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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

earths 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 found

in a variety of minerals; most important among these are vana-

dinite [Pb5(VO4)3Cl], and patronite (possibly VS4), 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 V5. It is associated

with organic complexes and is insoluble in reducing environ-

ments. 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 ev-

idence 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 in

water supplies. However, vanadium pentoxide dust causes gas-

trointestinal and respiratory disturbances. The United Nations* Approved by Standard Methods Committee, 1997.Joint Task Group: 20th EditionSee 3500-Al.

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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 in-

ductively 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 coupled

plasma methods are preferred for polluted samples. The electro-

thermal 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 acid

solution. Under the given conditions of concentrations of reac-

tants, 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 the

sample at 415 nm and comparing it with that of standard solu-

tions treated identically.

b. Interference:The substances listed in Table 3500-V:I willinterfere in the determination of vanadium if the specified con-

centrations are exceeded. This is not a serious problem for Cr6,

Co2, Mo6, Ni2, Ag, and U6 because the tolerable concen-

tration 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 min-

imize their interference; however, mercuric ion itself interferes if

in excess. Adding 350 g mercuric nitrate, Hg(NO3)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(NO3)2.

c. Minimum detectable concentration:0.025 g V in approx-

imately 13 mL final volume or approximately 2 g V/L.

2. Apparatus

a. Water bath,capable of being operated at 25 0.5C.

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.

2) 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 re-

agents, for dilutions, and as blanks.

a. Stock vanadium solution: Dissolve 229.6 mg ammonium

metavanadate, NH4VO3, in a volumetric flask containing ap-

proximately 800 mL water and 15 mL 1 1 nitric acid (HNO3).

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 intermedi-

ate vanadium solution with water to 100 mL; 1.00 mL 0.010 g V.

d. Mercuric nitrate solution: Dissolve 350 mg

Hg(NO3)2 H2O in 1000 mL water.

e. Ammonium persulfate-phosphoric acid reagent: Dissolve

2.5 g (NH4)2S2O8 in 25 mL water. Bring just to a boil, remove

from heat, and add 25 mL conc H3PO4. Let stand approximately

24 h before use. Discard after 48 h.

f. Gallic acid solut ion: Dissolve 2 g H6C7O5 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(NO3)2

* Whatman No. 42 or equivalent.

TABLE 3500-V:I. CONCENTRATION ATWHICHV ARIOUS I ONSI NTERFERE IN

THEDETERMINATION OFV ANADIUM

Ion

Concentration

mg/L

Cr6 1.0

Co2 1.0

Cu2 0.05

Fe2 0.3

Fe3 0.5

Mo6 0.1

Ni2 3.0

Ag 2.0

U6 3.0

Br 0.1

Cl 100.0

I 0.001

VANADIUM ( 3500-V)/Gallic Acid Method 3-105

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solution to each blank, standard, and sample. Place containers in

a water bath regulated to 25 0.5C and allow 30 to 45 min for

samples to come to the bath temperature.

b. Color development and measurement:Add 1.0 mL ammo-

nium persulfate-phosphoric acid reagent (temperature equilibrat-

ed), 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 succes-

sive samples at intervals of 30 s or longer to permit accuratecontrol 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, 250g Be/L, 240 g B/L, and 20 g Se/L in distilled water,

vanadium was measured in 22 laboratories with a relative stan-

dard deviation of 20% and no relative error.

7. Bibliography

FISHMAN, M.J. & M.V. SKOUGSTAD. 1964. Catalytic determination of

vanadium in water. Anal. Chem. 36:1643.

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