## 3500-V VANADIUM*

3500-V A. Introduction

1. Occurrence and Significance

Vanadium (V) is the first element in Group VB in the periodic table; it has an atomic number of 23, an atomic weight of 50.94, and valences of 2, 3, 4, and 5. The average abundance of V in the earth’s crust is 136 ppm; in soils it ranges from 15 to 110 ppm; in streams it averages about 0.9  g/L, and in groundwaters it is generally  0.1 mg/L. Though relatively rare, vanadium is foundin a variety of minerals; most important among these are vanadinite [Pb 5 (VO 4 ) 3 Cl], and patronite (possibly VS 4 ), occurring chiefly in Peru. Vanadium complexes have been noted in coal and petroleum deposits. Vanadium is used in steel alloys and as a catalyst in the production of sulfuric acid and synthetic rubber.

The dominant form in natural waters is V 5 . It is associatedwith organic complexes and is insoluble in reducing environments. It is considered nonessential for most higher plants and animals, although it may be an essential trace element for some algae and microorganisms. Laboratory and epidemiological evidence suggests that vanadium may play a beneficial role in the prevention of heart disease. In water supplies in New Mexico, which has a low incidence of heart disease, vanadium has been found in concentrations of 20 to 150  g/L. In a state where incidence of heart disease is high, vanadium was not found inwater supplies. However, vanadium pentoxide dust causes gastrointestinal and respiratory disturbances. The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 0.1 mg/L.

2. Selection of Method

The atomic absorption spectrometric methods (3111D and E),the electrothermal atomic absorption method (3113B), the inductively coupled plasma methods (3120 and 3125), and gallic acid method (3500-V.B) are suitable for potable water samples. The atomic absorption spectrometric and inductively coupledplasma methods are preferred for polluted samples. The electrothermal atomic absorption method also may be used successfully with an appropriate matrix modifier.



## 3500-V B. Gallic Acid Method

1. General Discussion

a. Principle: The concentration of trace amounts of vanadium in water is determined by measuring the catalytic effect it exerts on the rate of oxidation of gallic acid by persulfate in acidsolution. Under the given conditions of concentrations of reactants, temperature, and reaction time, the extent of oxidation of gallic acid is proportional to the concentration of vanadium. Vanadium is determined by measuring the absorbance of thesample at 415 nm and comparing it with that of standard solutions treated identically.

b. Interference: The substances listed in Table 3500-V:I will interfere in the determination of vanadium if the specified concentrations are exceeded. This is not a serious problem for Cr 6 , Co2 , Mo6 , Ni2 , Ag , and U6 because the tolerable concentration is greater than that commonly encountered in fresh water. However, in some samples the tolerable concentration of Cu2 , Fe2 , and Fe3 may be exceeded. Because of the high sensitivity of the method, interfering substances in concentrations only slightly above tolerance limits can be rendered harmless by dilution.

Traces of Br  and I  interfere seriously and dilution alone will not always reduce the concentration below tolerance limits.Mercuric ion may be added to complex these halides and minimize their interference; however, mercuric ion itself interferes if in excess. Adding 350  g mercuric nitrate, Hg(NO 3 ) 2 , per sam

ple permits determination of vanadium in the presence of up to 100 mg Cl  /L, 250  g Br  /L, and 250  g I  /L. Dilute samples containing high concentrations of these ions to concentrations below the values given above and add Hg(NO 3 ) 2 . c. Minimum detectable concentration: 0.025 g V in approximately 13 mL final volume or approximately 2  g V/L. d. Quality control (QC): The QC practices considered to be an integral part of each method can be found in Section 3020.

2. Apparatus

a. Water bath, capable of being operated at 25  0.5°C. b. Colorimetric equipment: One of the following is required: 1) Spectrophotometer, for measurements at 415 nm, with a light path of 1 to 5 cm. * Approved by Standard Methods Committee, 1997. Editorial revisions, 2011. Joint Task Group: 20th Edition—See 3500-Al.

TABLE 3500-V:I. C ONCENTRATION AT WHICH V ARIOUS I ONS INTERFERE IN

THE D ETERMINATION OF V ANADIUM

Ion Concentration
mg/ L

Cr6 1.0 Co 2 1.0 Cu 2 0.05 Fe 2 0.3 Fe 3 0.5 Mo 6 0.1 Ni 2 3.0 Ag 2.0 U 6 3.0 Br  0.1 Cl  100.0 I  0.001

12) Filter photometer, providing a light path of 1 to 5 cm and equipped with a violet filter with maximum transmittance near 415 nm.

3. Reagents

Use reagent water (see Section 1080) in preparation of reagents, for dilutions, and as blanks.

a. Stock vanadium solution: Dissolve 229.6 mg ammonium metavanadate, NH 4VO 3 , in a volumetric flask containing ap

proximately 800 mL water and 15 mL 1 1 nitric acid (HNO 3 ). Dilute to 1000 mL; 1.00 mL  100  g V. b. Intermediate vanadium solution: Dilute 1.00 mL stock vanadium solution with water to 100 mL; 1.00 mL  1.00 g V. c. Standard vanadium solution: Dilute 1.00 mL intermediate vanadium solution with water to 100 mL; 1.00 mL  0.010 g V.

d. Mercuric nitrate solution: Dissolve 350 mg Hg(NO 3 ) 2  H 2 O in 1000 mL water.

e. Ammonium persulfate-phosphoric acid reagent: Dissolve 2.5 g (NH 4 ) 2 S 2 O 8 in 25 mL water. Bring just to a boil, remove from heat, and add 25 mL conc H 3 PO 4 . Let stand approximately 24 h before use. Discard after 48 h.

f. Gallic acid solution: Dissolve2gH6C7 O5 in 100 mL warm water, heat to a temperature just below boiling, and filter through filter paper.* Prepare a fresh solution for each set of samples.

4. Procedure

a. Preparation of standards and sample: Prepare both blank and sufficient standards by diluting 0- to 8.0-mL portions (0 to 0.08  g V) of standard vanadium solution to 10 mL with water. Pipet sample (10.00 mL maximum) containing less than 0.08  g V into a suitable container and adjust volume to 10.0 mL with water. Filter colored or turbid samples. Add 1.0 mL Hg(NO 3 ) 2 solution to each blank, standard, and sample. Place containers in a water bath regulated to 25  0.5°C and allow 30 to 45 min for samples to come to the bath temperature.

b. Color development and measurement: Add 1.0 mL ammonium persulfate-phosphoric acid reagent (temperature equilibrated), swirl to mix thoroughly, and return to water bath. Add 1.0 mL gallic acid solution (temperature equilibrated), swirl to mix thoroughly, and return to water bath. Add gallic acid to successive samples at intervals of 30 s or longer to permit accurate control of reaction time. Exactly 60 min after adding gallic acid, remove sample from water bath and measure its absorbance at 415 nm, using water as a reference. Subtract absorbance of blank from absorbance of each standard and sample. Construct a calibration curve by plotting absorbance values of standards versus micrograms vanadium. Determine amount of vanadium in a sample by referring to the corresponding absorbance on the calibration curve. Prepare a calibration curve with each set of samples.

5. Calculation

mg V/L   g V (in 13 mL final volume) original sample volume, mL

6. Precision and Bias

In a synthetic sample containing 6  g V/L, 40  g As/L, 250


g Be/L, 240  g B/L, and 20  g Se/L in distilled water,vanadium was measured in 22 laboratories with a relative standard deviation of 20% and no relative error.

7. Bibliography

FISHMAN , M.J. & M.V. S KOUGSTAD . 1964. Catalytic determination of vanadium in water. Anal. Chem . 36:1643. * Whatman No. 42 or equivalent. VANADIUM (3500-V)/Gallic Acid Method

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