Immagini

DESCRIPTION

Exhibit 639 is a Grenet battery, based on potassium bichromate, consisting of a spherical glass bottle with a large and long neck and a bakelite tap with three holes. On the inside the positive electrode is made of two carbon strips connected together by a brass wire to their terminals on the tap top. The negative electrode is a zinc strip connected to a metal rod and terminal through the central tap hole (Disegno-1).

The strips are immersed in a dilute acqueous solution of water and sulfuric acid () and potassium bichromate (). The zinc strip can be raised above the solution when the battery is not in use. The molecules of sulfuric acid dissociate in hydrate ions Hhigh{+} and which transfer their negative charge on the zinc strip forming Zn SO₄ . The H⁺ ions transfer their positive charge on the carbon strips and the liberated hydrogen interacts with the solution forming chromic acid (H₂ CrO₄). The chemical reaction is the following:

With the sulfuric acid () chromic oxide () forms chromatic sulphate, while the potassium sulphate and the chrome form an undesired product, called the allum of chrome.

The allum of chrome (potassium chrome sulphate) crystallyzes on the electodes and weakens the battery; in modern batteries one uses chromic acid instead of potassium bichromate.

The battery voltage is about 2 volts, but falls rapidly; for this reason the battery is used for strong currents of short duration. The battery is known with different names: Grenet battery, Poggendorf battery or bichromate battery

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