Ac-electrochemical cell , which will be discussed today, was first described in the early century in the thesis of the famous Russian Academician VA electrochemists Kistyakovsky.
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In 1909 in London at the VII Congress of Applied Chemistry, Professor of St. Petersburg Polytechnic Institute Vladimir Kistyakovsky colleagues demonstrated externally very simple device.
The vessel with the liquid dropped two metal electrodes, they are drawn to the wire of the electric bell. It seems to be nothing unusual - galvanic cells were at that time well-known and ubiquitous. But what is it? Connected to the bell ringing device, then calms down, then rings again, as if
obeying someone's orders. Who controls them? A simple device does not behave as a conventional galvanic cells, which are known to provide a constant, DC only.Prominent scientists shake gray heads ...
Meanwhile, in the experiment, which showed Vladimir Kistyakovsky, was not unusual. However, in order to understand the theory of the phenomena, school knowledge of chemistry, physics and mathematics, perhaps small, but nevertheless understand how the ac-electrochemical cell, it is possible.
Let's do this first experienced take an iron nail and put him in a ten percent solution of sulfuric acid (hopefully, young chemists do not need to once again remind you that contact with acids should be very careful). Reaction begins, accompanied by bubbling hydrogen. Now plunge the same nail for a few seconds
in concentrated nitric acid and then again dissolved in sulfuric acid. At this time, bubbling of hydrogen is not observed. Iron is turned into a so-called passive state.Under the influence of a strong oxidizing agent - concentrated nitric acid - was formed on the surface of the nail thin, not discernible to the naked eye oxide film. It was she who protects the metal from the action of sulfuric acid.
As you can see, in these experiments the iron was either active or passive. But Professor
Kistyakovskij proposed a solution in which the simultaneous occurrence of several chemical and physical processes of iron becomes alternately the active and the passive. If such a solution is lower iron and
Graphite electrodes and connect them voltmeter, we find that the resulting electromotive force of the cell periodically (with a frequency of 11-16 c) changes. The cell provides a pulsating current.
But it is better to see once than to hear seven times. We invite you to repeat the experience Kistyakovsky, but more from the time of materials. Prepare first three solutions.
Solution number 1
5 ml of concentrated sulfuric acid dissolved in 100 ml of distilled water. ( Caution: It is necessary to add acid to water, not vice versa!)
Solution number 2
5 g 5 g of potassium dichromate, potassium dichromate (K2 Cr2 O7 ) soluble in 95 ml of distilled water.
Solution number 3
10 ml of concentrated sulfuric acid, is soluble in 166 ml of distilled water. ( Caution: add acid to water, but not vice versa! )
Mixed solution of number 1 and number 2 and cool to room temperature. The electrolyte is prepared.
As for our iron electrode electrochemical cell using a large nail or plate of mild steel. Graphite electrode can be learned from his time served by batteries.
To remove rust, iron electrode well cleaned with fine sandpaper and dive into the warm solution number 3, hold it there until the hydrogen bubbles. Now you are ready to experience.
Omit iron and graphite electrodes in an electrolyte and connect them to a voltmeter capable of measuring in the range of 0 to 2 V. (Very convenient for this purpose, a school demonstration voltmeter.) After a few seconds the meter pointer will move. If this does not happen again Place the electrode in warm sulfuric acid until hydrogen bubbles and try again. If at this time
bubbles appear, replace nail or steel plate to the other. Probably, they still remained rust or too much metal impurities, for example carbon. As soon as you pick the right nail, the long-awaited effect is obtained.
Built by us element, strictly speaking, it does not give a variable, and the ripple current. For AC plunge into the electrolyte two iron electrodes. Rather, the intervals of time at which each of the electrodes will be in an active or a passive state will not match. Therefore, positive and negative electrode will become alternately one or the other nail galvanic
element will give a real, albeit a low-frequency, alternating current.
Despite the fact that the ac-electrochemical cell appeared at the beginning of this century, the use of the technique he is still not found. Probably not the last role played in this he was not very good performance - instability in too low frequency of the current ...
But the achievements of modern electrochemistry allow us to hope that these disadvantages can be eliminated. Perhaps one of you guys tell me how to do it?
A. Davydov