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It may best be preserved in jars containing calcium chloride, but this must not be allowed to come into contact with the wire. It should, however, even then be examined carefully for rust before use. If pure ferrous ammonium sulphate is used as the standard, clear crystals only should be selected.
Owing to the carbonates that are always present in sewage, ferrous carbonate is also formed. The success of these laboratory experiments led me to a trial of the process on a larger scale, for hitherto only a gallon at any one time had been treated. Small brick tanks were erected at my wharf at Peckham and iron electrodes fitted to them.
PROCEDURE. Fill a glass-stoppered burette with the permanganate solution, observing the usual precautions, and fill a second burette with the ferrous sulphate solution prepared for use with the potassium bichromate. The permanganate solution cannot be used in burettes with rubber tips, as a reduction takes place upon contact with the rubber.
Limestones vary widely in composition from a nearly pure marble through the dolomitic limestones, containing varying amounts of magnesium, to the impure varieties, which contain also ferrous and manganous carbonates and siliceous compounds in variable proportions.
From 400,000 pounds of silver and 5,000 pounds of gold were obtained twelve pounds of platinum, two pounds of palladium, and several pounds of selenium. To obtain these the gold is first precipitated from the solution by ferrous chloride, all the other metals by iron turnings.
The solution has so deep a color that the lower line of the meniscus cannot be detected; readings must therefore be made from the upper edge. Run out into a beaker about 40 cc. of the ferrous solution, dilute to about 100 cc., add 10 cc. of dilute sulphuric acid, and run in the permanganate solution to a slight permanent pink.
The instrument is then removed to the dark room, and the plates developed by immersing them all at once in a solution consisting of four parts potassium oxalate and one part ferrous sulphate. After ten minutes they are removed, fixed, and dried. Their readings are then noted, and compared with those obtained with the silver chloride.
The processes of volumetric analysis are easily classified, according to their character, into: I. NEUTRALIZATION METHODS; such, for example, as those of acidimetry and alkalimetry. II. OXIDATION PROCESSES; as exemplified in the determination of ferrous iron by its oxidation with potassium bichromate.
Add 10 cc. of dilute sulphuric acid and titrate to a faint pink with the permanganate solution, adding it directly to the contents of the vacuum flask. Should the end-point be overstepped, the ferrous sulphate solution may be added. From the volume of the solution required to oxidize the iron in the wire, calculate the relation to the normal of the permanganate solution.
Potassium permanganate oxidizes ferrous salts in cold, acid solution promptly and completely to the ferric condition, while in hot acid solution it also enters into a definite reaction with oxalic acid, by which the latter is oxidized to carbon dioxide and water.
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