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A Simple Spectrophotometric Determination of
Phosphate in Sugarcane Juices, Water
and Detergent Samples
MAHADEVAIAH, M. S. YOGENDRA KUMAR,
MANSOUR S. ABDUL GALIL, M. S. SURESHA,
M. A. SATHISH and G. NAGENDRAPPA*
Department of Chemistry, University of Mysore, Manasagangotri, Mysore 570006, India.
Received 13 January 2007; Accepted 5 March 2007
Abstract: A simple spectrophotometric method is developed for the determination of phosphate in sugar cane juice, water and detergent samples. The method is based on the formation of phosphomolybdate with added molybdate followed by its reduction with sodium sulphide in aqueous sulfuric acid medium. The system obeys Lambert-Beer’s law at 715 nm in the concentration range 0.3-12.24 ppm. Molar absorptivity, correlation coefficient and Sandell’s sensitivity values were found to be 6.1x10^3 mol- cm,-1^ 0.999 and 0.0156 μg cm-2^ respectively. The results obtained were reproducible with acceptable standard deviation 3.7% and relative error 3.4%. For a comparison of the method phosphate present in various samples were also determined separately following an official method. The results of the developed method compare well with those of the official method.
Keywords : Phosphate, water, detergents, and spectrophotometry.
Introduction
Phosphorus is the eleventh most abundant element on the surface of the earth and is most commonly found as phosphate. It plays an important role in biochemical processes and is a key factor in the eutrophication of surface water^1 Increased phosphate concentrations are linked with increasing rates of plant growth^2. The analytical chemistry of phosphorus is very important in many fields, for example, medical and clinical science, agriculture, metallurgy and environmental science^3. Moreover, in recent years large quantities of phosphate have been used in beverages^4 , detergents^5 , fertilizers^6 and also in sugar industries7-9.
468 G. NAGENDRAPPA et al.
The literature7-9^ gives the importance of phosphate during the sugarcane juice clarification leading to cane sugar manufacture. In this context, the chemical action of phosphate during clarification of sugarcane juice assumes significant role particularly when the juice is deficient in natural phosphate content in the range of 300-350 ppm as P 2 O 5 which is essential for good clarification. In case of less phosphate content in sugarcane juice, soluble phosphate addition is the only alternative to achieve the target value by adding orthophosphoric acid, single super phosphate or triple super phosphate. Therefore, it becomes very important to determine the amount of phosphate in sugarcane juice. As a consequence, various phosphate determination procedures have been reported10-22^ which include for example, titrometry^10 , complexogravimetry^11 , colorimetry12,13, atomic absorption spectroscopy,^14 flow injection analysis^15 , HPLC^16 and spectrophotometry methods17-22. Among such methods spectrophotometry involving molybdovanadate^23 and ammonium molybdate^16 are most commonly used^16. In ammonium molybdate spectrophotometric method, different reductants have been employed such as tin(II) chloride23,25. ascorbic acid^26 and 1-amino-2- naphthol-4-sulfonic acid^27 , Some of these methods also involve complicated and expensive equipments^16 and need extraction procedure 20, 23^ and such techniques are usually not available in common laboratories. Though there are large number of methods10-22^ available for the determination of phosphate but such methods used in sugar cane juice for phosphate determination are unread, instead only a few methods 8, 9, 23, 25^ have been used. Therefore, it becomes very important7-8^ to determine the phosphate in sugar cane juice^7 and sodium sulphide is found to be a very effective reducing agent^28 but not commonly used for phosphate determination.^28 Considering the importance of phosphate and its determination in sugar cane juice an attempt is made here to develop the simple spectrophotometric method which is based on the formation of phosphomolybdate with added molybdate16, 24^ followed by its reduction with sodium sulphide^28 in aqueous acidic medium.
Experimental
Elico spectrophotometer model SL 171 (Hyderabad, India) with 1 cm matched quartz cells, Sartorius digital balance readable 0.0001g were used.
Reagents
All the chemical reagents used were of analytical grade and the water used was distilled water. Weighed amount of ammonium molybdate, 1.7081 g was dissolved in about 150 mL of warm water^27 ; slightly milky solution obtained was cooled to room temperature. It was then transferred into a 250 mL volumetric flask and diluted to the mark with water.
0.05g of sodium sulphide was transferred into a clean 100 mL beaker. It was dissolved in about 50 mL of water and then the solution was transferred into a 100 mL volumetric flask. The beaker was washed 3-4 times with water and washings were also transferred into the flask and the solution was diluted to the mark with water. The solution was standardized iodometrically^28 before use.
0.1145 g of disodium hydrogen phosphate was transferred into a 150 mL beaker^29. It was dissolved in water, then; the solution was transferred into a 250 mL volumetric flask and diluted to mark with water. The working solutions were prepared by diluting 5 mL of this solution to 50 mL with water.
Sulfuric acid (0.25N) was prepared by diluting the concentrated sulfuric acid (≈36N) with water.
470 G. NAGENDRAPPA et al.
method^25 are shown in Table 1and 2 which account for the complimenting nature of the results from both the methods. Therefore, the proposed method could be either a substitute to an official method^25 or it could also be an independent method for the determination phosphate in the sugarcane juice samples.
Table 1. Determination of phosphate in sugar cane juices. sample Phosphate in mg/l by developed method
Phosphate in mg/l by official method Sugarcane juice-1 172 170 Sugarcane juice-2 195 193 Water sample-1 52 51 Water sample-2 15 14
Table 2. Determination of phosphate in detergent powders
Company
Phosphate in % by developed method
Phosphate in % by official method Company-A 19.5 20. Company-B 25.2 25. Company-C 18.0 18.
Optimization
Effect of concentration of ammonium molybdate
First 5.528x10-3M ammonium molybdate solution was prepared29,31. Three series, each series having three labeled 10 mL volumetric flasks were arranged .To each flask of the first series 0.1 mL of 5.528x10-3^ M molybdate solution was added. To each flask of the second series 0.3 mL of 5.528x10-3^ M molybdate solution was added. To each flask of the third series 0.5 mL of 5.528x10-3^ M molybdate solution was added. To the flasks of each series, measured but various volumes (0.2, 0.5, 1.0 mL) of phosphate (30 ppm) solution, 1 mL of 2.0833x10-3^ M sodium sulphide solutions were added. All the solutions were diluted to the mark with water. The absorbance of the solutions were measured at 715 nm .The first series solutions containing 0.1 mL of molybdate showed absorbance 0.01, 0.02, and 0.05 for 0.2, 0.5,1.0 mL of phosphate. It yielded a straight line, but ran almost parallel to x-axis, indicating low sensitivity. A second series of solutions containing 0.3 mL of molybdate solutions showed absorbance value 0.02, 0.03and 0.05 for 0.2, 0.5and 1.0 mL of phosphate respectively. This also yielded a straight line but indicated low sensitivity when the absorbance is plotted against concentration of phosphate. A third series of solutions containing 0.5 mL of ammonium molybdate solutions showed absorbance values of 0.07, 0.09, and 0.15 for 0.2, 0.5 and 1.0 mL of phosphate respectively .The absorbance values were found to increase with concentration of phosphate and the colour of the solutions were stable up to 2h. Therefore, 0.5 mL of ammonium molybdate was more preferred than its volume used in the earlier two sets solutions for the determination of phosphate.
Effect of concentration of sulfuric acid
The effect of concentration of sulfuric acid on absorbance was investigated to achieve high absorbance via taking various volumes (1-3 mL) of 0.25 N of sulfuric acid ,0.5 mL of ammonium molybdate , 1 mL of phosphate (30 ppm) and 1 mL of 2.0833x10^3 M sodium sulphide solutions were added to the 10 mL volumetric flasks. Then, the solutions were
A Simple Spectrophotometric Determination of Phosphate 471
diluted to the mark with water. Absorbance of each solution was measured at 715 nm. The absorbance of the solutions were 0.02, 0.05and 0.15 respectively for 1,2 and 3 mL of 0.25 N sulfuric acid .The absorbance values indicated that the solution containing 3 mL of 0.25 N sulfuric acid appeared to be more sensitive. Therefore, it was used throughout the experiment to determine the phosphate.
Effect of concentration of sodium sulphide
The concentration influence of reducing agent was studied as above with different volumes (0.2, 0.5, 0.7, 1.0mL) of 2.0833x10-3^ M of sodium sulphide. The absorbance values were found to be 0.05, 0.1 and 0.16 respectively for sodium sulphide. Based on these results; 1.0 mL of this reagent was selected for the construction of calibration graph.
Effect of order of addition of the reagents
Different orders of the reagent addition were studied using optimised amounts of reagents following the recommended procedure .The results obtained have shown that the order of reagents addition do not effect absorbance values. But for maintaining the uniformity of the order of reagents addition as mentioned in the recommended procedure was followed throughout course of the determination of phosphate.
Calibration Graph
Under the optimum condition, a good linear relationship Figure 1 was found to exist between the absorbance of the system and concentration of phosphate 0.3-12.24 ppm with a straight line having slope 0.0336(Cal.) and intercept 0.4667.Molarabsotptivity, correlation coefficient, and Sandell’s sensitivity values were calculated and were found to be 6.103x10^3 mol-1cm,-1^ 0.999 and 0.0156 μg cm-2^ respectively. The precision and accuracy of the method were studied by analyzing a series of solution containing known amount of phosphate (0.3, 3.0 and 10.7 ppm) by using recommended volumes and concentration of the reagents. The precision of the method as expressed by relative standard deviation was less than 3.7% whereas the accuracy expressed by the calculated relative error was 3.4%.
0
0 5 10 15 Phosphate in ppm
Absorbance
Figure 1. Calibration graph for the determination phosphate under optimized experimental condition.
Effect of foreign ions
The effect of foreign ions were examined by using 3 ppm of phosphate and the results obtained are shown in Table 3. Most of the cations and anions commonly found in sugar cane juice7, 8 do not interfere but iron; copper and zirconium interfere at 20, 12, and 11 ppm respectively.
A Simple Spectrophotometric Determination of Phosphate 473
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