Determination
OF
Dissolved Oxygen
Proposed Modification of the Winkler Method RICHARD POMEROY'
AND
H. DARWIN KIRSCHMAN
University of California, Los Angeles, Calif.
Use of increased quantities of iodide in the Winkler method for determining dissolved oxygen diminishes interference b y organic matter, reduces loss of iodine vapor, and sharpens the end point of the titration. It is recommended that the hydroxide-iodide solution usually specified for this analysis, which is 12.5 N in alkali and 0.9 N in iodide, b e replaced b y one which i s 10 N in sodium hydroxide and 6 N in sodium iodide.
Table
Influence of Iodide Content on Reaction of Iodine with Organic Matter in Winkler Test (50 mg. of KI per sample added before acidification, additional amounts after acidification; titrated after 3 hours) Total R I Used per 250-hI1.Sample Grams 0.050 0.100 0.150 0.300 1.0 3.0
T
HE procedures generally followed for determination of dissolved oxygen in water call for the use of a hydroxideiodide solution of such composition and in such amount that the analysis of a 250- or 300-ml. sample of water uses 0.15 gram of potassium iodide or an equivalent amount of sodium iodide. It has long been evident that it may be desirable to use larger amounts of iodide, and some workers have modified their procedures accordingly, but the point has not heretofore been fully explored and the standard procedure has remained unchanged. The advantages of high iodide concentration are: reduced interference by other reducing agents, less loss of iodine vapor, and sharp end point. Table I.
Used Gram
Oxygen P.p.m.
0.140 0.140 0.140 0.140
Series A 0.000 0.050 0.200 0.500
8.07 6.96 5.28 4.70
0.127 0.254 0.635 1.270
Series B 0,200 0.200 0.200 0.200
5.96 6.87 7.64 8.01
Apparent Dissolved Oxygen .P.p.m. 1.60 1.73 1.88 2.05 2.19 2.27
would be expected to counteract this interference, just as it does with oxalate, and this is found to be the case. Thus when tests were made with aerated sewages using various concentrations of potassium iodide results were as shonn in Figure 1. The true concentrations of oxygen in these samples were riot knovn, but they must have been somewhat greater than the highest apparent values. The extent of interference by organic matter varies with the technique of the procedure.. If the precipitate of manganous and manganic hydroxides settles and compacts in the bottom of the bottle, and if the acid is added without prompt and complete mixing, local depletion of iodide occurs, thus permitting more extensive oxidation of organic matter. Interference is minimized by adding the acid promptly and with rapid mixing. The effect of delay in adding the acid has been noted by others but has been attributed to a reaction of oxygen with organic matter in the alkaline solution. This is not possible, for there is no oxygen in the m i x t u r e o n l y manganic hydroxide, which certainly does not act as an oxidizing agent in alkaline solution.
Oxalate Interference in Dissolved O x y g e n Test (NH4)aCzOr.Hz0 Apparent Dissolved
N a I Used, (250-M1. Sample) Grams
II.
REDUCTION O F INTERFERENCE BY O T H E R REDUCING A G E N T S
Theriault (4) showed that the use of excess oxalate in the permanganate method causes low results, and that this interference is diminished by increasing the concentration of iodide. Further data showing the magnitude of these effects as obtained in the authors' research are given in Table I. The evident explanation is that a t time of acidification the manganic hydroxide acts on any reducing agents which may be present. Iodide, oxalate, and all other reducing agents compete with one another for the oxidizing capacity of the manganic hydroxide. Concentration and rate of oxidation determine the relative extent of oxidation of each component. If oxalate is present in substantial amount and the iodide concentration is low, a considerable amount of manganic hydroxide will react with oxalate, causing low results in the determination. But if a high concentration of iodide is used, more iodide and less oxalate nil1 be oxidized. Thcriault and McSamue ( 5 ) showed that glucose and peptone also cause ON results, and it is to be expected that the miscellaneous organic matter generally present in polluted waters would hnve a similar effect. Bt thp same time high iodide concentrations I
GIPIMS K I Pen SANPLC
Figure 1.
Effect of Iodide on Apparent Dirro1ved;Oxygen
After adding the acid, there may often be a slow reaction of iodine with organic matter. This, too, is diminished by a high concentration of iodide. To illustrate this, the Winkler test was carried out on a gallon sample of aerated sewage, using 50 mg. of potassium iodide per 250 ml. of sewage. This was then used to fill several 250-ml. bottles t o which various amounts of potassium
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iodide were added. After 3 hours, these mixtures were titrated with 0.025 N thiosulfate. Results of the determinations are shown in Table 11. LOSS
OF IODINE VAPOR
As shown by Theriault (4), loss of iodine vapor may cause appreciable errors, which are likewise diminished by increasing the iodide concentration, since iodide holds the iodine in solution by the equilibrium reaction I- It Is-. When waters of low exygen content are analyzed, the error due to vaporization of iodine is slight, but in the analysis of waters of high oxygen content, there is less iodide left a t the end of the reaction and more iodine and triiodide ion. This causes a disproportionate increase of free iodine and hence loss of iodine vapor. When 150 mg. of potassium iodide are used in the analysis of 300 ml. of water, there is just enough iodide present to yield triiodide ion quantitatively if the water contains 16 p.p.m. of oxygen. The concenbration of free iodine and the consequent loss of vapor rapidly increase as the oxygen in the water increases up t o and beyond 16 p.p.m. At 8 p.p.m. of oxygen there is a twofold excess of iodide when 150 mg. of potassium iodide are used for a 300-ml. sample. If the potassium iodide content were t o be increased sevenfold, as is proposed in this paper, the excess of iodide would be thirteen times as great, which would reduce the concentration of free iodine to about one thirteenth, and this in turn would be expected to reduce the rate of escape of iodine vapor by a similar ratio.
+
-
SHARPNESS OF END POINT
Higher concentrations of iodide increase the sharpness of the end point in the titration. No quantitative data have been secured, but in comparing end points, the analyst gets the impression that their sharpness is roughly proportional t o the iodide concentration-that is, the color produced by a minute amount of iodine in the presence of starch is proportional t o the iodide content of the solution. In ordinary work this is not an important factor; but when high precision is desired, it is easily possible in relatively clean waters to obtain end points reproducible to 0.01 or 0.02 ml. of 0.025 N thiosulfate if liberal amounts of iodide are used.
Figure 9.
Solubility of Iodides in Hydroxide Solutiona
In view of the foregoing considerations, it seems generally desirable t o use more iodide than has been customary heretofore. In the past the cost of iodide salts may have been a large consideration, but this is relatively less important at present. The proposed change will increase the cost of a determination by about one cent, a small amount compared with the total cost of an analysis. The increase is fully justified in most dissolved oxygen work.
Vol. 17, No. 11
The most convenient way to use more iodide in the test is to increase the iodide content of the iodide-hydroxide solution. Vsing potassium salts, not much more than the 150 grams per liter n o y specified can be dissolved. An investigation was undertaken of the solubilities of potassium and sodium iodides in potassium and sodium hydroxides. Data for potassium and sodium iodides in sodium hydroxide and for potassium iodide in potassium hydroxide have been published (1, 2, 9). The results are shown graphically in Figure 2, which includes results for sodium iodide in potassium hydroxide. Precipitation of potassium iodide from the solution occurs as soon as the sodium hydroxide reaches a concentration of about 0.3 N . The dotted curve beyond this point represents the precipitation concentration rather than a true solubility. This curve is sketched from two experimental points and from the consideration that the precipitation curve must cross the line for potassium iodide in sodium hydroxide a t the point where potassium iodide concentration equals sodium hydroxide concentration and that it will thereafter approach, but not cross, the line for potassium iodide in potassium hydroxide.
Table 111.
Effect of Potassium Hydroxide Concentration in the Winklcr Anrlysis
Normality of KOH Used (1 M1. per 250-MI. Sample) 1.2 2.4
Ap arent Dissolved &,,en, P.p.m.
7.2 9.6 12.0 2 ml. of 12 H
9.00 9.01 9.01
4.8
4.73 7.45 8.99
8.99
The curves show that the only practical way t o prepare hydroxide-iodide solutions with substantially increased iodide content is to use sodium iodide and sodium hydroxide. The choice of a suitable composition for the reagent involves the question of required hydroxide concentration. A series of tests was run t o explore this point, with results as shown in Table 111. I n these tests 1 ml. of 4.3 N potassium hydroxide is necessary t o precipitate quantitatively the manganese in 1 ml. of the standard reagent, and it is evident that only a slight excess of potassium hydroxide over this amount is required for the quantitative determination of oxygen. If the iodide-hydroxide solution is made 10 N in sodium hydroxide and 6 N in sodium iodide, this should provide an ample excess of hydroxide t o care for any buffering constituents in the sample. A solution approximating this composition can be made by dissolving 900 grams of sodium iodide and 400 grams of sodium hydroxide pellets in 550 ml. of water, or enough water to make 1 liter of solution. The small amount of sodium carbonate which will be present as an impurity is insoluble in this solution and will slowly separate on standing. The solution may be clarified by filtration or decantation, but this is not necessary as the carbonate has no effect on the analysis. It is recommended that this solution be substituted for the hydroxide-iodide solution of composition specified in current manuals whenever maximum accuracy is desired, and that it be used for all samples containing much organic matter even if only approximate results are required. LITERATURE CITED
(1) Kirschman, H.D.,and Pomeroy, Richard, J . Am. Chem. SOC., 65, 1695 (1943). (2) Ibid., 66, 1793 (1944). (3) Pomeroy, Richard, and Kirsohman, H. D., Ibid., 66, 178 (1944). (4) Theriault, E.J., U.S. Public Health Bull. 151 (1925). (5) Theriault, E.J., and MoNamee, P. D., U.S. Pub. Health Re&., 48, 1363-77 (1933).
