Here is a way how to determine the oxygen content during gold leaching and Estimation of available Oxygen Content in Cyanide Solutions.
Until recently no simple and reliable method has been known for the estimation of free oxygen in working cyanide solutions so that the one here described supplies something that has long been needed. The matter takes on an added importance in the light of the results being obtained by the Crowe Vacuum Process for precipitation.
This method is based on the degree of coloration imparted to a solution of pyrogallic acid or some similar substance in the presence of caustic soda by the oxygen contained in the solution.
“Apparatus Required.—One dozen (nominal) 250 cc stoppered bottles, fairly regular in diameter and capacity: one 50 cc burette.
“Chemicals Required.—Sodium hydrate 2N solution (80 gm. per litre), pyrogallic acid or other suitable developer, brown dye.” (H. A. White.)
Preparation of standards for comparison
A suitable volume of distilled water must first be saturated with air and for this purpose the simplest method is probably that recommended by Sutton. Two large acid bottles are taken and half filled with distilled water; they are then well shaken for a few minutes or to save trouble may be put on the agitator wheel for an hour or so. The air in the two vacant spaces is then replaced by pouring the contents of one bottle into the other and vice versa after which the water is evenly divided between the two bottles and again subjected to agitation; this is repeated several times after which it may be considered to be saturated and the bottles are allowed to stand at rest for half an hour before being used. This water is then used as a standard by which to make up the standard colors for the comparisons. The point of saturation of water with atmospheric oxygen will of course vary with the temperature and pressure so in order to find the amount of oxygen present at saturation point under various conditions the following table by Roscoe and Lunt is referred to, after taking a reading of the temperature of the water and the barometric pressure.
The table is calculated on the basis of a barometric pressure of 760 millimeters but if the observed pressure is below this 1/76 of the value must be subtracted for each 10 millimeters difference while if the observed pressure is above 760 millimeters 1/76 of the value given must be added for every 10 millimeters difference. For instance, suppose that the temperature of the water after aeration is 21 deg. C. and the barometric pressure 620 millimeters, then from the table the number of cc of oxygen representing the saturation point at 21 deg. C. (for a barometric pressure of 760 millimeters) is 6.16. The difference in the barometric pressures is 760 — 620 = 140 and the correction is therefore 6.16 — (14/76 X 6.16) = 5.026, showing that under the conditions assumed the 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.
“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. Shake up till all the crystals are dissolved, and observe the colour.
“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 ½ 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.
“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 ½ 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.”