Table I. comparative FeO Values for Silicate Reference S t a n d a r d s
Table 11.Comparative Total Iron Values for Silicate Reference S t a n d a r d s
Proposed automated method Standard reference sample
FeO, %
Sa
Modified Wilson’s tihimetric C.V.* m e t h c d F e O , %
Granite GA Mica-Fe Basalt BR Diorite DR-N
1.39 0.01 0.72 1.39 18.85 0.10 0.53 18.75 6.72 0.04 0.60 6.66 5.49 0.01 0.18 5.45 Standard deviation of 5 repeat determinations. C.V. = Coefficient of variation.
titrimetric method (9) (the latter method has been used for several years in these labordories). Total iron is determined on aliquots of sample solution by first reducing the FeJ+ (and V5+), followed by addition of the buffer-2,2’-dipyridyl reagent. The reduction was incomplete in the presence of vanadate when the reducing agent was added t o the buffer as in Riley’s (10) method. In theory, it is not necessary for a reaction to go to completion when an AutoAnalyzer is used. However, reduction a t pH 4.9 gave inaccurate results and poor precision. Table I1 compares the determination of total iron by the proposed method with those done by Riley’s method (10). Except for the total iron value obtained for granite GA (2.56% Fez03), the relative error between the two methods is less than 1%.T o test whether this low value obtained for granite GA was caused by incomplete decomposition or by the method of determination, two powder aliquots were decomposed in a mixture of hydrofluoric and perchloric acids as described by Riley ( I O ) , followed by additions of vanadate and a mixture of HF/H3B03 t o give the same matrix as a normal test solution, and the determinations done using the Fez03 manifold shown in Figure 2. A total iron
Standard reference
Proposed automated method Fe203, %a
sample
Sb
C.V.C
Riley’s method F e 2 0 3 , 160
Granite GA Mica-Fe Basalt BR Diorite DR-N
2.56 0.02 0.62 2.71 26.02 0.04 0.14 25.94 13.07 0.05 0.41 12.84 9.53 0.05 0.50 9.64 a Total iron as Fez03. Standard deviation of 5 repeat determi. nations. Coefficient of variation.
value of 2.73% was obtained (as compared to 2.71% obtained by Riley’s method) showing incomplete decomposition to be the cause. This was confirmed when a total iron value of 2.72% was obtained after decomposition in a Teflon bomb. ACKNOWLEDGMENT The authors thank J. E. Thomas for helpful criticism of the manuscript. LITERATURE C I T E D H. N. S. Schafer. Ana/yst (London),91, 755 (1966). E. M. Donaidson, Anal. Chem., 41, 501 (1969). B. R. Sant and T. P. Prasad, Talanta, 15, 1483 (1968). J. L. Girardin and R. Thiel, Bull. Rech. Pau., 4, 513 (1970). W. J. French and S.J. Adams, Analyst (London).97, 828 (1972). A. D. Wilson, Analyst (London),85, 823 (1960). B. Bernas, Anal. Chern., 40, 1682 (1968). H. de la Roche and K. Govindaraju, Method. Phys. Anal., 7, 414 (1971). A. W. Hounslow and J. M. Moore, Geological Paper 66-1, Carleton University, Ottawa, Canada, 1966. (IO) J. P. Riley, Anal. Chirn. Acta, 19, 413 (1958). (1) (2) (3) (4) (5) (6) (7) (8) (9)
RECEIVEDfor review June 10, 1974. Accepted October 10, 1974.
New Automated Colorimetric Method for the Determination of Chloride Using Chromotropic Acid Badar K. Afghan, Ricky Leung, Achut V. Kulkarni,‘ and James F. Ryan Analytical Methods Research Section, Water Quality Research Division, Canada Centre for Inland Waters, Burlington, Ontario, Canada
Chloride occurs in almost all natural waters and enters bodies of water through natural sources such as minerals, or may be derived from other sources such as human and animal sewage, agricultural wastes, effluents from paper works, petroleum refineries, etc. The presence of chloride is generally not harmful to human beings until very high concentrations are reached ( I , 2 ) . However, the presence of chloride and other substances in waters used in some industries may have a pronounced effect ( 3 ) .Water containing concentrations of chloride as low as 0.5 mg/liter accelerates stress erosion in power reactors ( 4 ). Chloride also has an immediate effect on corrosion of steel a t concentrations as low as 3 m g h t e r (5, 6). In the majority of water quality laboratories in Canada and the USA, chloride is determined by the method based on displacement of thiocyanate ion from mercuric thiocyPermanent address, Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay, 400085 India. 556
anate by chloride ion, and the subsequent reaction of the liberated thiocyanate ion with ferric ion to form a colored complex (7, 8). This method utilizes toxic reagents such as mercuric thiocyanate and, because of the increasing concern to reduce pollution from heavy metals such as mercury, it is desirable to have a method which does not require the use of toxic reagents, particularly when analyzing a non-toxic parameter such as chloride. During our studies for the development of a colorimetric rnethod for formaldehyde, chloride in the the presence of nitrate gave a positive interference during the determination. However, when these ions were present individually, they did not produce any significant error (9). Under the experimental condition used during formaldehyde analysis, chloride ion catalyzed the conversion of nitrate to nitrite and the resultant nitrite reacted with,chromotropic acid to produce strongly colored species. Therefore, it was decided to optimize this reaction to determine chloride and possibly replace the existing method.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
used. To obtain reproducible results, it was necessary to arrange all large tubes on one side of the pump, descending to small tubes on the other side. The tray was filled with the samples and analyzed for nitrate as shown in Figure 1. T h e nitrate concentration of sample was determined by comparing the sample 963" to the calibration curve prepared from standards.
RESULTS Calibration Curves and Reproducibility. Calibration
Figure 1. Manifold for the detelmination of chloride using the proposed method
curves were run a t concentration ranges of 1-10 mgfiiter, 1-25 mg/liter, and 1-100 m g h t e r by changing the nitrate concentration and/or temperature of the color development. Identical calibration curves were obtained using synthetic lake water and actual lake water spiked with known amounts of chloride. Typical calibration curves are shown in Figure 2. The lower detection limit for chloride, using these particular reaction conditions was 0.25 mg/liter. All the measurements were made using a 505-nm narrow pass
lbl
I
i i\
-
-
.
~~~
~
CONCENTRATIOII
OF CHLORIDE ' m g / l I
Figure 2. ( a )Calibration curve for chloride in the range 1-30 mg/liter using 25 mglliter nitrate at 75 O C . Rate of sampling 20 samples per hour. ( b )Calibration curve for chloride in the range 1-100 mglliter using 100 mg/liter nitrate at 27 "C. Rate of sampling, 10 samples per hour
This paper describes an automated method to determine chloride using the above reaction. The method is capable of determining samples a t the rate of 20 samples/hour in a range of 0.25-100 mg/liter of chloride. The speed and sensitivity of t h e analysis can be further increased by variation of nitrate concentration, time of the color development and use of appropriate filter and range expansion.
EXPERIMENTAL Apparatus. Standard Technicon AutoAnalyzer modules were used. Reagents. Standard Chloride Stock Solution. A 1%stock solution of chloride was prepared by dissolving 16.5 grams of sodium chloride in distilled water and diluting the resultant solution to 1 liter. This solution was standardized using silver nitrate in the presence of potassium dichromate as indicator. Other solutions were prepared by appropriate dilution of the stock solution. Sodium Nitrate Stock Solution. A 1%stock solution was prepared by dissolving 13.8 grams of sodium nitrate in distilled water and diluting t o 1 liter. T h e 500 mg/liter working solution was prepared daily by appropriate dilution of the above solution. Chromotropic Acid Solution. Two and a half grams of monosodium salt of chromotropic acid was dissolved in concentrated sulfuric acid and made up to 1 liter with the concentrated acid. Procedure. The manifold and AutoAnalyzer equipment were connected as shown in Figure 1. The tubings for the sample aspiration and for filling the wash receptacle were ordinary standard tubing, while all the manifold pump tubes and transmission tubes were Acidflex. For sleeving purposes, green Acidflex tubing was
filter. The coefficient of variation a t 10 mg/liter of chloride was 1.2%.
Comparison of Colorimetric Methods and Analytical Application of Proposed Method. A comparison of the colorimetric method used in routine laboratories (7, 8 ) and the proposed method, was carried out by using various natural waters containing varying concentrations of major ions, organic matter, and minor components. The results are shown in Table I. Both methods gave comparable results. The proposed method can be directly applied to the analysis of natural waters, and possibly waste waters, in the range of 1-100 mgfliter of chloride. Other ranges can be covered by either dilution of samples or by changing the flow cell in the colorimeter. Table I also presents the data from analysis of some natural water samples.
DISCUSSION Effect of Nitrate Concentration. During our earlier work ( 9 ) ,the response for chloride was dependent upon the concentration of nitrate in the solutions. Therefore, initial experiments were carried out using a manifold identical to t h a t used in our earlier work (9). During the optimization of nitrate concentration, the concentration required for maximum color development was dependent upon the temperature used during the color development. Above 60 "C,
A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
557
Table I. Comparison of Colorimetric Methods for Determination of Chloride Chloride, m g l l i t e r Mercuric-thiocyanatq method
Proposed method
Amount
Found
Sample
Synthetic lake water
...
Lake water 1
28.6
Lake water 2
a
Totalg
Recovery, "6"
10.0 25 .O 50.0
9.9 25.4 49.7
99
25.0 50.0
55.0 80.0
102 101
10.0 25.0
15.6 30.3
99 101
25.0 50.0
51.0 77.1
99 101
5.0 10.0
8.0 13.9
96 104
...
...
26.3
River water 2
Added
...
5.2
River water 1
Amount
...
3.3
Fomd
...
Added
Totala
Recovery, %a
10.0 25.0 50.0
9.8 25.2 49.9
98 100 100
25.0 50.0
54.0 78.5
101 100
10.0 25.0
14.9 30.0
99 100
25.0 50.0
51.2 76.6
100 101
9.0 13.5
106 100
...
28.5
...
5.1
*..
26.0
...
3.5
5 .O 10.0
The results quoted are the average of four determinations.
U
64 0 4
4
S
T
0
6
10
20
30
40
3
FINAL CONCENTRATION OF NITRATE (rng 11)
40
Figure 3. Effect of nitrate concentration on color intensity during
chloride determination the optimum concentration required for maximum color development was approximately 25 mg/liter in the final solution. Higher concentrations of nitrate resulted in increased background absorbance and also in small incremental differences for similar concentrations of chloride. Although it was possible to obtain the highest response using 25 mg/liter of nitrate, the working range for chloride determination was relatively narrow, uiz., 0.25-30 mg/liter. I t was possible to obtain extended working range to cover concentrations of chloride between 1-100 mg/liter by working a t lower temperatures as well as by increasing the nitrate concentration in the manifold. For example, it was possible to obtain different calibration curves with varying working ranges, by either changing the temperature during the color development or the nitrate concentration in the system as shown in Figure 2. Typical results showing the effect of nitrate concentration on the color development using 25 mg/liter of chloride are shown in Figure 3. Effect of Acid Concentration. The color developed by chromotropic acid, nitrate, and chloride varied with acid concentration and did not take place in a solution containing less than 45% sulfuric acid. During these investigations, 25 mg/liter chloride solution was sampled for 10 minutes, 558
51
2o 1 c-._ --
50
A5
53
55
60
SULFURIC ACID CONCENTRATION
-65
7
70
(PERCENT1
Figure 4. Effect of sulfuric acid concentration on color development
and the steady state values were compared using various ratios of water to sulfuric acid so that final concentration was between 40-70% as shown in Figure 4. Therefore, 53.5% sulfuric acid was chosen for the determination. Effect of Temperature, Time of Color Development, and Reagent Concentration. Variation of temperature and time of color development had a pronounced effect on the intensity of color produced by the reaction of chloride, nitrate, and chromotropic acid. Optimum temperature and sensitivity of color reaction varied with the concentration of nitrate used in the manifold. A maximum sensitivity was achieved using 25 mg/liter of nitrate a t 75 "C. However, a working range was only between 0.25-30 mg/liter. The range of calibration curve can be altered to cover a wider range, uiz., 1-100 mg/liter, as shown in Figure 2. Time required for complete color development was the same irrespective of the temperature or concentration of nitrate used in the manifold. Full color development was attained by passing solutions through a 20-ft X 2.4-mm diameter coil. In the case where the high temperature (75 OC) was used during the color development, the solutions were cooled by passing through a jacketed mixing coil a t 20 "C prior to the color measurement.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
100
-
o1__._ ~
35:
~
~-~_
,
_~
4W
450
550
550
350
4w
WAVE LENGTH
450
5oc
550
nm
Figure 5. ( a ) Absorption spectrum of chrornotropic acid vs. chromotropic acid 4- 25 mg/liter nitrate at 75 OC (X). (6) Absorptbon spectrum of chrornotropic acid vs. chrornotropic acid 4- 25 rng/liter nitrate -I-4.65 rng/liter chloride at 75 O C (Y). (c) Absorption spectrum of X vs. Y
Studies were also carried out to find the optimum concentration for reagent required to analyze chloride in the range 1-100 mg/liter. During these experiments, the amount of chromotropic acid used in the manifold was varied by pumping various concentrations of chromotropic acid (between 0.05-1%) while the chloride concentration was kept constant. Optimum concentration wa5 between 0.25-0.5%. A high background color intensity was obtained using concentrations above 0.5% chromotropic acid. At the same time, the sensitivity of color reaction was also decreased considerably. Therefore, 0.25% chromotropic acid was chosen as the optimum during the determination. Effect of Various Contaminants. A possible interference of major and minor ions, normally found in water, was determined using the proposed method. Solutions containing 10- to 100-fold excess of individual ions, as well as 10 mg/liter chloride, were passed through the same procedure and the resuits were compared using the calibration curve in deionized water. None of the major or minor ions interfered in the proposed method. Similar results were also obtained using synthetic lake water containing all major and minor ions in the same solution. Organic compounds normally found in natural waters did not produce any significant effect. These included amino acids, carbohydrates, urea, etc. Amongst other halides, only bromide gave a similar response. Since the ratio of bromide to chloride in natural waters is very small, this interference is of no significance. However, it is possible to use this reaction for development of a very sensitive method for bromide. Spectral Characteristics. Absorption spectra of chromotropic acid-nitrate and chromotropic acid-nitrate containing chloride, using the proposed method, are shown in Figure 5 . Chromotropic acid absorbed in the UV-range with maximum absorbance a t 360 nm. On addition of nitrate, an additional peak occurred at 410 nm. However, in the presence of chloride, the intensity of both peaks increased considerably. In addition, a bathochromic shift of a second peak was also obtained with maximum absorbance a t 440 nm. During our investigations, all the absorption measurements were carried out using a 505-nm narrow pass filter instead of the filter corresponding t o maximum ab-
sorption because it was then possible to cover a concentration range between 0.25-100 mg/liter. In addition, by using the above filter, a small background absorbance, and hence a smoother ,base line, was obtained. Manifold. During the analysis, it was necessary initially to mix the sample with nitrate and then pump the resultant solution via the debubbler, through the manifold to obtain reproducible and reliable results. When the system was run in the usual mode, the concentration of sulfuric acid was varied a t the start and the end of the sample cycle as the probe was changed from the sample to wash receptacle and vice versa. This resulted in spikes in the base line and peaks and gave non-reproducible, erratic results. Because of the exothermic nature of the reaction, it was also necessary during the analysis to cool the solution immediately after the sample was mixed with sulfuric acid, prior t o passing it into the oil bath. It was also necessary to ensure proper mixing and homogeneity of solution prior to the measurement of color; otherwise a very noisy baseline was obtained. This was accomplished by passing the solution through a small coil containing glass beads prior to entry of the solution into the flow cell.
LITERATURE CITED (1)J. E. McKee and H. F. Wolf, "Water Quality Criteria," The Resources Agency Of California, State Water Resources Control Board, Publication 3-A, 1971,p 159. (2) "Canadian Drinking Water Standards," A Publication of the Department of National Health and Welfare, Ottawa, Canada, 1969,p 16. (3) E. K. Afghan, Technical Bulletin No. 52, Inland Waters Directorate, Department of the Environment, Ottawa, Canada, 1971. (4)Chern. Eng. News, 35 (37),77 (1957). (5) G.E. Hatch and 0. Rice, J. Arner. Water Works Ass., 51,719 (1959). (6)D. Warren, Proc. 75th Indus. Waste Conf., Purdue Univ., f n g . Extension Ser., 106,420 (1960). (7)W. J . Traversy, "Methods for Chemical Analysis of Waters and Wastewaters," Inland Waters Directorate, Department of the Environment, Ottawa, Canada, 1971. (8)"Methods for Chemical Analysis of Water and Wastes." Manual Published by the Environmental Protection Agency, Cincinnati, Ohio. (9)B. K. Afghan, A. V . Kulkarni, and J. F. Ryan, fnviron. Lett., 7, (I), 53
(1974).
RECEIVEDfor review July 29, 1974. Accepted November 21, 1974. ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 3, MARCH 1975
559
