The anions which form no complexes or insoluble precipitates with aluminum such as sulfate, nitrate, perchlorate, acetate, chloride, bromide, and iodide do not interfere. However, phosphate interferes quite seriously at pH 3.8. At this pH, the amount of phosphate cannot exceed more than 5 pmoles/25 ml. The phosphate interference can be diminished
by lowering pH. At pH 3.0, approximately 30 pmoles of phosphate/25 ml can be tolerated. RECEIVED for review November 29, 1968. Accepted Februcry 29, 1969. Work partially supported by the National Science Foundation, GP-8152.
Spectrophotometric Determination of Dissolved Oxygen in Water G . S. Sastry, R. E. Hamm, and K . H . Pool Chemistry Department, Washington State Uniuersity, Pullman, Wash. 99163
MANYMETHODS have been proposed for the determination of dissolved oxygen in water since the original work of Winkler (I). Literature on the subject can be broadly divided into: Winkler method and its modifications; colorimetric methods ; electrochemical methods; and others, such as radiometric methods. The first category of methods was well reviewed by Caritt and Carpenter (2). Their conclusion was that the Winkler method in its modified version is reliable for most samples. Various spectrophotometric analyses have been proposed, from one using the iodine color obtained from the Winkler method, to those using the oxidation of the leuco base of indigo-carmine (3). De Carvalho et al. proposed ( 4 ) the use of the Mn(II1)-EDTA complex in a spectrophotometric determination of dissolved oxygen. The method proposed was a highly empirical one because the resulting absorbances were not directly proportional to oxygen concentration, and were far lower than complete reaction should have given. The trans-1,2-diaminocyclohexane-tetraacetic acid complex of Mn(III), [Mn(III)CyDTAl, has been reported (5) to be more stable from both a kinetic and thermodynamic standpoint than the corresponding Mn(II1)-EDTA complex. This paper reports an investigation of the use of the Mn(II1)CyDTA complex for determining dissolved oxygen in water. EXPERIMENTAL
All reagents used were of analytical grade. The CyDTA used was obtained from K & K Laboratories and recrystallized before use. The 24 biological oxygen demand (B.O.D.) bottles used in this work were permanently numbered, along with their corresponding stoppers, to ensure proper fit. All the bottles of 300-ml nominal volume were individually calibrated before the work was started. Samples. To make samples containing various amounts of oxygen, either oxygen or nitrogen was bubbled through distilled water contained in 2-1. bottles. The water was immediately siphoned into four different B.O.D. bottles. Two of these were treated by the modified Winkler method and two by the proposed spectrophotometric method. Procedure: Modified Winkler Method. Twelve B.O.D. bottles containing six samples in duplicate were taken. Two ~
(1) L. W. Winkler, Ber. Deut. Che. Ges., 2 , 2843-55. (2) D. E. Caritt and J. H. Carpenter, J. Marihe Res., 1966, p 286. (3) A. H. Meyling and G. H. Frank, Analyst, 87, 60-2 (1962). (4) A. H. De Carvalho, J. G. Calado, and M. L. De Moura, Rec. Port. Quim.,5,15-19 (1963). (5) M. A. Suwyn and R. E. Hamm, Inorg. Chem., 6 , 139 (1967).
milliliters of 3M MnC12 followed by 2 ml of a solution 8N NaOH 4M NaI were added with separate syringes whose needles were dipping into the sample. The bottles were stoppered and shaken vigorously. The precipitate was allowed to settle approximately two thirds of the way to the bottom, and shaking was repeated. After the precipitate was allowed to settle the second time, 2 ml of 5M HsS04 were added with a syringe; this time the acid was allowed to trickle down the side. Each bottle was stoppered and again vigorously shaken. Fifty milliliters of the solution were removed with a pipet. The remaining solution in the bottom was titrated with standard thiosulfate using Thyodene (Fisher Scientific Co.) as the indicator. The oxygen concentration obtained in this way was taken as the standard value of dissolved oxygen in the sample. Spectrophotometric Method. The other 12 bottles containing the same six duplicate samples used in the Winkler method were treated with 2 ml of 3M MnClz solution followed by 2 ml of 8N NaOH. The bottles were vigorously shaken and the precipitate allowed to settle. Solid CyDTA, 0.5 g, was added to a typical sample bottle, the bottle was shaken, and 1.4 ml of 5M H2S04 were added and again shaken, The red-purple color of Mn(II1)CyDTA complex appeared at this point. The undissolved CyDTA was filtered off using a medium porosity sintered glass filter. The absorbance at 500 mp was determined with a Cary Model 14 spectrophotometer using 1-cm cells. The absorbance remained constant (within observation error) for more than 10 min. The absorbance of a typical sample decreases, following pH adjustment, at a rate of approximately 3% per hour at room temperature. Blanks. By bubbling purified nitrogen through a sample for an hour, a blank sample was prepared. The Winkler blank corresponded to 0.22 ppm oxygen (ppm as used herein refers to milligrams of O2 per liter solution), and the spectrophotometric blank was 0.010 absorbance unit. All results listed were corrected for both blanks.
+
RESULTS
Effect of pH. Addition of quantities of sulfuric acid to the spectrophotometric samples different from 1.4 ml recommended above led to the results summarized in Table I. The absorbance is sensitive to acidity in the pH range 2.0 to 2.3. At pH values greater than 2.3, the absorbance is reasonably constant. If less than 1.4 ml of sulfuric acid was added, the Mn(OH)* precipitate persisted. In all determinations, 1.4 ml of sulfuric acid solution was added. The resulting pH was always within the range 3.0 i- 0.2. Absorbances of duplicate samples were typically within 0.005 absorbance unit of each other. VOL. 41, NO. 6, MAY 1969
857
Table I. Effect of Varying Quantities of 5 M H2S04 Added to Aliquots of the Same Sample All aliquots treated alike according to procedure outlined in text up to acidification Vol of 5M HzS04added, ml PH Absorbance 2.2 2.0 1.8 1.6 1.4
2.00 2.20 2.30 2.50 3.00
Cu(I1) Fe(II1)
Cation concn, ppm
Dissolved oxwen found. uum Proposed spectroWinkler photometric method method
10 10 10 10
10 Ca(I1)
10 10
10 20
20 20
7.55
6.74 7.55 7.33 6.57 8.90 7.62 6.84 8.91 6.97 9.38
7.50 6.74 7.50 7.25 6.57 8.45 7.00 6.25 8.00 6.45 8.80
Effect of Concentration of CyDTA. Different quantities of CyDTA ranging from 0.2 to 1.0 g were added to different bottles containing the same sample. Absorbances were independent of excess CyDTA concentration. Effect of Time of Addition of HzS04. The time interval between addition of solid CyDTA and the H2S04 solution was varied from near zero to 30 minutes. The resulting absorbance was independent of the time interval, indicating no appreciable reduction of oxidized manganese by CyDTA in alkaline solution. Data Analysis. Data obtained from the modified Winkler method and the proposed spectrophotometric method were used to construct a calibration curve. Over 20 data points (each representing a mean of duplicates) were included in a linear least squares fit of the data. Relative standard deviation of the slope of the line was 0.83z over the range of 0 to 11 ppm oxygen. The equation relating absorbance at 500 mfi and concentration of dissolved oxygen is:
The net stoichiometry of the proposed method indicates that 4 mol of Mn(II1)CyDTA should be produced for each mole of dissolved oxygen. Using the slope of the calibration curve and the assumed 4 to 1 molar ratio, a molar absorptivity for Mn(II1)CyDTA was calculated to be 339M-1 cm-’. Suwyn and Hamm (5) reported the molar absorptivity of Mn(II1)CyDTA to be 345M-1 cm-l. Hence, the 4 to 1 assumed stoichiometry is confirmed. 858
a
e
0.270 0.290 0.310 0.312 0.320
Table 11. Effect of Potentially Interfering Cations on Oxygen Concentration Found
Cation added
a
O a i
ANALYTICAL CHEMISTRY
p p m Dissolved Oaygsn
Figure 1. Calibration curve for proposed spectrophotometric determination of dissolved oxygen Absorbance of Mn(I1I)CyDTA at 500 mp in 1.00-cm cuvette us. concentration of dissolved oxygen as obtained from a modified Winkler analysis
Interferences. A systematic investigation of the possible interference of the anions common to natural water, chloride, and sulfate was precluded by the presence of large quantities of each added with the reagents. However, if either interfered seriously with the colorimetric analyses, the calculated molar absorptivity of Mn(II1)CyDTA would be expected to be grossly different from Suwyn and Hamm’s value. Iron(III), copper(II), and calcium(I1) were tested for possible interferences and the results are given in Table 11. Iron(II1) gives positive errors by the Winkler method due to oxidation of iodide by iron(II1). For this reason, iron(II1) was added only to the samples used for the spectrophotometric determination. Mn(I1I)CyDTA is known to react with oxidizable material but generally at a rather slow rate (6, 7). This would cause more rapid decrease of the absorbance. Rate of reaction of this oxidant [Mn(III)CyDTA] with any reductant would have to be investigated for each specific sample. DISCUSSION Note that iron(II1) can be tolerated in the proposed spectrophotometric method, whereas it seriously interferes in the Winkler method. The source of the calcium(I1) interference is uncertain. The calibration curve departs from linearity at oxygen concentrations greater than 11 ppm, as shown in Figure 1. Still, the proposed spectrophotometric method is useful for solutions containing more than 11 ppm dissolved oxygen, but accuracy is less than that attainable with lower oxygen concentrations. RECEIVED for review February 3, 1969. Accepted March 6 , 1969. Work supported by a grant-in-aid from the Washington State University Research Committee and in part by a grant from the National Science Foundation. (6) M. A. Suwyn and R. E. Hamm, Znorg. Chem., 6,142 (1967). (7) Zbid., p 2150.
