Analytical Determination of Phosphorous
Phosphorous is a polyvalent non-metal grouped in the nitrogen group of the periodic table of the chemical elements. Elemental phosphorous exists in white and red forms. Whit phosphorous reacts with air to glow due a phenomenon called phosphorescence (a kind of chemiluminiscence). With an atomic number of 15 and denoted by the chemical symbol P, phosphorous is used by all plants. In P-deficient soils it is given as phosphate fertilizers. Algae thriving on soils and water bodies are due to eutrophication (1) and indicate high phosphate levels. In biological systems phosphorous in a dynamic state both as free as unique inorganic phosphate (Pi) andor as bound in mostly organic forms such as with nucleotides and ATPADP and many other molecules.
The major use of phosphates is as agricultural fertilizers. Phosphorous has no synthetic source and is dependent upon mining only and is used and wasted as never earlier. Therefore there is warning signal of the impending shortage of phosphate very soon as early as in the next three decades phosphate sources. There is an international urge to look for alternate sources as well recycling of the used phosphates. In the either situations the critical requirement is that of a reliable cheap and a rapid technique for determination of phosphorous.
The analytical chemistry of phosphorus is very important in many fields, for example, medical and clinical science, agriculture, metallurgy and environmental science. Moreover, in recent years large quantities of phosphate have been used in beverages, detergents, fertilizers6 and also in sugar industries.
Phosphorus is designated as an important plant nutrient and as a result comprises a significant part of agricultural fertilizers and phosphate production has sharply risen in the past half a century. However, phosphorus-based compounds typically have a low solubility. The issue of phosphorus determination has attracted considerable academic research. Pardo et al (2) outline the growth in phosphate concentrations in water and attribute this to increased eutrophication. Environmental problems created by phosphorus and many other inorganic and organic pollutants have generated a great interest in improving methods for accurate and rapid qualitative and quantitative determination of phosphorus.
Definition
In water phosphorous can exist in different forms. The total phosphorus concentration denotes the aggregate of the percentages of these of these percentages. According to Pardo et al (2), labile phosphorus, phosphorus is usually linked with aluminium, iron and manganese oxides and hydroxides. In contrast, phosphorus is often linked with calcium, both in organic and residual phosphorus with calcium, organic phosphorus and residual phosphorus. The aggregation of labile phosphorus and phosphorus connected to AlFeMn oxyhydrates is referred to as non-apatite inorganic phosphorus (NAIP) and calcium associated phosphorus is also named apatite phosphorus (AP). These forms of phosphorus are the most important fractions and this paper will limit its scope to determining these forms of phosphorus.
The need and Contexts for Phosphorus Analysis
In such diverse situations the need for phosphorus analysis is only underestimated and understated in policy matters. The complexity and diversity of the needs are so very wide that a generalized approach may never be possible. There are the environmental issues concerning, water, air, soils and mineral sites. The quantity ranges required for analysis in such situations would be in milligrams or grams. On the other hand in a biological context would be in milligrams or much less. In clinical researches the accuracy and quantities would be much finer. In a purely analytical sense a different strategy would be required for very high purity detection by more sophisticated and expensive methods but for a high precision
Thesis Statement
The purpose of this paper is to demonstrate some commonly used laboratory techniques employed to classify the percentage of phosphorus present and its concentration. Various techniques of differing complexity exist to achieve this outcome and the paper will classify the best methods that provide precise means of detecting phosphorus using modern chemical experimental equipment.
Literature Review
Various phosphate determination procedures have been developed for a long time and used reported (3). Some of the popularly used methods are titrometry, complexogravimetry, colorimetry, atomic absorption spectroscopy, flow injection analysis, HPLC and spectrophotometry methods. Spectrophotometry uses molybdovanadate and ammonium molybdate are most commonly used. In ammonium molybdate spectrophotometric method, different reductants have been employed such as tin(II) chloride, ascorbic acid and 1-amino-2-naphthol-4-sulfonic acid.
Spectroscopy methods are popular because of the sensitivity of the method and ease of converting organic phosphorous into a blue colored complex by several reducing agents. These methods or modifications are based on the original report by Bell, R. D. and Doisy (5). Many modifications of this original method are available (e.g., 4, 6, later 7, 8 and many more). Some of the most recent methods based on this principle but with modifications are 9, 12,12a, 12b, 12c and many more.
One of the oldest methods most popularly used method was described by Fiske and Subbarow (4). They used the reduction of phosphomolybdic acid by hydroquinone suggested earlier by Bell and Daisy (5) modified later by Briggs (6), many later modifications) for blood and urine analysis for phosphorus. This method has stayed so long with many modifications (see 7, 8 chen et al brenblaum, and many more). Both the two earlier papers differed. In the Briggs procedure during the synthesis of phosphomolybdic acid and its subsequent reduction by hydroquinone, the concentration of the latter was reduced to 150th of that suggested by Bell and Daisy (51920). But this alteration was not without its weaknesses such as despite the prolonged treatment with hydroquinone from 5 min in Bell and Daisy to 30 min by Briggs(6), the intensity of the blue color is not as much as seen in the earlier procedure. The basic principle of the process is the conversion of phosphate to phosphomolybdic acid followed by its reduction by sulphuric acid to result in a blue substance. Among the various reducing substances Fiske and Subbarow (4) found the use of 1, 2, 6- aminonaphthol sulphonic acid to be superior and this produced very encouraging results. This substance acts rapidly in excess of equimolar quantities of the sulfonic acid and also giving accurate results in short time.
Fiske and Subbarow (4) conjectured the possibility of interfering substances during extraction of phosphorus from organic substances and also the eventualities of the formation other interfering compounds in the earlier processes. He analyzed various reducing substances which were available.
In the standard method for phosphorus analysis only phosphate, called the reactive phosphate is measured. The reason is that what is actually measured is phosphomolybdate which is the product of molybdate reaction itself. In other words phosphorus which is available for reactions for further analysis is called the reactive phosphorus. In many samples phosphorus may not be available in that form. Therefore the samples have to be treated with other process such as digestionextraction, filtration, purification and so on before the concentrations can be determined through the colorimetric methods. Different phosphorus species (are distinguished from each other empirically by filtration), and then a series of digestions that selectively convert phosphorus to phosphate. After the digestion phosphate is measured. Reactive phosphate is then determined in each digest
For total phosphorus determination sample is digested to convert all phosphorus compounds to phosphate. The digest is filtered and phosphate is then measured, usually by molybdenum blue. The most widely used method is that of Fiske and Subbarow with many modifications. The modifications are in the extraction procedures or chemicals and also the use of reducing agent. The modifications can also be large interferences by heavy metals such as arsenic.
Molybdate ascorbate method is one of the most widely used procedures. As and recommended by American Public Health Association (9) there are following basic steps are described
Digestion This is to facilitate availability of phosphorous bound in organic forms as orthophosphate. Perchloric acid digestion is very strong and requires long time and is therefore suggested for difficult water samples with heavy sediments. Digestion of some samples is also facilitated by UV light treatment along with persulfate oxidation in an automated digestion-determination by flow injection method. The Nitric acid-Sulfuric acid is generally suggested for most samples.
Colorimetric Method As mentioned above the basic principle are using blue complexes with molybdate for reading color reading in a colorimeterspectrophotometer. The choice of the exact method to be used depends upon the expected range of concentrations in the samples. For very small quantities ranging between 0.01 to 6 mg PL ascorbic acid or the stannous chloride suggested by Murphy and Riley (8) method is the choice. For lower ranges than this, an extraction step has to be incorporated to remove the interfering substances. The popularity of the ascorbic acid method has been great enough for developing an automated method and its availability presently.
For routine analysis in the range of 1 to 20 mg PL, vanadomolybdophosphoric acid method is most useful choice.
Comparison of Efficacy of Methods There are few research reports which provide a comparative analysis of the efficiency of on method with another. Chamberlain and Shapiro (10) compared the phosphorus concentrations of 13 water samples as determined by several chemical procedures. After employing the algal biomass assay, they found that there were appreciable differences among these methods. They attributed arsenic interference as the reason and not the phosphate hydrolysis compounds. The reason for this is that arsenate forms the same blue color as phosphate but arsenic concentrations as much as 100 gL do not interfere. They therefore recommend the use of arsenate insensitive extraction procedure. But it is not known if the arsenic concentrations are higher.
It has to be pointed out that pH strongly affects the color of molybdate blue complex. While arsenic is serious interfering substance barium, lead and silver form precipitate as phosphates easily but the effects were not very significant. Similar observations have been mentioned for silica.
Allarino (12) compared three methods of Mehlich (12a, Olsen and Bray for estimating available P on Iowa calcareous soils or across soils of varying soil pH. Their results demonstrate that the Olsen and the Mehlich-3 methods are more reliable tools than the Bray-P1 method for estimating available P on Iowa calcareous soils or across soils varying in soil pH. Because these methods seem to have similar ability to the Bray-P1 for estimating available P on neutral and slightly acid soils the results suggest that either the Olsen or Mehlich-3 methods would be much better than the Bray-P 1 test when a single soil-test for P is used for routine analysis of Iowa soils. It is noteworthy, however that there was a small proportion of calcareous soils in which estimations of available P by any of the three methods were unsatisfactory. Further research is being conducted to explain these results.
The Mehlich-3 extractant proposed as a universal extractant is an attractive method for routine soil testing because of its reliability for neutral or slightly acid soils. But there is a lack of information on correlations and crop-responses of the phosphorus concentrations.
Future
With the phosphorus availability becoming bleak there is a stronger growing need for a widely test for phosphorus analysis including the extractiondigestion procedure.
Interfering substances have been unpredictable and therefore difficult to anticipate the diversity and ranges of available concentrations. This is particularly important because of the urgency of the environmental contamination problems. Different elemental and their compounds together only confound and confuse the prospects and situation of analytical approaches. While many techniques are available they are all confined to laboratories that are well equipped and manned by trained technical experts.
A field level portable phosphorus test method for rapid analysis is very necessary.
There is no standardized comparison of all the tests available presently. The diversity of soils, waters and organic samples for analysis is wide enough to warrant at least some tests. In the case of phosphorus one possibility for improving laboratory efficiency would be to use a single extractant for multiple purposes
Categories:
0 comments:
Post a Comment