Phosphorus Analysis Methods; Overview for Molybdovanadate Method.

By : arifin_pararaja

The phosphate ion is a polyatomic ion with the empirical formula PO43− and a molar mass of 94.973 g/mol; it consists of one central phosphorus atom surrounded by four identical oxygen atoms in a tetrahedral arrangement. The phosphate ion carries a negative three formal charge and is the conjugate base of the hydrogenphosphate ion, HPO42−, which is the conjugate base of H2PO4, the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric acid. It is a hypervalent molecule (the phosphorus atom has 10 electrons in its valence shell). Phosphate is also an organophosphorus compound with the formula OP(OR)3

A phosphate salt forms when a positively-charged ion attaches to the negatively-charged oxygen atoms of the ion, forming an ionic compound. Many phosphates are not soluble in water at standard temperature and pressure.

More precisely, considering the following three equilibrium reactions:

H3PO4 H+ + H2PO4

H2PO4 H+ + HPO42−

HPO42− H+ + PO43

In dilute aqueous solution, phosphate exists in four forms. In strongly-basic conditions, the phosphate ion (PO43−) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO42−) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H2PO4) is most common. In strongly-acid conditions, aqueous phosphoric acid (H3PO4) is the main form.

Phosphorus are classified as

Orthophosphate

Orthophosphates applied to agricultural or residential cultivated land as fertilizers are carried into surface waters with storm runoff and to a lesser extent wit melting snow.

Condensed phosphates (pyro-, meta-, and other polyphosphate)

Phosphate can form many polymeric ions such as diphosphate (also pyrophosphate), P2O74−, and triphosphate, P3O105−. The various metaphosphate ions have an empirical formula of PO3 and are found in many compounds.

Organically bound phosphates.

Organic phosphates are formed primarily by biological process. Organophosphate is an ester of phosphoric acid are important in biochemistry and biogeochemistry.

Small amounts of orthophosphate or certain condensed phosphates are added to some water supplies during treatment.

The other of phosphates :

organophosphorus compounds

Phosphine – PR3

Phosphine oxide – OPR3

Phosphinite – P(OR)R2

Phosphonite – P(OR)2R

Phosphite – P(OR)3

Phosphinate – OP(OR)R2

Phosphonate – OP(OR)2R

Phosphate – OP(OR)3, such as triphenyl phosphate.

Phosphorus analyses embody two general procedural steps:

1. Coversion of the phosphorus from of interest to dissolved orthoposphate

2. Colorimetric determination of dissolved orthophosphate.

The separation of phosphorus into its various forms is defined analytically but the analytical differentiations have been selected so that they may be used for interpretive purposes.

Precision and bias data for manual phosphorus methods.

Method

Phosphorus concentration

Relative Standard Deviation

%

Relative Error

%

Ortho-P

(µ/L)

Poly-P

(µ/L)

Total

(µ/L)

Vanadomolybdophosphoric acid

100

75.2

21.6

600

19.6

10.8

7000

8.6

5.4

+ Acid hydrolysis

80

106.8

7.4

300

66.5

14.0

3000

36.1

23.5

+ Persulfate

210

55.8

1.6

990

23.9

2.3

10230

65

0.3

+ Sulfuric-nitric acid

210

65.6

20.9

990

47.3

0.6

10230

7.0

0.4

+ Pherchloric acid

210

33.5

45.2

990

20.3

2.6

10230

11.7

2.2

Stannous chloride

100

25.5

27.8

600

14.2

8.0

7000

7.6

4.3

+ Acid hydrolysis

80

60.1

12.5

300

47.6

21.7

3000

37.4

22.8

+ Persulfate

210

28.1

9.2

990

14.9

12.3

10230

11.5

4.3

+ Sulfuric-nitric acid

210

28.1

9.2

990

14.9

12.3

10230

11.5

4.3

Ascorbic acid

100

9.1

10.0

600

4.0

4.4

7000

5.2

4.9

Remarks :

  1. The digestion method to determine total phosphorus must be able to oxidize organic matter effectively to release phosphorus as orthophosphate
  2. The perchloric acid method, the most drastic and time – consuming method, is recommended only for particularly difficult samples such as sediments.
  3. The nitric acid-sulfuric acid method is recommended for most samples.
  4. By far the simplest method is persulfate oxidation technique. Persulfat oxidation is coupled with ultraviolet light for a more efficient digestion in an automated in -line digestion/ determination by flow injection analyisis.
  5. The vananadomolybdophosphoric acid method is most useful for routine analysis in the range of 1 to 20 mg P/L.
  6. The stannous chloride method or the ascorbic acid method is more suited for the range of 0.01 to 6 mg P/L.
  7. An extraction step is recommended for the lower levels of this range and when interferences must be overcome.
  8. Ion chromatography and capillary ion electrophoresis are useful for determination of orthophosphate in undigested samples.

The range of phosphorus concentration with any methods.

Methods

Phosphorus concentration

Persulfate UV Oxidation

0.02 to 2.50 and 1.0 to 125.0 mg/L

Phosver™ 3 with Acid Hydrolysis

0.06 to 3.50 mg/L PO43-

Acid Hydrolyzable Digestion

LR (Low Range)

Ascorbic acid rapid liquid

(LR) 19 to 3,000 µg/L PO43-

Ascorbic acid1

1.5 to 15.0 mg/L PO43-

Ascorbic acid2

6.0 to 60.0 mg/L PO43-

Amino acid

0.23 to 30.00 mg/L PO43-

Molibdovanadate3

0.3 to 45.0 mg/L PO43-

Molibdovanadate4

1.0 to 100.0 mg/L PO43-

Molibdovanadate rapid liquid

(HR) 0.3 to 45.0 mg/L PO43-

PhosVer 3 (Ascorbic acid)

0.02 to 2.50 mg/L PO43-

Acid persulfate digestion

mg/L PO43- or mg/L P

PhosVer® 3 with Acid persulfate digestion

0.06 to 3.50 mg/L PO43- or 0.02 to 1.10 mg/L P

Molibdovanadate with Acid persulfate digestion

(HR) 1.0 to 100.0 mg/L PO43-

Ascorbic acid with Acid persulfate digestion5

1.5 to 15.0 mg/L PO43-

Ascorbic acid with Acid persulfate digestion6

6.0 to 60.0 mg/L PO43-

Remarks :

1 = UniCell™ Vials (Hach metods; 806 phosphate HCT 121)

2 = UniCell™ Vials (Hach metods; 804 phosphate HCT 122)

3 = Reagent Solution or AccuVac® Ampuls (Hach method 8114)

4 = Test ‘N Tube™ Vials (Hach method 8114)

5 = UniCell™ Vials (Hach metods; 806 phosphate HCT 121)

6 = UniCell™ Vials (Hach metods; 804 phosphate HCT 122)

Note : Overview for Molybdovanadate Method.

Summary of method

In the molybdovanadate method, orthophosphate reacts with molybdate in an acid medium to produce a mixed phosphate/ molybdate complex. In the presence of vanadium, yellow molybdovanadophosphoric acid is formed. The intensity of the yellow color is proportional to the phosphate concentration. Test results are measured at 430 nm.

Instrumentation

Colorimetric equipment; One of the following is required :

Spectrophotometer, for use at 400 to 490 nm

Filter photometer, provided with a blue or violet filter exibiting maximum transmittance between 400 and 470 nm.

The wavelength at which color intensity is measured depends on sensitivity desired, because sensitivity varies tenfold with wavelengths 400 to 490 nm. Ferric iron causes interference at low wavelengths, particularly at 400 nm. A wavelength of 470 nm usually is used. Concentration ranges for different wavelengths are :

P Range (mg/L)

Wavelength (nm)

1.0 – 5.0

400

2.0 – 10

420

4.0 – 18

470

Interfering subtances and suggested treatments

Large amounts of sample turbidity may cause inconsistent results in the test because the acid present in the reagents may dissolve some of the suspended particles and because of variable desorption of orthophosphate from the particles.

The following may interfere when present in concentrations exceeding these listed below :

Interfering substance

Interference levels and suggested treatment

Arsenate

Only interferes if the sample is heated*

Iron, ferrous

Blue color caused by ferrous iron does not interfere if iron concentration is less than 100 mg/L

Molybdate

Causes negatif interference above 1000 mg/L

Silica

Only interferes if the sample is heated*

Sulfide

Causes a negatif interference.

pH, extreme or highly buffered samples

May exceed buffering capacity of the reagents. Samples may require pretreatment. Sample pH should be about 7.

Fluoride, Thorium, Bishmut, Thiosulfate or thiocyanate

Causes a negatif interference.

Temperature, Cold (less than 20oC

Causes a negatif interference.

Temperature, Hot (greater than 25oC

Causes a positif interference.

Note : The following do not interfere in concentrations up to 1000 mg/L such as : Pyrophosphate, tetraborate, selenate, benzoate, citrate, oxalate, lactate, tartrate, formate, salicylate, Al3+, Fe3+, Mg2+, Ca2+, Ba2+, Sr2+, Li+, Na+, K+, NH4+, Cd2+, Mn2+, NO3, NO2, SO42-, SO32-, Pb2+, Hg+, Hg2+, Sn2+, Cu2+, Ni2+, Ag+, U4+, Zn4+, AsO3, Br, CO32-, ClO4, CN, IO3, SiO44-.

* Gentle warming of the sample to room temperature will not prevent this substance from interfering.

Sampling and storage :

  1. Collect samples in plastic or glass bottles that have been cleaned with 1:1 Hydrochloric Acid Solution (HCl 1:1) and rinsed with deionized water. Phosphate contamination is common because of its absorption on glass surfaces.
  2. Do not use commercial detergents containing phosphate for cleaning glassware used in this test. Preferably, reserve the glassware only for phosphate determination, and after use, wash and keep filled with water until needed. If this is done, acid treatment is required only occasionally.
  3. For best results, analyze the samples immediately after collection. If prompt analysis is impossible, preserve the samples for up to 48 hours by filtering immediately and storing at 4 oC (39 oF). The sample should be at room temperature before analysis.

Safety and pollution prevention:

Good safety habits and laboratory techniques should be used throughout the procedure.

The final sample will contain molybdenum. In addition, the final samples will have a pH less than 2 and are considered corrosive (D002) by the Federal RCRA.

Consult the Material Safety Data Sheet (MSDS) for information specific to the reaegent used.

Reference:

Alaerts,G. Dr. Ir; Santika Sumestri, Sri ; 1987: “ Metode Penelitian Air “; Surabaya: Usaha Nasional

Ardi, Sumarna, Drs, BSc;dkk. 1992; ” Pengantar Kimia Analisis II ” ; Bogor : Deperindag; SMAKBo

Busch, D.H; Shull. H; Conley R.T.; 1928 : “ Chemistry 2nd Edition”. Boston: : Allyn and Bacon Inc

Eaton, Andrew, et al. 2005. Standard Methods for Examination of Water and Wastewater. 21st Edition. American Public Health Association. Marryland USA.

Hach. 1997-2004. DR/890 Colorimeter Procedures Manual. USA : Hach Company.

Hach. 1997-2003. DR/4000 Spectrophotometer Procedure. USA : Hach Company.

Hach. 2002. Water Analysis Handbook. 4th Edition. USA : Hach Company.

Jr. Day Clyde, M: Selbin, Joel; 1987 : “ Kimia Anorganik Teori “ Jogjakarta : Gadjah Mada University press

Sienko. J. Michell; Plane. A. Robert; 1961 : “ Chemistry. 2nd Edition’. New York: Mc. Graw Hill Book Company Inc.

Sukardjo, Prof. Dr.; 1990 : “ Kimia Anorganik “, Jakarta : Rineka Cipta

Vogel.1979.”Textbook of Macro and Semimicro Qualitative Inorganic Analysis”. London : Longman Group Limited.

Wardio,BE, dkk; 1978, “ Kimia Analisa 1 ”, Jakarta : Depdikbud.

www.chem.is.try.org

www.wikipedia.or.id

www.smk3ae.wordpress.com

8 thoughts on “Phosphorus Analysis Methods; Overview for Molybdovanadate Method.

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