SELENIUM 217
4. CHEMICAL AND PHYSICAL INFORMATION
4.1 CHEMICAL IDENTITY
Information regarding the chemical identity of selenium and selenium compounds is presented in
Table 4-1.
4.2 PHYSICAL AND CHEMICAL PROPERTIES
Selenium is a non-metal element with atomic number 34 and an atomic mass of 78.96 (Lide 2000).
Selenium belongs to Group 6 (Group VIA) of the periodic table, located between sulfur and tellurium,
and resembles sulfur both in its various forms and in its compounds. The six stable isotopes of selenium
are
74
Se,
76
Se,
77
Se,
78
Se,
80
Se, and
82
Se. These isotopes occur naturally with approximate abundances of
0.87, 9.02, 7.58, 23.52, 49.82, and 9.19%, respectively (Hoffmann and King 1997). Artificial radioactive
isotopes of selenium have also been created by neutron activation. The gamma-emitting isotope
75
Se has
been used in diagnostic applications of medicine (Hoffmann and King 1997). Selenium exists in several
allotropic forms. Three are generally recognized, but as many as six have been claimed (Lide 2000). The
stable form at ordinary room temperatures is the grey or hexagonal form with a melting point of 220.5 EC
(Lide 2000). The other two important forms are red (monoclinic) with a melting point of 221 EC and
amorphous selenium, which exists in black and red forms. Black amorphous selenium is vitreous and is
formed by the rapid cooling of liquid selenium. Red amorphous selenium is colloidal and is formed in
reduction reactions (Hoffmann and King 1997). Important selenium oxidation states are -2, 0, +4, and +6.
The chemical properties of selenium are similar to sulfur. Selenium combines with metals and many
nonmetals directly or in aqueous solution. The selenides resemble sulfides in appearance, composition,
and properties (Hoffmann and King 1997). Selenium may form halides by reacting vigorously with
fluorine and chlorine, but the reactions with bromine and iodide are not as rapid. Selenium does not react
directly with hydrogen fluoride or hydrogen chloride, but decomposes hydrogen iodide to liberate iodine
and yield hydrogen selenide (Hoffmann and King 1997). Selenium reacts with oxygen to form a number
of oxides, the most stable of which is selenium dioxide.
Information regarding the physical and chemical properties of selenium and selenium compounds is
located in Table 4-2.
SELENIUM 218
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Selenium and Selected Compounds
a
Characteristic
Selenium
Hydrogen
selenide
Selenic acid
Selenious
acid
Synonyms
Elemental
selenium;
selenium base;
selenium dust;
colloidal selenium;
selenium
homopolymer
b
;
selenium alloy
Dihydrogen
selenide;
hydrogen
selenide [H
2
Se];
selenium
anhydride;
selenium
dihydride;
selenium
hydride; selane
Selenic acid,
liquid
DOT,b
Monohydrated
selenium
dioxide;
selenous
Registered trade
name(s)
C.I. 77805;
VANDEX
b
Chemical formula Se H
2
Se H
2
SeO
4
H
2
SeO
3
Wisewesser line
notation
SE H2 SE H2.SE-04 H2SE-03
Identification
numbers:
CAS
NIOSH RTECS
EPA hazardous
waste
OHM/TADS
DOT/UN/NA/IMCO
shipping
HSDB
NCI
7782-49-2
VS7700000
No data
7216880
UN 2658
b
4493
No data
7783-07-5
MX1050000
No data
No data
UN 2202;
Hydrogen
selenide;
anhydrous
548
No data
7783-08-6
VS6575000
No data
No data
UN 1905; IMCO 8.0
675
No data
7783-00-8
VS7175000
U204
c
No data
No data
6065
No data
SELENIUM 219
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Selenium and Selected Compounds
a
Characteristic
Sodium selenate
Potassium
selenate
Sodium selenide
Sodium
selenite
Synonyms Disodium selenate
Selenic acid,
dipotassium salt
b
Disodium
monoselenide
b
Disodium
selenite;
disodium
selenium
trioxide;
selenious
acid disodium
salt; sodium
selenium
oxide
Registered trade
name(s)
P-40
b
; Sel-Tox
SS02 and SS-20
c
No data No data
Chemical formula Na
2
SeO
4
d
K
2
SeO
4
Na
2
Se
b
Na
2
SeO
3
d
Wisewesser line
notation
NA2 SE-04
b
KA2 SE-04 NA2 SE NAS SE-03
Identification
numbers:
CAS
NIOSH RTECS
EPA hazardous
waste
OHM/TADS
DOT/UN/NA/IMCO
shipping
HSDB
NCI
13410-01-0
No data
No data
No data
No data
No data
No data
7790-59-2
VS6600000
No data
No data
No data
No data
No data
1313-85-5
b
WE0350000
b
No data
No data
No data
No data
No data
10102-18-8
VS7350000
No data
7217299
UN 2630
b
768
No data
SELENIUM 220
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Selenium and Selected Compounds
a
Characteristic
Selenium dioxide
Selenium
trioxide
Selenocystine
Selenomethionine
Synonyms
Selenious
anhydride; selenium
oxide; selenium
oxide [SeO
2
];
selenous acid
anhydride
No data
Selenium
cystine
b
;
3,3-diselenodi-
DL-alanine
b
;
seleno-DL-
cystine
b
; DL-
selenocystine
b
Methionine, seleno
b
;
2-amino-4-
(methylselenyl)
butyric acid;
2-amino-4-(methylsel
eno)
Registered trade
name(s)
No data No data
Chemical formula SeO
2
SeO
3
e
C
2
H
4
NO
2
(CH
2
)Se
2
(CH
2
)C
2
H
4
NO
2
(CH
3
)Se(CH
2
)2C
2
H
4
NO
2
Identification
numbers:
CAS
NIOSH RTECS
EPA hazardous
waste
OHM/TADS
DOT/UN/NA/IMCO
shipping
HSDB
NCI
7446-08-04
VS8575000
V204
c
7800105
No data
677
No data
13768-86-0
f
No data
No data
No data
No data
No data
No data
1464-43-3
b
AY6030000
b
No data
No data
No data
No data
No data
1464-42-2
ES100000
No data
No data
No data
No data
No data
SELENIUM 221
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Selenium and Selected Compounds
a
Characteristic Selenium sulfide Selenium disulfide
Synonyms
Selenium monosulfide;
selenium sulfide [SeS];
selensulfid (German); sulfur
selenide (SSe)
Selenium disulphide; selenium
sulfide
b
; sulfur selenide
Registered trade name(s) No data
Exsel; Selsun Blue; Selsum
b
;
Seleen
Chemical formula SeS SeS
2
b
Wisewesser line notation SE S SE S2
b
Identification numbers:
CAS
NIOSH RTECS
EPA hazard waste
OHM/TADS
DOT/UN/NA/IMCO shipping
HSDB
NCI
744-34-6
VTO525OOO
V205
b
8400272
No data
679
NCI-C50033
7488-56-4
VS8925000
V205
8400272
UN 2657
No data
No data
a
All information obtained from HSDB 2001, except where noted
b
RTECS 2001
c
EPA 1980a, 1980b (40 CFR 261.33)
d
Budavari et al. 1996
e
Lide 2000
f
ChemIDplus 3003
CAS = Chemical Abstracts Service; DOT/UN/NA/IMCO = Department of Transportation/United Nations/North
America/International Maritime Dangerous Goods Code; EPA = Environmental Protection Agency;
HSDB = Hazardous Substances Data Bank; NCI = National Cancer Institute; NIOSH = National Institute for
Occupational Safety and Health; OHM/TADS = Oil and Hazardous Materials/Technical Assistance Data System;
RTECS = Registry of Toxic Effects of Chemical Substances
SELENIUM 222
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Selenium
Hydrogen
selenide
Selenic acid Selenious acid
Molecular weight 78.96 80.98 144.97 128.97
Color/form Red, grey, or black Colorless
b
White
hexagonal
prisms;
hygroscopic
b
White hygroscopic
prisms
b
Physical state Solid Gas Solid Solid
Melting point
221 EC(red);
220.5 EC (grey);
180 EC (black)
b
-65.73 EC 58 EC 70 EC
(decomposes)
b
Boiling point
685 EC -41.3 EC 260 EC
None, loses water
upon heating
Density (g/cm
3
) 4.39 (red); 4.81
(grey); 4.28 (black)
b
2.12 (-42 EC) 2.9508 (15 EC) 3.004 (15 EC)
Odor Unknown; upon
combustion, smells
like rotten
horseradish
Disagreeable odor No data No data
Odor threshold:
Water (mg/m
3
)
Air
No data
No data
No data
No data
No data
No data
No data
No data
Solubility:
Water
Insoluble
377 mL/100 mL at
4 EC;
270 mL/100 mL at
22.5 EC;
0.73 mL/100 mL at
20 EC
c
Very soluble in
hot water
90 parts dissolve
in 100 parts of
water at 0 EC; 400
parts in 100 parts
at 90 EC
Organic solvent(s) Insoluble in alcohol,
slightly soluble in
carbon disulfide
(2 mg/100 mL,
room temperature),
soluble in ether
Soluble in carbon
disulfide, carbonyl
chloride
Decomposes in
alcohol
b
Very soluble in
alcohol
Partion coefficients:
Log K
ow
Log K
oc
No data
No data
No data
No data
No data
No data
No data
No data
Vapor pressure
1 mmHg at 356 EC
(grey)
1,330 mmHg at -
30 EC; 3,420 mmHg
at 0.2 EC; 9,120
mmHg at 30.8 EC
No data
2 mmHg at 15 EC;
4.5 mmHg at
33 EC; 7mmHg at
40.3 EC
Henry’s Law
constant
Not applicable No data Not applicable Not applicable
Autoignition
temperature
No data No data No data No data
SELENIUM 223
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Selenium
Hydrogen
selenide
Selenic acid Selenious acid
Flashpoint No data Not applicable No data No data
Flammability limits No data No data No data No data
Conversion factors No data ppm selenium to mg
Selenium/m
3
in air
(20 EC): ppm
selenium x 3.23=mg
selenium/m
3
to ppm
selenium in air
(20 EC): mg
selenium/m
3
x
0.31=ppm selenium
(v/v)
No data No data
Explosive limits Unknown
d
No data No data No data
SELENIUM 224
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Sodium selenate
Potassium
selenate
Sodium
selenide
Sodium selenite
Molecular weight 188.94 221.15 124.94 172.94
Color/form White crystals Colorless crystals or
white powder
Crystalline;
turns red on
exposure to air
and
deliquesces
White tetragonal
crystals
b
Physical state Solid Solid Solid Solid
Melting point No data No data
>875 EC
No data
Boiling point No data No data No data No data
Density (g/cm
3
) 1.61
b
3.07
2.625 (10 EC)
No data
Odor No data No data No data No data
Odor threshold:
Water (mg/m
3
)
Air
No data
No data
No data
No data
No data
No data
No data
No data
Solubility:
Water
Organic solvent(s)
Very soluble in
water
No data
Soluble in about 1
part of water
No data
Decomposes in
water
No data
Freely soluble in
water
No data
Partition coefficients:
Log K
ow
Log K
oc
No data
No data
No data
No data
No data
No data
No data
No data
Vapor pressure No data No data No data No data
Henry’s Law
constant
No data No data No data No data
Autoignition
temperature
No data No data Not flammable
e
Not flammable
e
Flashpoint No data No data Not flammable
e
Not flammable
e
Flammability limits No data No data Not flammable
e
Not flammable
e
Conversion factors No data No data No data No data
Explosive limits No data No data No data No data
SELENIUM 225
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Selenium dioxide
Selenium
trioxide Selenocystine Selenomethionine
Molecular weight 110.96 126.96
b
334.12
c
196.11
Color/form Lustrous,
tetragonal needles;
yellowish-green
vapor
White crystals
b
No data Transparent,
hexagonal sheets or
plates; metallic luster
or crystals
Physical state Solid Solid No data Solid
Melting point
340 EC; sublimes at
315 EC
b
118 EC
b
No data
DL form: 265 EC
(decomposes); L
form: 266–268 EC
Boiling point None
b
Sublimes
b
No data Not applicable
Density (g/cm
3
)
3.954 (15 EC)
3.44
b
No data No data
Odor Pungent sour smell No data No data No data
Odor threshold:
Water (mg/m
3
)
Air
0.0002
e
No data
No data
No data
No data
No data
No data
No data
Solubility:
Water (g/100 mL)
Organic solvent(s)
(parts/100 parts
solvent)
38.4 at 14 EC;
in methanol: 10.16
at 11.8 EC; in 93%
ethanol: 6.67 at
14 EC; in acetone:
4.35 at 15.3 EC; in
acetic acid: 1.11 at
13.9 EC; soluble in
benzene
Soluble in water
No data
No data
No data
No data
No data
Partition coefficients:
Log K
ow
Log K
oc
No data
No data
No data
No data
No data
No data
No data
No data
Vapor pressure 12.5 mm Hg at
70 EC; 20.2 mm Hg
at 94 EC; 39.0 mm
Hg at 181 EC;
760 mm Hg at
315 EC; 848 mm
Hg at 320 EC
No data No data
No data
No data
No data
Henry’s Law
constant
Not applicable Not applicable No data No data
Autoignition
temperature
Not flammable
e
Not flammable
e
No data No data
Flashpoint Not flammable
e
Not flammable
e
No data No data
SELENIUM 226
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Selenium dioxide
Selenium
trioxide
Selenocystine Selenomethionine
Flammability limits Not flammable
e
Not flammable
e
No data No data
Conversion factors ppm (v/v) to mg/m
3
in air (20 EC):
ppm (v/v) x
4.53=mg/m
3
;
mg/m
3
to ppm (v/v)
in air (20 EC):
mg/m
3
x 0.22=ppm
(v/v)
No data No data No data
Explosive limits No data No data No data No data
SELENIUM 227
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical and Chemical Properties of Selenium and Selenium
Compounds
a
Property Selenium sulfide Selenium disulfide
Molecular weight 111.02
f
143.08
f
Color/form Orange-yellow tablets or powder
f
Bright red-yellow
powder
f
Physical state Solid
f
Solid
f
Melting point
118–119 EC (decomposes)
f
<100 EC
f
Boiling point No data No data
Density (g/cm
3
)
3.056 (0 EC)
f
No data
Odor No data No data
Odor threshold:
Water (mg/m
3
)
Air
No data
No data
No data
No data
Solubility:
Water
Organic solvent(s)
Insoluble
Insoluble in ether; decomposes in alcohol
f
Insoluble
No data
Partition coefficients:
Log K
ow
Log K
oc
Not applicable
Not applicable
No data
No data
Vapor pressure Not applicable Not applicable
Henry’s Law constant Not applicable Not applicable
Autoignition temperature No data No data
Flashpoint No data No data
Flammability limits No data No data
Conversion factors No data No data
Explosive limits No data No data
a
All information obtained from Budavari et al. 1996, unless otherwise noted.
b
Lide 2000
c
RTECS 2001
d
NIOSH/OSHA 1981
e
Weiss 1986
f
Lide 1993
Note: The gray metallic form is the most stable form of selenium (Budavari et al. 1996).
