Water Ex-tractable Phosphate by Standard Spectrophotometric and FIA - Term Paper Example

Water extractable phosphate by standard spectrophotometric and FIA (Flow injection analysis methods) For the analysis of water extractable phosphate from soil, you have been asked to find and evaluate a Standard colorimetric method and to perform a few trial measurements. 1) Describe, using a flow diagram the complete process of this analysis, showing all conditions, timings, etc. Phosphorus in soil is found in form of phosphate that consists on an atom of phosphorus linked to four oxygen atoms. Chemically, there are three forms of phosphate in soil (Crowther 2009) and (SASSA 2007): a) Soluble inorganic phosphates or orthophosphates, PO4-3 b) Non soluble inorganic phosphates c) Organic phosphates, where phosphate group is part of organic compounds, such as: nucleic acids, phospholipids, etc. Colorimetric analysis of water soluble phosphates, is based on the reaction of these compounds to form coloured complex in solution, which colour intensity is spectrophotometrically measured. This method is selective for this kind of phosphates, so, for total phosphates determination, in soil, a conversion to orthophosphate is required for all phosphates that is done by a digestion method after an extraction of all phosphates (Nutrient 2000) and (Water quality program 2006). In aqueous extracts, could there be colloidal particles of non soluble phosphates or of other compounds. Care must be taken to eliminate to the maximum those particles, because, they could alter the absorbance reading of soluble phosphates giving incorrect readings. Commonly the way to eliminate particles is to filter the extract through a 0.45 m membrane filter. A research work have been accomplished to study the effect of colloids in the absorbance measurements of water extracts from soil, and a method consisting on the addition of NaCl to the water extract, followed by 0.45 m filtering, is given to reduce interference by colloidal particles (Koopmans, Chardon & van der Slam 2005). Orthophosphates can be analyzed by the next colorimetric methods, each of they have different detection limits (Lim 1991) and (Water quality program 2006): - Ascorbic acid method, 0.3-6.0 mg/l P, depending on light path through the solution; - Automated ascorbic acid reduction method, 0.001 to 10.0 mg/l P - Vanadomolybdophosphoric acid method, 1 to 20 mg/l P - Stannous Chloride Method, 0.001 to 6 mg/l P These methods are based on the production of complex with ammonium molybdate in acidic solution (Martinez & Baeza 2008). PO4-3 + 3 NH4+ + 12 MO4-2 + 24 H+  (NH4)3POMo12O40 + 12 H2O Generally, the complete process of phosphates colorimetric analysis of soils has the next stages: a) Sampling. Adequate sampling is important for the analysis be representative. Considerations on sampling and different procedures are given on (Carter 1993) and (Jones 1999). b) Storing and preparation of the sample. The conditions of sample storing should be controlled for example in temperature, oxygen, humidity. Also the sample is prepared for analysis, mainly grinded and dried (Carter 1993) and (Jones 1999). c) Extraction. There are several procedures to extract phosphorus from soil, the selection of the method depends on what fraction of phosphorus is the wanted to be analysed (Crowther J. 2009), (Jones 1999), (SASSA 2007), (Sobeck & Ebeling 2007). About water extraction, several methods are described, varying mainly the ratio soil to water that is according to the purpose of the analysis, (Jones 1999) and (Rita 2008). d) Digestion. Is the process of converting all the phosphates in the form appropriate that reacts with the colorimetric reagent, and also to eliminate interferences which could be caused by other present species (Nutrient 2000) and (Water quality program 2006) e) In solution colorimetric reaction of the sample with colorimetric reagents. f) Colorimetric measurement of the coloured solution and the blank. This is done by a spectrophotometer according to the required wave length absorbed by the coloured specie. g) Calculation of concentration. In order to calculate the concentration of the analyte, a calibration curve should be constructed, based on the absorbance measurement of at least four standards of known concentration in the appropriate range. It is important to mention, that it is not recommendable to extrapolate over the range of the calibration curve, as sometimes to higher concentrations of the coloured specie the curve could have a lineal deviations. The most used method is that of ascorbic acid, it is described in the showed below flow diagram (Hodges 2000) and (Clesceri, Greenberg & Eaton 1998), which is based on the following facts: “Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of orthophosphate-phosphorus to form an intensely colored antimony-phospho-molybdate complex. This complex is reduced to an intensely blue-colored complex by ascorbic acid. The color is proportional to the phosphorus concentration. The complex is not stable and thus analysis must be performed within 30 minutes of adding the ammonium molybdate and antimony potassium tartrate” (Nature 2000). This method is describe in detail also in (Sobeck & Ebeling 2007). Equipment needed for this analysis is a spectrophotometer with a phototube that produces the required length, 880 nm (Hodges 2000). Interferences: on this method, As can interfere on concentrations over 0.1 mg As/ l, because, it also form a blue complex with molybdate (Clesceri, Greenberg & Eaton 1998). 2) A calibration curve was constructed using the following values. Concentration mg/l 1 5 10 15 Spectrometer reading 3 8 13 16 Produce de curve and describe it, in terms of; working range, application to the Beer-Lambert Law and its analytical validity. What could you do to increase your confidence in the curve when using it to evaluate the simples from the soils. Calibration curve: Application to the Beer-Lambert law Beer-Lambert law is as follows (Kebbekus & Mitra 1998): A = log P/Po = abC; where: A: absorbance spectrometer reading P/Po = T-1: transmitance inverse a: constant of proportionality, known as absorptivity b: length of radiation path inside the solution of analyte This law produces a straight line that cross the origin, and the slope is ab For the given curve, ab = 0.93 cm * l/mg cm The curve does not cross the origin, as it should (A = 0 when c = 0). This could be caused because of one or more of the next reasons: a) The standards or the blank were not prepared correctly, b) The A value at zero concentration, 2.7963, could represent the absorbance of the blank, or impurity or the reagents in solution, and it should be subtracted of the absorbance reading of each standard. c) The maximum absorbance length given by the spectrophotometer, is not the same maximum value indicated in the literature for the substance in studio, that in this case is 880 nm, as indicated in the above described method. Working range and analytical validity The Beer-Lambert indicates that concentration is directly proportional to absorbance, so, calibration curve should be a straight line. The obtained curve has a R2 de 0.9775, and it presents a negative deviation alter third value, it could have been caused because of instrumental errors, such as pholychromatic radiation or parasite radiation that is not absorbed. So, working range, for a straight line could be until third point if is considered that R2 de 0.9775 is low. This way, it is concluded that validity range is until 10 mg/l. Because the curve does not cross the origin, should be wary of its validity, unless it is discussed in detail what was the mistake and a correction can be done. Care must be taken to avoid the turbidity of the simple and subtract the absorbance of the blank from the reading given for the simple. It is necessary always to verify the purity of the reagent, and to rinse with acidic water the used glass material, to eliminate waste of detergents, which may increase the reading. 3) You have had the chemistry of the method accepted but the number of samples needed to be analysed has been set at 150. Due to the Lab time that this would required, you have decided to use a flow injection analysis variation of the method you described above. This uses the same chemistry (the same reactions etc). Show, using an annotated block diagram, the FIA system and conditions you would use. You may be able to find an exact published FIA version of this analysis, but if not, design your own using information gained from the lecture and other sources. Briefly summarise the advantages and disadvantages of using FIA over the Standard methodology. N.B. If you use 1 mm id tubing and, a 1 cm3 per minute flow rate then the liquid would move about 150 along the tube in one minute. FIA system consists of a laminar flow stream of reagent solution, which is injected a certain volume of sample, using an injection valve, trying to avoid mixing of different samples controlling the injection interval. (AnalChemis Resources 2000). Other controlled conditions are described: “The sample plug on reaching the detector, which may be a spectrophotometric or electrochemical device, is detected as a peak with a sharp front and a tail. The degree of dispersion of the injected plug may be controlled by several means, including the flow dynamics of the system, reactor path length, and reagent flow rate. Thus the sensitivity of detection is maximized by optimizing factors such as reagent concentrations, pH, temperature, reactor path length, and reagent flow rate” (AnalChemis Resources 2000). Before the reading, there must be a steady baseline in the reading. Fluid regime should be laminar. Each sample is injected a minimum of three times, and the readings are statistically treated to determine average PO43– concentration, deviation and coefficient of variation. The reagents prepared are the same as for the manual version of this analysis. Parameter of manifold, should be optimized for sensibility, determination rate and reagent consumption and repeatability should be achieve, a suggestion is to begin with a ratio of 1:1 for sample and reagent carrier, and repeatability should be achieve (Alves 2006) by a experimental design method. More time of residence in the coil, results in major sensibility, but, high interval of sample injection makes slow the analysis velocity. So, an appropriate point should be found. Considering, a reagent-sample ratio of 1:5, from the above described standard method (8 ml to 50 ml), and considering a time of 2 minutes and a coil length of 150 cm, with a tube of 1mm id, then the total flow required, F, is: F = (150 cm * (( * (0.1 cm)2)/4)) / 2 min = 0.59 cm3 / min (total flow) So, sample flow is = 0.49 cm3 / min And reagent flow is = 0.1 cm3 / min Interval of injection should allow the reading get back to the baseline, in (AnalChemis Resources 2000) suggest from 15 to 45 seconds for this analysis, so to begin could be every 30 seconds. This parameter should be probed with a standard solution of phosphate to improve the results and repeatability. Advantages Human errors are eliminated for handling of reagents. There is a reduction in the cost per analysis and the generation of waste. FIA reduces the analysis time, which makes more number of tests in less time. Under proper conditions, high reproducibility is obtained and can provide analytical information in real time, for example in the flow of a river, thus, storage of samples an the risk of obtaining wrong values due to the instability of the sample are eliminated. Disadvantages It is sometimes difficult to achieve good mixing between the flow of reagents and flow injection of the sample, which may give erroneous absorbance readings. A complete mixing can be achieved by increasing the velocity, whenever laminar flow is maintained, to avoid the successive samples are not mixed together (Alves 2006). References list Alves, M. S. D. C. N. 2006. Flow systems for the trace determination of phosphate in waters base on spectrophotometric vanadomolybdate method. M.Sc. Cranfield University. AnalChemis Resources 2000. Analytical Chemistry: FIA –Determination of phosphate by Flow Injection Analysis. [Online] Chem. Dept. UWI. St. Augustine Campus., Available at: http://delloyd.50megs.com/labscripts/phosphate.html [Accessed April 20 2009]. Carter, M.R. 1993. Soil sampling and methods of analysis. Canada: Lewis Publishers. Clesceri, L.S., Greenberg, A.E. & Eaton, A.D. 1998. Standard methods for the examination of water and wastewater. 20th ed. Maryland: United Book Press. Crowther, J. 2009. Soil/sediment analysis: background to analytical methods. [Online] Lampeter Archaeological Services. Available at: http://www.lamp.ac.uk/uwlas/pdf/uwlas_soil_methods.pdf [Accessed 23 April 2009]. Hodges, S.C. 2000. Methods of phosphorus analysis for soils, sediments, residuals and wasters. [Online] Southern cooperative series bulletin #396, Available at: http://www.sera17.ext.vt.edu/Documents/Methods_of_P_Analysis_2000.pdf [Accessed 22 April 2009]. Jones, J.B. 1999. Soil analysis Handbook of reference methods. .Georgia: CRC Press. Kebbekus, B. B. & Mitra, S. 1998. Environmental chemical analysis. London: Blackie Academic & Professional. Koopmans, G.F., Chardon, W.J. & van der Slam C. 2005. Disturbance of water-extractable phosphorus determination by colloidal particles in a heavy clay soil from the neatherlands. J. Environ. Qual. 34, pp 1446-1450. Lim, S. 1991. Determination of phosphorus concentration in hydroponics solution. [Online] Varian, UV Instruments at work, Available at: https://www.varianinc.com/media/sci/apps/uv52.pdf [Accessed April 21 2009]. Martinez, J.M. & Baeza, A. 2008. Metodologia analítica: Curvas de vaildacion A = f( C ). [Online] Facultad de Quimica, UNAM. Available at: http://quimica.unam.mx/blogs/jmmartinez/files/2008/03/5-determinacion-de-fosfatos-en-detergentes.pdf [Accessed April 21 2009]. Nutrient Removal Project 2000. Phosphorus measurements. [Online] Nutrient Removal Project home page, Available at: http://ceeserver.cee.cornell.edu/mw24/cee453/NRP/Phosporus%20Determination%20using%20the%20Colorimetric%20Ascorbic%20Acid%20Technique.doc [Accessed April 22 2009]. Rita, A., 2008. Hach soil manual. [Online] Agriculture for Kurd’s home page, Available at: http://www.soil.a4kurd.com/pdf/english/HACHsoilmanualsummary.pdf [Accessed April 22 2009]. SASSA 2007. Analytical methods: Quantitative phosphate. [Online] SASSA Home page, Available at: http://www.sassa.org.uk/index.php/Analytical_Methods:Quantitative_Phosphate#Sampling_and_sample_preparation [Accessed 21 April 2009]. Sobeck, S.A. & Ebeling, D.D. 2007. Comparison of mass spectrometry, colorimetry, and inductively coupled plasma. [Online] Mass spectrometric analysis for phosphate in soil extracts, Available at: http://www.asdlib.org/eUGHUploads/49_eUGH_publication.pdf [Accessed 22 April 2009]. Water quality program 2006. Water resources characterization DSS: phosphorus.[Online] CSREES North Carolina, Available at: http://www.water.ncsu.edu/watershedss/info/phos.html [Accessed April 21 2009]. Bibliography Hill, G.C.1989. Chemistry in context. 3rd ed. Walton on Thames, England: T. Nelson. Jackson, M. L. & Barak, P. 2005. Soil chemical analysis. Wisconsin: UW-Madison Libraries Parallel Press. Kluiber, R.W. 2009. Phosphate analysis. [Online] Tutorial General Chemistry Laboratory, Available at: http://genchem.rutgers.edu/phosA.html [Accessed 21 April 2009] Skoog, A. D. & Holler, F. J. 2006. Principles of instrumental analysis. Brooks Cole. Walton, H.F. & Reyes, J. 1973. Modern chemical analysis and instrumentation. Marcel Dekker Inc. Read More