More permanent coatings for iron are paint, japan, enamel, and less corrodible metals. Paint is used on the structural steel of bridges, as well as on wagons, agricultural machinery, and other outdoor hardware; also on water-pipes and some other indoor articles. Carriage hardware, tea trays, the handles of scissors, and many other small articles are japanned, i.e. covered with a lacquer, which is then baked on, polished, and varnished.

Enamelware is now very commonly used in the kitchen. This is iron covered with a glaze similar to that used on porcelain and chinaware. It is put on in a molten condition, and solidifies on cooling. Enamelware is particularly satisfactory for culinary vessels, as it resists the action of the acids contained in foods, as well as that of the oxygen of the air. In using enamelware care should be taken not to crack the enamel by shock or by too rapid heating or cooling of the dry vessel.

Among the metals used to cover iron to protect it from the air are:

(a) Zinc. Iron covered with zinc is said to be " galvanized." The word is derived from the name of Galvani, the discoverer of the electric current, but the modern processes of galvanizing iron consist simply in immersing the cleaned iron in molten zinc, passing the sheet between rollers, and allowing to cool.

(b) Tin. Ordinary tinware is made of "tinplate," which is sheet iron covered with tin by a process similar to that used with zinc.

(c) Nickel. Iron is sometimes nickel-plated, the nickel being welded to the iron.

The magnetic oxide of iron, Fe3O4, formed by the action of atmospheric oxygen, or by that of very hot steam, on hot iron, adheres closely to the iron. In this respect it differs from rust, but it resembles the oxides of other metals. This fact has been utilized to prevent rusting. The iron is heated

in a furnace and superheated steam is blown in upon it. Iron with a blue finish has been so treated.

When once the covering layer is broken at a single point, tinware and nickelware will rust more rapidly than iron which has not received a protective coating. The same is true of iron covered with magnetic oxide, and probably also of enamelware. Galvanized iron, however, when similarly injured, does not corrode as fast as unprotected iron. The zinc appears to exert a protective action, even upon the exposed parts of the iron. This is one reason why galvanized iron is preferred to tinware for outdoor use. On the other hand, zinc is acted upon by vegetable acids. Hence galvanized iron is not suitable for culinary vessels or for receptacles for soft fruits, milk, or any acid food.

66

Rust Stains on Fabrics

Linen, cotton, and other textiles not infrequently become soiled with iron rust. Iron-rust stains are sometimes called iron mold," possibly on account of some confusion with mildew, which is really a mold. Being composed of ferric oxide, which is insoluble in water and in alkalies, such stains are not removed by the ordinary washing processes. Like other basic oxides, however, iron oxide is converted into soluble salts by the action of suitable acids.

Experiment 61.

Materials:

Oxalic acid.

Acid potassium oxalate.

Heat about 10 cc. (test tube) of ferric chloride solution to boiling and add sodium hydroxide solution. What is the precipitate? Write equation for the reaction by which it was produced.

Allow the precipitate to settle, pour off the supernatant liquid, add water, shake, allow to settle, and again pour off. (This is called washing by decantation.) Finally add a half test tube of water, shake thoroughly, and divide into five equal portions in test

tubes. To these add respectively: (1) dilute hydrochloric acid, (2) dilute sulphuric acid, (3) acetic acid, (4) oxalic acid, dissolved in hot water, (5) acid potassium oxalate, dissolved in hot water. Which of these acids dissolve the precipitate? Write equations for reactions. (In the last reaction potassium oxalate is formed as well as ferric oxalate.)

The acid chosen for the removal of rust stains from textile fabrics must be one that will do the work quickly and thoroughly but without injury to the textile itself.

Those most commonly employed are oxalic acid, H2C2O4, and its acid potassium salt, KHC2O4. This acid salt is commonly known as "salt of lemon" or "salts of lemon," although actually oxalic acid does not occur in lemons. Salt of sorrel is a more appropriate name. The action of this salt is less vigorous · - both on the rust and on the fabric than that of free oxalic acid.

Any of the acids used to remove rust stains may injure the fabric if not thoroughly washed out. As the fabric dries, the acid solution becomes more and more concentrated until it reaches a concentration at which it acts upon the textile fibers and weakens them. This may occur even with the volatile acid, hydrochloric, which may reach the concentration of 20 per cent hydrochloric acid before drying off completely. Goods which have been treated with acid for the removal of rust stains should be washed immediately in pure water, and afterwards in water containing a little ammonia.

I

CHAPTER XX

STRONG AND WEAK ACIDS AND BASES

Experiment 62.*

Materials:

3 equal pieces of magnesium ribbon, each weighing about 0.04 gram.

Lead foil, e.g. tea lead.

3 eudiometer tubes, 50 cc.

3 dishes, e.g. glass evaporating dishes.

Stands and clamps.

Normal (or approximately normal) solutions of hydrochloric, acetic, and formic acids.1

Fill one eudiometer tube with each acid solution, invert it in a dish of the same acid, and clamp it with the mouth a little below the surface of the liquid in the dish. Attach each of the pieces of magnesium ribbon to a piece of lead foil (e.g. by passing it through a slit in the latter) so that the magnesium cannot rise in the liquid. Bring the anchored pieces of ribbon quickly beneath the mouths of the eudiometers and compare the rates at which the hydrogen gas collects in the eudiometers.

Experiment 63.

Materials:

The normal acid solutions used in Experiment 62.
3 pieces of marble of about equal size and form.

1 If the laboratory reagents are on the normal system, the two former can be made by diluting the reagent dilute hydrochloric and acetic acids. A normal solution of formic acid may be made from pure formic acid (specific gravity 1.22) by diluting 37.7 cc. to 1 liter; and from acid of specific gravity 1.06 by diluting 173.6 cc. to I liter. Normal acetic acid may be made by diluting 57.1 cc. of glacial acetic acid to 1 liter. Normal hydrochloric acid may be made as follows: Dilute 105 cc. concentrated acid or 165 cc. of acid of specific gravity 1.10 to I liter. Compare the solution so obtained with a normal solution of sodium carbonate 53 grams of the pure, dry salt to 1 liter - by adding to the latter two or three drops of methyl orange solution and running in the acid from a burette until the well-mixed solution is just red. Then dilute the acid solution to such a strength that 10 cc. of it will exactly neutralize 10 cc. of the normal sodium carbonate solution.

Put the three pieces of marble in separate test tubes and pour one acid on each. Compare the rates at which gas (carbon dioxide) is evolved, and also the rates at which the marble is dissolved. Do the three acids arrange themselves in the same order with respect to their activity in dissolving marble as they did with respect to their activity in dissolving magnesium?

If we

Some acids are much more active than others. take two quarts of water and dissolve in one enough hydrochloric acid, in the other enough acetic acid, to yield one cubic foot of hydrogen gas by their action on zinc or magnesium, the hydrochloric acid will produce the gas much more rapidly than the acetic, although ultimately the same quantity will be set free from both. Hydrochloric acid also excels acetic in the speed of its action upon metallic oxides (such as magnesia, cupric oxide, etc.) and upon marble.

Compared in these and many other ways, hydrochloric acid is always found to be more active than acetic. For this reason it is designated a strong acid, whereas acetic acid is classed as weak. The relative strengths of other acids may be similarly compared, and by methods not very different the relative strengths of bases may be compared. The results of such comparison lead to the following classification:

Acids:

Strong: Hydrochloric, Nitric, Sulphuric.
Moderately strong: Oxalic, Tartaric, Citric.
Weak: Acetic, Palmitic, Stearic, Oleic.
Very weak: Carbonic, Boric (or Boracic).

Bases:

Strong: Sodium hydroxide (caustic soda), Potassium hydroxide (caustic potash), Calcium hydroxide (slacked lime in aqueous solution, limewater).

Moderately strong: Magnesium hydroxide.
Weak: Ammonium hydroxide.