This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
1. A ready means of testing a water for organic matter is to add a few drops of solution of permanganate of potash to the water, and allow it to stand for a short time. The color which the water receives from the test will remain if it be entirely free from organic matter, but will gradually disappear if organic matter be present. The more organic impurity present, the sooner the pink color will change.
This solution of permanganate of potash communicates a bright violet-rose color to the water when first added. If, however, decomposed organic matter be present in a degree hurtful to health, this color is changed to a dull yellow; or, if a still larger quantity exists in the water, the color will in time entirely disappear. Where the color is rendered paler, but still retains a decided reddish tinge, then we may infer that, although putrefying organic matter is present, it is so in such minute quantities as are not likely to be immediately hurtful. The smaller the quantity of this test applied, the sooner will the result be shown. It is also essential to test the water previously for iron, as, if present, it will mislead, as the indications will be the same as if organic matter were present. One drop or two to the test glass is the quantity to be added to the water. It should be allowed to stand for two hours; if, however, the change in color takes place before the expiration of this time, it is a stronger indication of the impurity of the water - the rule being that the quicker and more perfect the discoloring of the water tested, the greater is the quantity of decomposing organic matter present; if, also, upon the addition of a few more drops, a change in color is manifested, it is a sign that a very large and dangerous quantity of putrefying organic matter is present.
The solution of permanganate of potash is made by dissolving 1 grain of permanganate potassium crystals in 90 grains (1 drachm and a half) of distilled water. Keep in a glass-stoppered bottle. It will keep a year if properly protected from light and air.
2. Prepare the following solution:
Permanganate of potassium .. 1 grain.
Distilled water...90 grains. Commercial solution of potassa .. 70 grains. Keep in a glass-stoppered bottle in a dark place.
Fill one of the test-tubes nearly full with the water to be tested, and the other to exactly the same height with distilled water, and to each of these must be added with a dropping tube exactly the same number of drops of the permanganate solution. About two drops will be required. The solution taken up in the dropping tube and not used must not be returned to the stock bottle, but must be thrown away, and the tube immediately washed out. The test tubes are now agitated to mix their contents and then set in the rack, with a piece of cotton wool stopping the mouth of each. A sheet of white paper is set at the back, and the tubes are left for at least 24 hours and the changes of color noted from time to time. The distilled water, if good, will retain its beautiful pink color with very little precipitate for two or three days, and, being placed in close proximity to the water to be tested, will make manifest, by contrast, any change of color this last may undergo. The first change will be from pink to scarlet, then dull scarlet, then muddy scarlet, and finally color is lost altogether. If these changes take place quickly, the drinking water is bad; if slowly, the case is more hopeful; if not until after 24 hours, decidedly good.
3. An authority has the following about Permanganate of Potash and Organic Matter:
The union of oxygen with dead organic matter always occurs when the two are brought together under favorable circumstances, and the disappearance of the one may be made to reveal the presence of the other. The solution of permanganate of potash has an intensely deep purple color, which is owing to the oxygen it contains. Whenever this solution is brought in contact with easily oxidizable substances, it loses its oxygen and consequently its color. If, therefore, enough of the solution be added to a suspected water to impart a distinct tint, and the color disappears, it is certain that something is present which is capable of taking the oxygen from the permanganate. Whether this is organic matter or something else is uncertain without the application of other tests. The only other substances which are apt to occur in water, and are capable of effecting the change, are ferrous salts, nitrites and hydrogen sulphide. If these are known to be absent, and the color of the permanganate disappears, it may be decided that organic matter is present. But if either of these occurs, the test has no value.
The methods for detecting nitrites and iron, which is most always, when present, in the form of a ferrous salt, are appended; also the method of detecting hydrogen sulphide. Sometimes, however, iron occurs in water as a ferric salt. This does not affect the permanganate; but the method given for detecting iron makes no distinction between its two classes of salts. To distinguish them is too difficult, except for the chemist.
It is another drawback to the permanganate test that it does not act on albuminous substances, urea, kreatin, sugar, gelatine or fatty matters. So that a water might be very badly polluted and yet give no indication of it with this test. Cases are recorded where sickness resulted from the use of water supposed to be good, because it did not affect the permanganate. Other instances are recorded where good water was condemned from the application of this test. From what has been said, it will be seen that this test alone is reliable only when iron, nitrites and hydrogen sulphide are known to be absent, and at the same time the color of the solution disappears. It is often valuable as a confirmatory test, and for that purpose it is described here.
The solution is easily prepared by dissolving the crystals of permanganate of potash in pure water. To apply the test, take two tumblers, of clear glass; fill one with water of known purity, and the other with the water to be tested; then add a drop of the solution to each, and compare the change in color. Those who have been accustomed to work by this method are guided by the following rules: If decomposing organic matter be present in a degree hurtful to health, the pink color is changed to a dull yellow; or, if a still larger quantity exists in the water, the color will in time entirely disappear. Where the color is rendered paler, but still retains a decided reddish tinge, then, although putrefying organic matter is present, it is so in such minute quantities as are not likely to be immediately hurtful. The quicker and more perfect the decoloration of the water tested, the greater is the quantity of decomposing organic matter. The following preparation of permanganate is a more delicate and perhaps a more reliable test than the simple solution:
Caustic potash .. 4 parts by weight.
Permanganate of potash . . .1 part "
Distilled water . . . .160 parts "
If it is found inconvenient to weigh the very deliquescent caustic potash, the liquor potassae of commerce may be substituted. Then the formula is:
Liq. potassae....70 parts.
Distilled water....90 "
Permanganate of potash .. 1 part.
If the solution is kept in a glass-stoppered bottle in a dark place, it will remain good for a year or more. This test is applied in the same manner as the simple solution. It is claimed that water of average good quality, with this test, will keep its color well for forty-eight hours. If it becomes decidedly paler in twenty-four hours, it is hardly fit-to use.
Those who employ this method do not claim for it scientific accuracy, but think, in the absence of opportunity for more careful analysis, a ready and reliable conclusion may be reached. We think the claim for reliability is too strong, on account of the same reasons that were given under the description of the simple solution.
It would be interesting and profitable for any one purposing to use the permanganate test in either form, to collect samples of water from several sources - wells, springs, brooks and stagnant pools - and to apply the test to them, comparing the results. It would be well to do the following also: Add a little sulphate of iron to water distinctly colored with permanganate. The color will quickly disappear. Repeat the experiment, using nitrite of potash, having prepared some by boiling a solution of saltpetre with zinc. The effects of hydrogen sulphide may be seen by doing the experiment with sulphide of soda.