had been equilibrated with laboratory air and contained 0 to 20 μM of added nitrite, there was no discernible change in the concentration of oxygen in the presence of the added azide (Fig. 1). All the observed concentrations fell within ±0.3% of the average value, within what may be readily attributed to the analytical uncertainty and the bottle to bottle variations in the equilibrium with laboratory air. Since the highest reported concentration of nitrite in seawater is still below 15 μM (Kamykowski and Zentara, 1991), this slight modification should be more than adequate for removing the interference of nitrite and should be incorporated into the standard methods for the determination of oxygen in seawater. The Winkler reaction scheme is standardized by generating a stoichiometrically defined amount of tri-iodide ion from a known quantity of iodate through the Dushman reaction between iodate and excess
iodide under acidic condition such that:
In a handbook for the analysis of fresh water, Golterman et al. (1978) commented in a note that azide interferes slightly in this reaction and may result in an error of 1% in the determination of oxygen. As a result, azide should not be used for the removal of the interference of nitrite if oxygen is to be determined at the highest accuracy.
However, these comments were made without supporting data or references. This possible source of error was re-examined here by determining the concentrations of dissolved oxygen in the absence of nitrite in triplicates in four samples of artificial seawater with and without the addition of azide. The concentrations in these samples
were standardized against artificial seawater with added iodate with and without the addition of azide. The standardizations were carried out in triplicates at three concentrations of added iodate. The results are shown in Table 2. In all cases, the standard deviations of the triplicates were within ±0.2% indicating that the addition of azide did not affect the precision of the analyses. In the samples that were processed through the Winkler reaction scheme without the addition of azide, if azide was added in the standardization with iodate, the concentrations of dissolved oxygen found were consistently slightly higher than those standardized against iodate without the addition of azide by an average of 0.13%. This difference was barely detectable but it was consistent with the slight interference of azide in the Dushman reaction and the subsequent carry-over of the interference as a small systematic error in estimating the concentration of oxygen in the Winkler reaction scheme as suggested by Golterman et al. (1978). However, under the experimental conditions used here, the error was much less than 1% and was of a similar magnitude as the analytical uncertainty in the analyses. If azide was not added in the standardization with iodate, the concentrations of dissolved oxygen found in the samples that were processed through the Winkler reaction scheme with and without the addition of azide were basically indistinguishable from each other. The average ratio in the concentrations found in these two methods was 0.9995 (Table 2). The deviation from unity of 0.05% was within the analytical uncertainty of the method. Thus, there was no evidence that azide interferes in the Winkler reaction scheme. If there were an interference,it was confined to the Dushman reaction used in the standardization. Hence, azide can be used for the removal of the interference of nitrite.For the highest accuracy, as a precaution, the standardization of the Winkler reaction scheme should be carried out in nitrite-free deionized water or artificial seawater, rather than in natural seawater, with added iodate without the addition of azide. This practice is already
the recommended procedure for circumventing the interference of iodate in natural seawater (Wong and Li, 2009).
3.4. Effect of the interference of nitrite on historical data of oxygen In the historical data of oxygen, in the open oceans, a correction for the presence of nitrite is needed probably only at the nitrite maxima. In coastal waters, the effect of nitrite needs to be considered. Although the molar ratio between nitrite present and apparent oxygen is somewhat variable, a ratio of 0.4±0.1 mol mol−1 seems like a reasonable estimate. Thus, if the concentration of nitrite is available, the correction is rather straight forward. Because of the uncertainty in the molar ratio, the correction will degrade the precision in the estimated concentration of oxygen by about ±10−1 to 10−2 μM in most cases. Nevertheless, while these corrections can be readily made, in order to avoid confusion, such corrections on the published values should be discouraged until an internationally accepted standard practice can be put in place. Acknowledgments This work was supported in part by the National Science Council,Taiwan through grant number NSC98-2611-M-001-004-MY3, and by the Academia Sinica through a thematic research grant titled “Atmospheric Forcing on Ocean Biogeochemistry (AFOBi)”. I thank Yao-Chu Wu and Kuo-Yuan Li for making the oxygen measurements and the three reviewers for their constructive comments
had been equilibrated with laboratory air and contained 0 to 20 μM of added nitrite, there was no discernible change in the concentration of oxygen in the presence of the added azide (Fig. 1). All the observed concentrations fell within ±0.3% of the average value, within what may be readily attributed to the analytical uncertainty and the bottle to bottle variations in the equilibrium with laboratory air. Since the highest reported concentration of nitrite in seawater is still below 15 μM (Kamykowski and Zentara, 1991), this slight modification should be more than adequate for removing the interference of nitrite and should be incorporated into the standard methods for the determination of oxygen in seawater. The Winkler reaction scheme is standardized by generating a stoichiometrically defined amount of tri-iodide ion from a known quantity of iodate through the Dushman reaction between iodate and excess
iodide under acidic condition such that:
In a handbook for the analysis of fresh water, Golterman et al. (1978) commented in a note that azide interferes slightly in this reaction and may result in an error of 1% in the determination of oxygen. As a result, azide should not be used for the removal of the interference of nitrite if oxygen is to be determined at the highest accuracy.
However, these comments were made without supporting data or references. This possible source of error was re-examined here by determining the concentrations of dissolved oxygen in the absence of nitrite in triplicates in four samples of artificial seawater with and without the addition of azide. The concentrations in these samples
were standardized against artificial seawater with added iodate with and without the addition of azide. The standardizations were carried out in triplicates at three concentrations of added iodate. The results are shown in Table 2. In all cases, the standard deviations of the triplicates were within ±0.2% indicating that the addition of azide did not affect the precision of the analyses. In the samples that were processed through the Winkler reaction scheme without the addition of azide, if azide was added in the standardization with iodate, the concentrations of dissolved oxygen found were consistently slightly higher than those standardized against iodate without the addition of azide by an average of 0.13%. This difference was barely detectable but it was consistent with the slight interference of azide in the Dushman reaction and the subsequent carry-over of the interference as a small systematic error in estimating the concentration of oxygen in the Winkler reaction scheme as suggested by Golterman et al. (1978). However, under the experimental conditions used here, the error was much less than 1% and was of a similar magnitude as the analytical uncertainty in the analyses. If azide was not added in the standardization with iodate, the concentrations of dissolved oxygen found in the samples that were processed through the Winkler reaction scheme with and without the addition of azide were basically indistinguishable from each other. The average ratio in the concentrations found in these two methods was 0.9995 (Table 2). The deviation from unity of 0.05% was within the analytical uncertainty of the method. Thus, there was no evidence that azide interferes in the Winkler reaction scheme. If there were an interference,it was confined to the Dushman reaction used in the standardization. Hence, azide can be used for the removal of the interference of nitrite.For the highest accuracy, as a precaution, the standardization of the Winkler reaction scheme should be carried out in nitrite-free deionized water or artificial seawater, rather than in natural seawater, with added iodate without the addition of azide. This practice is already
the recommended procedure for circumventing the interference of iodate in natural seawater (Wong and Li, 2009).
3.4. Effect of the interference of nitrite on historical data of oxygen In the historical data of oxygen, in the open oceans, a correction for the presence of nitrite is needed probably only at the nitrite maxima. In coastal waters, the effect of nitrite needs to be considered. Although the molar ratio between nitrite present and apparent oxygen is somewhat variable, a ratio of 0.4±0.1 mol mol−1 seems like a reasonable estimate. Thus, if the concentration of nitrite is available, the correction is rather straight forward. Because of the uncertainty in the molar ratio, the correction will degrade the precision in the estimated concentration of oxygen by about ±10−1 to 10−2 μM in most cases. Nevertheless, while these corrections can be readily made, in order to avoid confusion, such corrections on the published values should be discouraged until an internationally accepted standard practice can be put in place. Acknowledgments This work was supported in part by the National Science Council,Taiwan through grant number NSC98-2611-M-001-004-MY3, and by the Academia Sinica through a thematic research grant titled “Atmospheric Forcing on Ocean Biogeochemistry (AFOBi)”. I thank Yao-Chu Wu and Kuo-Yuan Li for making the oxygen measurements and the three reviewers for their constructive comments
การแปล กรุณารอสักครู่..

had been equilibrated with laboratory air and contained 0 to 20 μM of added nitrite, there was no discernible change in the concentration of oxygen in the presence of the added azide (Fig. 1). All the observed concentrations fell within ±0.3% of the average value, within what may be readily attributed to the analytical uncertainty and the bottle to bottle variations in the equilibrium with laboratory air. Since the highest reported concentration of nitrite in seawater is still below 15 μM (Kamykowski and Zentara, 1991), this slight modification should be more than adequate for removing the interference of nitrite and should be incorporated into the standard methods for the determination of oxygen in seawater. The Winkler reaction scheme is standardized by generating a stoichiometrically defined amount of tri-iodide ion from a known quantity of iodate through the Dushman reaction between iodate and excess
iodide under acidic condition such that:
In a handbook for the analysis of fresh water, Golterman et al. (1978) commented in a note that azide interferes slightly in this reaction and may result in an error of 1% in the determination of oxygen. As a result, azide should not be used for the removal of the interference of nitrite if oxygen is to be determined at the highest accuracy.
However, these comments were made without supporting data or references. This possible source of error was re-examined here by determining the concentrations of dissolved oxygen in the absence of nitrite in triplicates in four samples of artificial seawater with and without the addition of azide. The concentrations in these samples
were standardized against artificial seawater with added iodate with and without the addition of azide. The standardizations were carried out in triplicates at three concentrations of added iodate. The results are shown in Table 2. In all cases, the standard deviations of the triplicates were within ±0.2% indicating that the addition of azide did not affect the precision of the analyses. In the samples that were processed through the Winkler reaction scheme without the addition of azide, if azide was added in the standardization with iodate, the concentrations of dissolved oxygen found were consistently slightly higher than those standardized against iodate without the addition of azide by an average of 0.13%. This difference was barely detectable but it was consistent with the slight interference of azide in the Dushman reaction and the subsequent carry-over of the interference as a small systematic error in estimating the concentration of oxygen in the Winkler reaction scheme as suggested by Golterman et al. (1978). However, under the experimental conditions used here, the error was much less than 1% and was of a similar magnitude as the analytical uncertainty in the analyses. If azide was not added in the standardization with iodate, the concentrations of dissolved oxygen found in the samples that were processed through the Winkler reaction scheme with and without the addition of azide were basically indistinguishable from each other. The average ratio in the concentrations found in these two methods was 0.9995 (Table 2). The deviation from unity of 0.05% was within the analytical uncertainty of the method. Thus, there was no evidence that azide interferes in the Winkler reaction scheme. If there were an interference,it was confined to the Dushman reaction used in the standardization. Hence, azide can be used for the removal of the interference of nitrite.For the highest accuracy, as a precaution, the standardization of the Winkler reaction scheme should be carried out in nitrite-free deionized water or artificial seawater, rather than in natural seawater, with added iodate without the addition of azide. This practice is already
the recommended procedure for circumventing the interference of iodate in natural seawater (Wong and Li, 2009).
3.4. Effect of the interference of nitrite on historical data of oxygen In the historical data of oxygen, in the open oceans, a correction for the presence of nitrite is needed probably only at the nitrite maxima. In coastal waters, the effect of nitrite needs to be considered. Although the molar ratio between nitrite present and apparent oxygen is somewhat variable, a ratio of 0.4±0.1 mol mol−1 seems like a reasonable estimate. Thus, if the concentration of nitrite is available, the correction is rather straight forward. Because of the uncertainty in the molar ratio, the correction will degrade the precision in the estimated concentration of oxygen by about ±10−1 to 10−2 μM in most cases. Nevertheless, while these corrections can be readily made, in order to avoid confusion, such corrections on the published values should be discouraged until an internationally accepted standard practice can be put in place. Acknowledgments This work was supported in part by the National Science Council,Taiwan through grant number NSC98-2611-M-001-004-MY3, and by the Academia Sinica through a thematic research grant titled “Atmospheric Forcing on Ocean Biogeochemistry (AFOBi)”. I thank Yao-Chu Wu and Kuo-Yuan Li for making the oxygen measurements and the three reviewers for their constructive comments
การแปล กรุณารอสักครู่..
