TABLE 3.-OXYGEN DISSOLVED BY DISTILLED WATER. 5-30°C. water would contain 5.026 cc of oxygen at saturation point. To obtain the result in milligrams of oxygen multiply the above figure by 1.429 which is the weight in milligrams of 1 cc of oxygen at normal temperature and pressure. The figure thus obtained in this case is 7.18 milligrams. H. A. White continues, "Fill (with a siphon), completely, one of the 250 cc (nominal) bottles with the water and add 0.100 gm. of solid "pyro"-the densely crystalline form sold as "Pyraxe" is preferred-and shake all the crystals below the water surface. Then add from a burette, dipping below solution level, 1 cc of the 2N NaHO and immediately close the bottle with the stopper in such a manner as to exclude any air bubble at all. up till all the crystals are dissolved, and observe the colour. Shake "This colour is then matched with the solution of a brown dye. I find that 'Diamond' brown requires a little acid, methyl orange and a small amount of chromate of potash to match the shade exactly. Make up about three litres of the dye solution, adjusted till another 250 cc bottle-full of it will exactly match the water tested when both are held together to the light. "A somewhat better match, preserving the tint well on dilution, may be made by using the brown colour obtained by shaking up a solution of pyrogallol or eikonogen with excess of air till further darkening has ceased. In this case some slight change of tint takes place in a day or two. "Take eight of these nominal 250 cc bottles and determine the exact capacity, which will be very close to 300 cc, and number 1 to 8. Into No. 1 pour the matched dye solution and insert the stopper without leaving any air bubble. This will represent the amount of oxygen found in the water sample, and the other bottles will have poured into them a proportionate quantity of the dye solution till the series represents 7, 6, 5, 4, 3, 2, 1 and 1⁄2 mgm. of oxygen per litre respectively. "If the water sample does not come out sufficiently near 7.0 mgm. per litre, the following figures will have to be varied to correspond; and the capacity of the bottles, if more than 2 cc or 3 cc away from 300 cc, will also naturally affect the calculation. 66 'Assuming 300 cc bottles and 7.0 mgm. oxygen per litre in the sample water— Into No. 1 bottle pour 300 cc matched dye solution and fill with water. Into No. 2 bottle pour 257 cc matched dye solution and fill with water. Into No. 3 bottle pour 214 cc matched dye solution and fill with water. Into No. 4 bottle pour 172 cc matched dye solution and fill with water. Into No. 5 bottle pour 129 cc matched dye solution and fill with water. Into No. 6 bottle pour 86 cc matched dye solution and fill with water. Into No. 7 bottle pour 43 cc matched dye solution and fill with water. Into No. 8 bottle pour 21.5 cc matched dye solution and fill with water. In any case the standards are adjusted to represent from 7 down to 1⁄2 mgm. per litre of oxygen, and are so marked, preferably, with a diamond on top of the stopper. The four remaining bottles are marked A, B, C and D in the same way." "Any solution may now be tested by plunging a bottle, held vertically, well below solution level, jarring it and moving to allow all air bubbles to escape, then dropping in the stopper while still under solution. The stopper may be given a downward twist when removed from the solution and the bottle will then be sufficiently air-tight for the few moments which elapse before testing. Using a siphon to fill the bottle is obviously a more accurate method, but that given is sufficient and more easily applied. Solutions in contact with pulp must be settled in a two-litre bottle filled in the above manner, and the sample bottle filled therefrom by a siphon. "In testing the sample of solution, the stopper is gently removed with an upward twist so as not to lose any liquid. The 0.100 gm. pyrogallol (of which a sufficient number of lots is ready weighed out) is then dropped into the bottle and shaken down below solution level; 1 cc of 2N NaHO is then run in from the burette with point below solution level, and the stopper instantly replaced in such a way that no air bubble is left. After shaking till all crystals are dissolved, the sample bottle is placed between the two standards it more nearly resembles, and thus the oxygen may be determined to 0.5 mgm. per litre. If the colour is not exactly the right tint it will become so within 15 minutes—some solutions retain a sort of purplish tint with pyrogallol for a short time." Assay of Cyanide Solutions for Gold and Silver. Method No. 1.-Evaporate a known quantity in a square boat made of lead foil, scorify, and cupel. This is not reliable for low grade solutions because the amount that can be taken for assay is too small. Method No. 2.-Evaporate a known quantity in a large porcelain dish whose bottom is covered with a thin layer of litharge. When dry remove the litharge, carefully clean the dish with small pieces of filter paper dipped in dilute hydrochloric acid, the paper being dried, burnt, and added to the litharge. The latter is then fluxed and fused down in a crucible. The method may be used for amounts of solution up to 20 assay tons and is reliable though tedious. Method No. 3 (Virgoe).-To a known amount of solution add weak copper sulphate solution in slight excess, that is, until the clear filtrate shows a faint blue tint, then acidify with hydro chloric or sulphuric acid, and filter. It is also advisable to add a little sodium sulphite solution to ensure that the last traces of gold are thrown down. Should the solution be very weak in cyanide it is advisable to strengthen it to about 0.05% KCN before precipitating. For 10 assay tons of solution Clennell1 recommends Copper sulphate (10% CuSO4.5H2O). Sulphuric acid (10% H2SO4) . . 20 cc 20 cc 10 cc These are added in the order given stirring after each addition. The precipitate is then filtered, but washing is unnecessary. When sufficiently drained 60 grams of the following flux are placed in the same filter paper: Borax.. Charcoal. 30 parts 30 parts 1 part The paper is then folded up and fused down in a small crucible, the resulting button of lead being cupelled. Method No. 4 (Chiddey-Magenau).-To 10 A.T. of solution (which if very weak in cyanide should be strengthened up to about 0.5% by addition of the proper quantity of concentrated solution) add 10 or 12 cc of a 10 per cent. solution of lead acetate and about 0.3 to 0.4 gram of zinc dust: stir and bring to a boil. Then add 15 cc of strong HCl and leave on the hot plate until the excess zinc has all dissolved, when the beaker should be removed and allowed to cool. Decant the solution and wash the spongy lead several times in large volumes of water. The lead may then be collected and pressed into a ball between the fingers. This ball is then folded up in lead foil in which a vent has been left for escape of steam, and cupelled. Colorimetric Estimation of Gold in Cyanide Solution. The following method was devised by C. W. Dowsett, Mill Superintendent of the Dome Mines in Ontario. 1 The Cyanide Handbook, page 517 (Second Edition). Take approximately 1000 cc of solution in a tight stoppered jar. A Mason fruit jar or pickle bottle is convenient. Add 2 to 3 drops of lead acetate. (Clear saturated solution.) Add a pinch of zinc dust. (Approx. 2 grams.) Add 10 to 55 cc saturated NaCN solution to bring up to about 0.1% KCN. Pour out into a large evaporating dish. Add 10 cc aqua regia and evaporate to 3 cc. Pour into small test tube and cool thoroughly. (Cooling is very important.) The presence of gold will be indicated by a purplish ring or, if the tube is shaken, by a purplish tinge throughout. The whole operation may be performed over a spirit lamp or similar flame and does not take over 5 minutes. The presence of as little as 0.02 dwt. gold per ton of solution is very plainly shown. Colorimetric Estimation of Silver in Cyanide Solutions.-A few drops of a 10% solution of sodium sulphide added to 25 to 50 cc of the solution to be tested, gives a pure white precipitate of zinc sulphide in the absence of silver. The precipitate becomes brownish in the presence of silver, and the depth of color is a very close indication of the amount present. 4 |