tated by the sodium carbonate so formed. If the amount of sodium carbonate is not sufficient to decompose all of these bodies, a sufficient quantity should be added with the sodium hydroxide to effect the complete decomposition. The precipitate is allowed to settle or filtered off. In cases in which the feed water is heated before it enters the boiler, it may only be necessary to add to the water sodium carbonate in quantity sufficient to decompose the calcium and magnesium chlorides and sulphates, since the heat alone will suffice to throw down the carbonates. Care should be taken in these precipitations that no more sodium hydroxide is added than is sufficient for the decomposition, since any excess would tend to corrode the boiler. Clark's process consists in treating the water with calcium hydroxide (lime-water). This precipitates the calcium and magnesium carbonates by depriving the water of its free carbonic acid. It has, of course, no effect upon the calcium sulphate. It is to be noted that the proportion of calcium hydroxide which is to be added must be calculated from the amount of free carbonic acid existing in the water, and not from the amount of carbonates to be removed. The precipitate will usually require at least twelve hours for complete subsidence, but after three or four hours the water will be sufficiently clear for some purposes. If a filter press is used, as in Porter's process, the time required for clarification is very much shortened. Another advantage of this process is the use of a solution of silver nitrate, in order to determine more conveniently the proportion of calcium hydroxide which is to be employed. The lime is first slaked and dissolved in water, and the water to be softened run in and thoroughly mixed with it. From time to time small portions are taken out in a cup and a few drops of a solution of silver nitrate added. As long as the lime is in excess a brownish coloration is produced. When this has become quite faint, and just about to disappear, the addition of the water is discontinued, and, after a short time, the water is filtered by means of the press. Soluble phosphates added to a water, precipitate completely in a flocculent condition any calcium, magnesium, iron or aluminum. This reaction can be best applied by using the tri-sodium phosphate (Na,PO + 12H2O), which is now a commercial article. By reason of the facility with which this substance loses a portion of its sodium to acids, it acts not only as a precipitant to the above materials, but will neutralize any free mineral acid present in the water. From evidence submitted by those who have used the process on the large scale, it appears that not only is no hard scale formed, but that scale already existing prior to its use is gradually disintegrated and removed with the sludge. Experiments indicate that no injury results from an excess of the material; but the economical employment of the method, especially with very hard waters, can only be based upon a correct analysis, and an estimation of the phosphate required for the precipitation. In many cases the composition of the water will be such that a partial precipitation will be sufficient for all purposes. In regard to the quality of water for technical other than steam-making purposes, such as brewing, dyeing, tanning, etc., no detailed methods or standards can be laid down. The nearest approach to purity that can be secured in the supply will be of the greatest advantage. The more objectionable qualities will be large proportion of organic matter, especially if it distinctly colors the water, and excessive amounts of iron or free acid. For the removal of these the same processes are applicable as are indicated in the directions for the purification of the water for other purposes. 98 DIBDIN'S TABLE OF DISSOLVED OXYGEN. DIBDIN'S TABLE OF OXYGEN DISSOLVED BY WATER AT VARIOUS EXTENDED TO GIVE THE WEIGHT OF OXYGEN CORRECTED TO 0°C. AND 760mm. PRESSURE. TEMPERATURES. TEMPERATURE FAHRENHEIT. 41° 42 TEMPERATURE The table is calculated for a barometric pressure of 760 mm., and would require corrections for variations from this, but such corrections are mostly within the limits of experimental error. |