In this way a sufficiently accurate estimate of the quantity of soda required to eliminate the impurities of the feed water can be made and the due proportion added to the feed water.
By exercising a little judgment, the use of pure chemicals, with well cleaned vessels, test tubes, etc., the following reagents will determine the character of the most important elements which injure the iron surfaces of a steam boiler.
Carbonic acid is indicated by baryta water.
Sulphates are indicated by chloride of barium.
Chlorides are indicated by nitrate of silver.
Lime salts are indicated by oxalate of ammonia.
Organic matter is indicated by chloride of mercury.
The “base” of the better class of the various patented boiler compounds is tannin (whence tannic acid) and some form of alkali, and if the compounds were to be deprived of these two elements they would be absolutely worthless.
Where they contain, as some certainly do, sal-ammoniac, muriatic, hydrochloric and sulphuric acids, they cannot but act as boiler destroying agents.
Tannin or tannic acid is the principal ingredient used in preparing leather. It is found in a great variety of plants—sassafras root has it in large proportion, the gall nut and the bark of various trees, especially the oak produce it.
It is the presence of this acid that gives their only value to very many “compounds,” tan bark, gum catechu (which sometimes contains one-half part of tannic acid), etc. The acid seems to have but little effect where large quantities of sulphate of lime are present, but in waters where carbonate of lime predominates its detersive qualities are more marked.
The records of the Patent Office show that one boiler compound contains 23 per cent. of catechu, and others, 60, 81, 5, respectively, by which may be inferred the large quantity of this agent, which has been sold in combination with other chemicals, principally soda.
Note.
While the product of water steeped in clean tan bark may be favorable in its action upon boiler incrustation, it has been found to be very unsafe, in practice, to use the “tan liquor” taken from the vats. The danger arises from the fact that sometimes during the process of tanning leather, the required acidity cannot be produced by natural fermentation when sulphuric acid is added, in order to bring the liquor to its required strength—in due course, this corrosive substance acts injuriously on the boiler.