Besides, 10.0mL potassium permanganate solution was transferred into a 250 mL conical flask, in which. 0.5 g potassium iodide and 1 mL H2SO 4 solution (1:3).
Journal of Ocean University of China (Oceanic and Coastal Sea Research) ISSN 1672-5182, April 30, 2006, Vol.5, No.2, pp.137-140 http: // www. ouc. edu. cn/xbywb/ E-maih xbywb@mail, ouc edu. cn
A New Spectrophotometric Method for Measuring COD of Seawater LIU Ying, JI Hongwei~, XIN Huizhen, and LIU Li Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266003, P. R. China
(Received January 26,2005 ; accepted December 14, 2005) Abstract This research studied a new spectrophotometric method for measuring the chemical oxygen demand (COD) of seawater. In this method, the COD was measured using a spectrophotometer instead of titrating with sodium thiosulphate. The measuring wavelength was selected to be 470 nm, and the COD of three standard glucose solutions (COD = 0.5, 1.5 and 2.5 mgL 1, respectively) and two seawater samples (from the South Yellow Sea and Jiaozhou Bay) were measured using the spectrophotometric method and titrimetric method respectively. The results showed that the spectrophotometric method was
somewhat better than the titrimetric method. The relative standard deviation (RSD) of the spectrophotometric method was less than 2.7%, and the recovery of seawater samples ranged from 96.3% to 103.8%. In addition, the spectrophotometric method has other advantages such as expeditiousness, operation simplicity, analysis automatization, etc. Therefore the spectrophotometric method can be used to measure the COD of seawater with satisfactory results. Key words
chemical oxygen demand (COD); spectrophotometric method; seawater
1 Introduction Chemical oxygen demand (COD) is defined as the amount of oxygen equivalents consumed in oxidizing the organic compounds of samples by strong oxidizing agents such as dichromate or permanganate. It can reflect the pollution degree of water and is one of the most important parameters to assess pollution in water monitoring (Huang, 1989; Zhang, 1989). At present, the COD of freshwater is usually measured using titrimetric method after oxidization by dichromate or acidic permanganate, and that of seawater using titrimetric method after oxidization by permanganate in alkaline condition ( F u and Xu, 1997; Administration of Quality and Technical Supervision of the People's Republic of China, 1999). Because the titrimetric method has some drawbacks, such as unsteadiness of sodium thiosulphate, high consumption of chemicals, long analysis time, impossibility of automatic measurement, e t c , new analytical methods have been developed. Now some spectrophotometric methods are used to measure the COD in freshwater and urban and industrial waste water systems (Chen and Cheng, 2002; Sibel, 2003; Houda, 1999; Wang and Liang, 2000; Zhu and Zeng, 2003; State Environmental Protection Administration of China, 1989). However, there have been few reports on Corresponding author. Teh0086-532-82032482-8122 E-mail: hwji@mail. ouc. edu. cn
measuring COD of seawater using spectrophotometric method. For example, Xu and Zhang ( 2 0 0 3 ) measured COD in seawater by measuring the absorbance of permanganate, mainly investigating the influence of medium acidity and the measuring time interval. This research was designed to develop a new spectrophotometric method of determining COD of seawater by measuring the absorbance of I2 standard solution rather than that of permanganate. The results showed that this new spectrophotometric method is suitable for the measurement of COD of seawater.
2 Materials and Methods 2.1
Apparatus
and Reagents
The model o{ the U V - V I S spectrophotometer is HP8453 of Agilent Co. The iodine used was guarantee grade and the other chemicals were all of analytical grade. The analytical solutions were prepared according co the national standard ( T h e Administration of Quality and Technical Supervision of the People's Republic of China, 1999). COD standard solution: First, 0.234 3 g glucose (dried at 60 - 70 ~ for 2 h prior to use) was dissolved in redistilled water and adjusted to a final volume of 500 mL. T h e theoretical COD of this solution was 500 m g L -1. Then, 1.0 mL, 3.0 m L and 5.0 m L of this solution were diluted to 1 000 mL respectively, and three glucose standard solutions with COD of 0.5, 1.5 and 2.5 mgL -1 respectiv-
Journal of Ocean University of China
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ely were obtained. Potassium hydrogen phthalate ( KCsHsO 4 ) was dried for 2 h at 105 0(2 and 0.425 1 g of it was dissolved in redistilled water and adjusted to 1000 mL. The theoretical COD of this solution was 500 m g L -1 . Subsequently, 1.0 mL, 3.0 m L and 5.0 m L of this solution were diluted to 1 000 m L respectively, and 3 potassium hydrogen phthalate standard solutions with COD of 0.5, 1.5 and 2.5 m g L -1 respectively were obtained (Zhang and Liu, 2003). 2 . 2 Methods 2.2.1
Titrimetric method
Vol. 5, No. 2, 2006
where m is the concentration of sodium thiosulphate (molL 1 ), V1 is the volume of sodium thiosulphate consumed in titrating the blank solution ( m L ) and V2 is the volume of sodium thiosulphate consumed in titrating the sample solution ( m L ) . For the spectrophotometric method, the concentration of COD was calculated using the formula COD ( m g L 1) = 160(61C1-VC2), where V is the volume o f the sample solutions for absorbance measurement ( m L ) , C1 is the concentration of I2 in the blank solution from the calibration curve ( m o l L - I ) , and C2 is the concentration of I2 in the sample solution from the calibration curve ( m o l L - I ) .
The titrimetric method was the same as stipulated in the national standard ( T h e Administration of Quality and Technical Supervision of the People's Republic of China, 1999).
3 Results and Discussion
2.2.2
The absorbances of the six I2-KI standard solutions were read at different wavelengths (400 n m - 510 nm) with the results shown in Fig. 1 and Table 1.
Spectrophotometric method
First of all, 12.700g I2 and 40.0g K I were dissolved in 10 L of distilled water, yielding an I2-KI standard solution ( 0 . 0 0 5 m o l L - i ) . Then, 1.0 mL, 2.0 mL, 3.0 mL, 4.0 mL, 5.0 m L and 6.0 m L of such solution were transferred into six test tubes respectively, and were adjusted to 50 mI. with distilled water. The absorbances of these solutions were read on a spectrophotometer at different wavelengths by a 1 cm thick cuvette every 10 nm from 400 nm to 510 nm (Chen, 1997). Distilled water was used as the reference. Besides, 10.0mL potassium permanganate solution was transferred into a 250 m L conical flask, in which 0.5 g potassium iodide and 1 m L H 2 S O 4 solution ( 1 : 3 ) were added, and the flask was placed in darkness for two minutes. 50 m L distilled water was added, the absorbance of the solution thus formed at the optimal wavelength was read on a spectrophotometer. The concentration (C1) of I2 of the blank solution from the calibration curve was obtained. The procedure of sample digestion is the same as stipulated in the standard procedure ( T h e Administration of Quality and Technical Supervision of the People's Republic of China, 1999). After the digestion step, the absorbance at the selected wavelength was measured using a spectrophotometer, then the concentration ( C 2 ) of I2 in the sample solution was obtained from the calibration curve. For the titrimetric method, the concentration of sodium thiosulphate was calculated according to the formula: 6mKiO3 • 15.00 lY/Na2S203 --
VNa2S203
the concentration of COD was calculated from the formula C O D ( m g L -1) = m (V1 - V2) • 8/100 • 1 000
=rn(V1-
V2) x 80,
3 . 1 Determination of the Optimal Wavelength
2.5 2 ~ 1.5 ~ 1 0.5 0 400 410 420 430 440 450 460 470 480 490 500 510
Wavelength/run Fig.1 Relationship between absorbance and wavelength of I2-KI standard solutions. 1. C ( I 2 ) = l . 0 0 •
2. C ( 1 2 ) = 2 . 0 0 •
3. C ( l z ) = 3 . 0 0 •
4molL 1 4. C ( I 2 ) = 4 . 0 0 •
4molL 1
5. C ( I 2 ) = 5 . 0 0 x 1 0 4molL l; 6. C ( I 2 ) = 6 . 0 0 •
4molL 1.
Table 1
Absorbances of I2-KI standard solutions at different w a v e l e n g t h s
Concentrationt/ ( 10 -4 molL-I )
Absorbance 450 nm 460 nm
430 nm
440 nm
1.00
0.098
0.094
0.087
0.078
2.00
0.236
0.213
0.185
0.157
0.132
0. 404
0. 354
0. 299
0. 246
0. 199
4.00
0.585
0.506
0.417
0.336
0.266
5.00
0. 774
0. 663
0. 539
0. 428
0. 332
6.00
0. 960
0. 817
0. 658
0. 516
0. 397
3.00
470 nm 0.069
Linear
r
0.9973
0.9979
0.9988
0.9995
0.9999
Regression Equation
a
0.1744
0.1462
0.1153
0.0884
0.0659
b
0.1010
0.0705
0.0393
0.0158
0.0018
Note: t concentration of the I2-KI standard solutions.
Fig. 1 and Table 1 show that the absorbance of I2K I standard solutions decreases with the increase of wavelength and there is a good linear relationship between the absorbance and the concentration of I2-KI from 430 nm to 470 nm, which are in good agreement
L I U Y. et al.: A New Spectrophotometric Method for Measuring COD of Seawater w i t h t h a t r e p o r t e d previously ( C h e n , 1 9 9 7 ) . T h e line a r i t y ( r = 0.999 9) is the best at 470 n m ; thus 470 n m was selected as the o p t i m a l w a v e l e n g t h .
3 . 2 Determination of the Standard Substance P o t a s s i u m h y d r o g e n p h t h a l a t e and glucose were selected as the s t a n d a r d substances, from w h i c h t h e s t a n d a r d solutions w i t h COD of 0.5, 1.5 and 2.5 m g L 1 w e r e p r e p a r e d respectively. True CODs of those solutions were measured using s p e c t r o p h o t o m e t r i c method. T h e results are listed in Table 2. Table 2
Oxidizing efficiencies of the standard solutions of KHP and glucose
CODt/ MeasuredCOD/(mgL 1 ) ( mgL - 1) KHP Glucose 0.5 -0.29 1.5 0.16 0.78 2.5 0.40 1.29 Note: t COD of standard solutions.
Oxidation efficiency/% KHP Glucose -58.0 10.7 52.0 16.0 51.6
COD of standard/ ( mgL -1)
oxidized relatively. W h e n t h e COD of f r e s h w a t e r is m e a s u r e d b y d i c h r o m a t e or p e r m a n g a n a t e in acidic conditions, p o t a s s i u m h y d r o g e n p h t h a l a t e is often selected as a s t a n d a r d substance. But Table 2 suggests t h a t it is not suitable to choose p o t a s s i u m h y d r o g e n p h t h a l a t e as t h e s t a n d a r d substance w h e n the oxidizing a g e n t is p o t a s s i u m p e r m a n g a n a t e in alkaline conditions. It also shows t h a t even glucose can not be oxidized c o m p l e t e l y b y alkaline p o t a s s i u m p e r m a n g a n a t e , t h e o x i d a t i o n efficiency being m e r e l y about 5 1 . 6 % - 5 8 . 0 % and being in accord w i t h t h a t r e p o r t e d previously ( C h e n and Chen, 1994). T h e r e f o r e w e chose glucose as t h e s t a n d a r d substance.
3 . 3 Determination of COD of the Standard Solutions and Seawater Samples
T h e oxidation efficiency of p o t a s s i u m h y d r o g e n p h thalate is much lower t h a n t h a t of t h e glucose. One of t h e reasons is t h a t p o t a s s i u m h y d r o g e n p h t h a l a t e has a p h e n y l structure, t h u s can not easily be oxidized b y p e r m a n g a n a t e in alkaline conditions. O n the o t h e r hand, glucose has a chain structure, t h u s can be easily Table 3
139
In o r d e r to verify the reliability of t h e spectrophot o m e t r i c m e t h o d , we m e a s u r e d t h e COD values of t h e t h r e e glucose s t a n d a r d solutions ( C O D = 0.5, 1.5 and 2.5 m g L -1) and t w o s e a w a t e r samples ( o n e of w h i c h from Jiaozhou Bay: 2004/07/12, 36~ 120~ and the o t h e r from t h e S o u t h Yellow Sea: 2 0 0 4 / 0 7 / 1 3 , 3 5 ~ 120~ b y the s p e c t r o p h o t o m e t r i c m e t h o d and t h e t i t r i m e t r i c m e t h od. T h e results are listed in Tables 3 and 4.
CODs of glucose standard solutions measured using the two methods
Method+ 1
2
Measured COD/(mgL 1) 3 4
Average
RSD/ %
Oxidation efficiency/%
0.5
T SP
0.28 0.28
0.29 0.30
0.29 0.29
0.27 0.30
0.28 0.29
3.4 2.7
56.0 58.0
1.5
T SP
0.75 0.77
0.79 0.80
0.76 0.78
0.77 0.77
0.77 0.78
2.2 1.8
51.3 52.0
2.5
T SP
1.26 1.27
1.31 1.30
1.30 1.28
1.27 1.32
1.28 1.29
1.8 1.7
51.2 51.6
Note: t T: titration; SP: spectrophotometry. Table 4 Samples The South Yellow Sea Jiaozhou Bay
CODs of seawater measured by the two methods
Methodt T SP T SP
1 0.72 0.77 1.05 1.10
2 0.75 0.74 1.07 1.07
Measured COD/(mgL 3 0.74 0.75 1.04 1.13
1) 4 0.76 0.76 1.10 1.08
RSD/% Average 0.74 0.76 1.07 1.10
2.3 1.7 2.5 2.4
Note: t T: titration; SP: spectrophotometry. Tables 3 and 4 s h o w t h a t the average COD values from the two m e t h o d s are nearly t h e same. A c c o r d i n g to F - t e s t and T - t e s t analysis (confidence level 95 % ), t h e r e is no significant difference b e t w e e n t h e t w o m e t h o d s , b u t the precision of t h e s p e c t r o p h o t o m e t r i c m e t h o d is s o m e w h a t b e t t e r t h a n t h a t of the t i t r i m e t r i c m e t h o d . T h e r e f o r e w e can use the s p e c t r o p h o t o m e t r i c m e t h o d to measure t h e recovery of C O D of s e a w a t e r by a s t a n d a r d a d d i t i o n e x p e r i m e n t .
3 . 4 The Influence of Volatility of 12 Solution Because I2 is volatile, t h e absorbance of a single I2 s t a n d a r d solution is not a p p r o p r i a t e for t h e measurem e n t . In o r d e r to p r e v e n t t h e effect of volatility of I2, K I was a d d e d to improve t h e s t a b i l i t y of I2 as I2 and K I can p r o d u c e I3- 9 Table 1 s h o w s t h a t t h e calibration curve was not affected b y t h e volatility of I2. T h e re-
140
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J o u r n a l o f Ocean University o f China
sult is t h e same as t h a t r e p o r t e d previously ( C h e n , 1997; Z h a n g et al., 2 0 0 3 ) . W e can therefore choose I2 + K I solutions to measure t h e absorbance.
3 . 5 Recovery of COD of Seawater by a Standard Addition Experiment F i r s t 1 m L of the s t a n d a r d solutions ( C O D = 50 m g L -1) were a d d e d into 2 s e a w a t e r samples and t h e n the COD of t h e t w o samples were m e a s u r e d by the spec-
trophotometric method. The results are listed in Table 5. Table 3 shows t h a t glucose cannot be oxidized completely b y potassium p e r m a n g a n a t e in alkaline conditions. T h e mean oxidation efficiency is 54.0 % . Therefore t h e oxidation efficiency was t a k e n as 54 % in calculating the recovery, w h i c h was equivalent to an a d d e d s t a n d a r d substance of 27.00 tag. Table 5 shows t h a t t h e e x p e r i m e n t a l recovery ranges from 96.3% to 103.8 % , suggesting t h a t the s p e c t r o p h o t o m e t r i c method can be used to m e a s u r e COD of s e a w a t e r .
Table 5 Samples/( 100 rnL)
COD of seawater/ (mgL -1)
The South Yellow Sea 0.76
Jiaozhou Bay
1.09
The results of recovery experiment COD measured/ COD added//~g COD recoverd//xg (mgL 1) 1.01 26.01 27.00 1.03 28.03 (50.00 x 54.0%) 1.02 27.02 27.00
Recovery/% 96.3 103.8 100.1
1.34
26.34
97.6
1.35
27.35
101.3
1.35
27.35
101.3
(50.00•
4 Conclusion T h e e x p e r i m e n t a l results show t h a t 1 ) although t h e r e is no significant difference b e t w e e n the speet r o p h o t o m e t r i c m e t h o d and the t i t r i m e t r i c m e t h o d at 9 5 % confidence level, the s p e c t r o p h o t o m e t r i c m e t h o d is s o m e w h a t b e t t e r t h a n t h e t i t r i m e t r i c m e t h o d ; 2 ) the relative s t a n d a r d deviation ( R S D ) of t h e speet r o p h o t o m e t r i c m e t h o d is less t h a n 2 . 7 % and t h e recovery of s e a w a t e r samples ranges from 96.3% to 103.8 % , m e e t i n g t h e r e q u i r e m e n t s of COD measurem e n t of s e a w a t e r ; 3 ) the s p e c t r o p h o t o m e t r i c m e t h o d has some a d v a n t a g e s over the t i t r i m e t r i c m e t h o d such as simple d e t e r m i n a t i o n procedure, acceptable precision, a u t o m a t i c analysis, etc; and 4 ) c o m p a r i n g the results w i t h those r e p o r t e d previously ( X u and Zhang, 2 0 0 3 ) , it can be claimed t h a t a new m e t h o d in this respect has been developed in this s t u d y . T h e r e f o r e , it can be concluded t h a t the s p e c t r o p h o t o m e t r i c m e t h o d can be used to measure the COD of s e a w a t e r w i t h satisfactory results.
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