Acids, Alkalis and Salts

by George Henry Joseph Adlam

Verdigris, or basic copper acetate, is a valuable pigment. It is made by interposing cloths soaked in vinegar between plates of copper. After the action has been allowed to go on for a long time, the plates are washed with water and the verdigris is scraped off. The finest verdigris is made in France in the wine-producing district around Montpellier. Here, instead of cloths soaked in vinegar, the solid residue from the wine presses is spread in layers between the copper plates. The product made in this way is called vert de Montpellier.

Fig. 11. DUTCH PROCESS FOR WHITE LEAD

Verdigris, like all the copper compounds, is extremely poisonous. It is very liable to be formed on the surface of copper utensils used for cooking purposes.

Lead Acetate, or sugar of lead, is used in large quantities in the colour industry for making various reds and yellows. It is prepared by dissolving the metal or its oxide (litharge) in acetic acid.

The slow action which acetic acid vapour has upon the metal lead finds a very interesting application in what is known as the Dutch process for the manufacture of white lead[4] for paint. The metal is cast into grids or spirals, which are placed on the shoulders of the specially made pots sketched in [Fig. 11]. A little dilute acetic acid is poured into each of the pots, which are then arranged side by side on a thick layer of tan bark, stable manure, or other material which will heat by fermentation. The first layer of pots is then boarded over; another layer of pots is placed upon this, and so on, tier upon tier, until the shed is quite full. The heat developed by the fermenting material vaporizes the acetic acid, and this vapour corrodes the lead, forming basic lead acetate. The carbon dioxide which is also produced during fermentation converts the acetate into the carbonate, which falls as a heavy white powder into the pots.

Future Supply of Acetic Acid. When all the operations involved in the production of acetic acid from wood, from the felling of the tree to the final separation of the glacial substance, are taken into consideration, it will be readily understood how it is that this acid has never been cheap when compared with other acids used on an equally large scale. In addition to this, the competition for wood for paper-making and for the very numerous cellulose industries is rapidly increasing. It is, therefore, not surprising to learn that chemists have turned their attention towards the discovery of newer and cheaper methods of making acetic acid.

Such a process seems to have been worked out in Germany. The starting-point is acetylene gas made by the action of water on calcium carbide. When this gas is passed through sulphuric acid containing suspended mercuric oxide or dissolved mercury salt, the acetylene is oxidized first to aldehyde and then to acetic acid.

If this process should prove to be successful, it will form the starting-point of a new and important industry, for, apart from the large amount of acetic acid which is used in commerce, there is the production of the very important solvent known as acetone, which can be made from acetic acid by a very simple operation.

Tartaric Acid. Grape juice contains a large quantity of potassium hydrogen tartrate dissolved in it; when the liquid is fermented and alcohol is formed, this salt crystallizes out because it is not soluble in alcohol. After the new wine has been poured off, the salt is found as a brownish crystalline residue adhering to the sides of the vat. Also the salt goes on crystallizing after the wine is put into barrels, and forms an incrustation on the sides. This is called the lees or sediment of wine. In commerce, the substance is known as argol (sometimes spelt argal), and also tartar of wine.