some extent, producing CuOH + H+, but not all the electrochemical reaction goes into making H+. At the cathode, you produce H2 and OH-. So there is more OH- produced than H+. You probably need to insert some fine-tipped pH probes and maybe redesign the experiment, e.g. do it in a capillary and freeze it at some point, analyse for Cu as a function of distance. Capillary, because that would cut down on convection, which you have to avoid here. A groove along the glass would do as well.

Just one point here - he is using de-ionized water, so it has very low conductivity to start with. Any reactions going on are caused by very small impurities and at a very high overvoltage (so in this case dissolution of Cu might be as good reaction as any).

Characterizing what kind of impurities he has there might be important to understand what's going on, they might be organic for example. Maybe there is some organic Cation radical stuff coing on, that would produce some *OH- which could even build peroxides while recombining. OR - I think I got it - it is the indicator itself that is getting oxidized/reduced, because it is probably the highest concentration impurity present in the system! So resulting acidity depends on the dynamics of the secondary reactions of oxidized/reduced state of the indicator!

If water would be distilled and de-gased to remove CO2 it should not have any noticeable reaction at all at 20V. So it looks like this is the "impurity-governed" experiment.

Regards, Evgenij

Dieter Britz wrote:

When I was a high school student I discovered I could make a pH indicator from the juice of inkweed berries. It remained purple at one end of the scale and changed to green at the other. Inkweed is found growing in rocky waste ground such as around a heap of illegally dumped builder's brick rubble. :-)

I presume that inkweed is a European weed introduced here (Australia), but to describe it just picture a small dock plant with glossy fruit that looks like a mulberry and undergoes the same red-->black colour transition as it matures but instead of being spherical the fruit is cylindrical about 5 cm in length.

ObElectronics: I have previously posted on my experiments with the fascinating electrolytic rectifer: a beaker of sodium bicarbonate solution and two metal electrodes--this might interest experimenters.

I thought that cation have grater mobility than anions?

WayneL