Praseodymium

Typical wastewaters

• phosphate fertiliser manufacture (phosphogypsum slurry) Pr³⁺ precipitating as fluoride and phosphate near the outfall 21 t of Pr to the Rhine estuary in 1994 from one plant; sediment near the outfall 30 ppm
• rare earth separation plant wastewater Pr³⁺ in an ammonia nitrogen rich effluent (300 to 5,000 mg/L NH₄-N in Chinese separation plant wastewater, 2005 estimate) no praseodymium concentration read
• mine drainage (coal) Pr³⁺ and the PrSO₄⁺ ion pair in acid sulfate water; total REE averaged 282 µg/L over 141 sites

1 · Identity

Symbol, number

Pr, 59

Oxidation states in water

+3 only (Pr³⁺)

Note

The metal's reactions are in the book entry; in water praseodymium shares the trivalent carbonate and phosphate chemistry of the lanthanide row.

2 · Occurrence in water

Natural sources

Weathering of monazite and bastnaesite with the other light lanthanides; mobilised in acid water and held on phosphate and carbonate in neutral water.

Anthropogenic sources

Phosphogypsum slurry from phosphate fertiliser manufacture: 21 t of Pr to the Rhine estuary in 1994 from one plant, sediment near the outfall 30 ppm Pr; rare earth processing wastewater (ammonia nitrogen 300 to 5,000 mg/L in Chinese separation plant wastewater, 2005 estimate).

3 · Speciation

Trivalent Pr³⁺ throughout the natural pH and Eh range. Below about pH 6, and in sulfate rich water such as mine drainage, the free ion and the PrSO₄⁺ ion pair dominate (sulfate complexes above 90 percent at pH 3.1 to 3.4 in the EPA groundwater study). From neutral to alkaline pH the carbonate complexes PrCO₃⁺ and Pr(CO₃)₂⁻ take over, and they bind the heavy lanthanides more strongly than the light ones (at pH 5.1 to 6.1 the carbonate share rose from 5.0 percent for La to 19.2 percent for Lu). Phosphate, carbonate and fluoride solids and sorption to iron and manganese oxides cap the dissolved concentration at ng/L in oxic neutral water, and the light lanthanides ride partly on colloids.

Solubility

Controlled by the phosphate, carbonate and fluoride solids and by sorption; the RIVM report puts total dissolved lanthanum and cerium in Dutch surface water at about 1 µg/L or less and the free ion fraction at 0.3 fM to 9 pM. No solubility product for a praseodymium solid was read this session.

Hydrolysis

Hydrolysis of Pr³⁺ is minor in natural water; the hydroxide Pr(OH)₃ forms only at high pH, so lime or caustic neutralisation of acid mine water strips the element with the iron and aluminium hydroxides rather than as its own hydroxide (no hydrolysis constant read this session).

Complexation

Sulfate at low pH, carbonate at neutral to alkaline pH (the sources read); humic substances compete for the element in organic rich water and are out-competed by carbonate in alkaline water. No stability constants were read this session.

Precipitates

PrPO₄ (hydrated phosphate), Pr₂(CO₃)₃, PrF₃ near fluoride rich discharges, Pr(OH)₃ at high pH; co-precipitated on iron, aluminium and manganese hydroxides.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Filtration defines the result: 0.2 or 0.45 µm filtrates still carry colloid bound lanthanides, and only ultrafiltration (10 kDa) separates the truly dissolved pool, which matters most for the light lanthanides. Acidify after filtration. Report the shale-normalised pattern so that anthropogenic anomalies (gadolinium, lanthanum, samarium) are visible.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

No human health guideline anywhere read; no acute or chronic human toxicity data for this element were read this session.

Bioaccumulation

Field bioconcentration factors in Rhine estuary amphipods fall from the light to the heavy lanthanides (La 28,840 to Lu 4,786; Pr 38,905); carp muscle takes up little (BCF 0.22 to 1.10 for Ce, La, Nd, Pr, Sm) while internal organs reach 634 to 978. Anthropogenic La and Sm in the Rhine are taken into mussel shells, anthropogenic Gd is not, so speciation decides bioavailability.

Ecotoxicity

RIVM (2000) environmental risk limits, not discharge limits: MPC 9.1 µg/L in fresh surface water and 1.00 µg/L in salt water (negligible concentrations 0.17 and 0.09 µg/L), derived as lowest LC₅₀ divided by 1000 plus a background set at the detection limit (0.08 µg/L). Acute data behind it: Daphnia magna 48 h EC₅₀ 9.0 mg/L, zebrafish 96 h LC₅₀ 25 mg/L; chronic: no chronic test listed. Field bioconcentration factor in amphipods (porewater basis) 38,905.

Flags

• The RIVM MPCs, where they exist, are environmental risk limits from a 2000 report (LC₅₀ divided by 1000, background at the detection limit), not permit limits.
• The seawater range is one western Pacific station.

Gaps

• WHO GDWQ and EU DWD 2020/2184 Annex I were not read this session (eur-lex returned the articles without the annexes), so their absence of a rare earth parameter is not asserted here; only the US EPA table was read.
• No GCC discharge standard was read; no GCC row is written.
• World average river water concentrations (Gaillardet et al. 2003) were not reachable; river figures come from the Rhine and Dutch waters only.
• No source read this session gives municipal wastewater concentrations for this element beyond the gadolinium literature.
• ICP-MS oxide interference corrections between lanthanides are standard practice but no method text describing them was read, so none is written.
• No stability constants or solubility products for this element's carbonate, phosphate or hydroxide were read; the speciation is qualitative, taken from the EPA groundwater study and the RIVM report.
• No praseodymium specific river or groundwater concentration was read; the Rhine estuary survey reports Pr only in sediment (30 ppm near the outfall).

Sources

Identity

Name and symbol

Praseodymium, Pr

Atomic number

59 protons

Position

no group (f-block) · period 6 · f-block · lanthanide

CAS number

7440-10-0

Atomic structure

Atomic mass

140.907 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f³
[Xe] 6s²⁴f³

Electrons per shell

2, 8, 18, 21, 8, 2

Valence electrons

5 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,204 K (930.85 °C)

Boiling point

3,793 K (3,519.85 °C)

Density

6.77 g/cm3

Appearance

grayish white

Thermal conductivity

12.5 W/(m·K)

Electrical resistivity

poly: 0.700 µΩ·m

Electrical conductivity

1.43 MS/m

Crystal structure

double hexagonal close packed

Molar heat capacity

27.2 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.13 (Pauling Scale)

Ionisation energy

5.464 eV
1st 527, 2nd 1,020, 3rd 2,086 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 203, covalent 203, van der Waals 239 pm

Ionic radius

Pr³⁺ 99; Pr⁴⁺ 85 pm

Reactivity

A soft rare-earth metal somewhat more resistant to air than lanthanum, cerium or neodymium, but still reactive: it develops a flaking green oxide coating in air, corrodes completely within about a year, and is stored under mineral oil or sealed in plastic.

with water

Reacts slowly with cold water and quickly with hot water to praseodymium(III) hydroxide and hydrogen: $\ce{2 Pr + 6 H2O -> 2 Pr(OH)3 + 3 H2}$.

with oxygen, air

Tarnishes slowly in air to a green oxide layer that spalls off; heated to about 150 C it burns to the mixed-valence oxide: $\ce{12 Pr + 11 O2 -> 2 Pr6O11}$.

with acids

Dissolves readily in dilute acids to yellow-green Pr3+ solutions and hydrogen: $\ce{2 Pr + 3 H2SO4 -> 2 Pr^3+ + 3 SO4^2- + 3 H2}$.

with halogens

Reacts with all the stable halogens to the trihalides, mostly green: $\ce{2 Pr + 3 Cl2 -> 2 PrCl3}$.

Typical compounds

• Pr₆O₁₁ praseodymium(III,IV) oxide black oxide from burning the metal; didymium glass
• Pr₂O₃ praseodymium(III) oxide from reducing Pr6O11 with hydrogen
• PrCl₃ praseodymium(III) chloride green trihalide
• PrF₃ praseodymium(III) fluoride green fluoride; reduced for the metal
• PrF₄ praseodymium(IV) fluoride rare tetravalent compound
• Pr(NO₃)₃ praseodymium(III) nitrate soluble salt; yellow glass and glaze colourant

Occurrence, production and use

Crustal abundance

9.2 milligrams per kilogram

Oceanic abundance

6.4×10-7 milligrams per liter

Occurrence and sources

The element occurs along with other rare-earth elements in a variety of minerals. Monazite and bastnasite are the two principal commercial sources of the rare-earth metals. It was prepared in relatively pure form in 1931.

• monazite (phosphate) and bastnaesite (fluorocarbonate) the two principal sources, with the other lanthanides; heavy-mineral sands and the Mountain Pass carbonatite deposit
• rare earth metals (mainly lanthanum, cerium, praseodymium, neodymium) carried with phosphate rock into wet-process phosphoric acid and discharged with phosphogypsum reported emission to water on disposal of phosphogypsum: 2,200 g and 360 g of rare earth metals per tonne P2O5 at two plants (HDH-1 and HDH-2 processes, 1996/97 data, plants since closed because of the discharge to sea); Table 5.8 of the LVIC-AAF BREF, PDF p253, printed p225; the ledger's chemical chapter holds this under the phosphoric-acid hub

Extraction, production

Ion exchange and solvent extraction from monazite and bastnaesite

praseodymium is usually sold with neodymium as a mixed neodymium-praseodymium oxide

Reduction of anhydrous praseodymium chloride with calcium to the metal

$\ce{2 PrCl3 + 3 Ca -> 2 Pr + 3 CaCl2}$

no balanced equation printed by the source

Uses

Praseodymium's primary use is as an alloying agent with magnesium to create high-strength metals that are used in aircraft engines. Praseodymium also makes up about 5% of Misch metal, a material that is used to make flints for lighters. Praseodymium forms the core of carbon arc lights which are used in the motion picture industry for studio lighting and projector lights. Praseodymium is added to fiber optic cables as a doping agent where it is used as a signal amplifier. Praseodymium salts are used to give glasses and enamels a yellow color. Praseodymium is also a component of didymium glass, which is used to make certain types of welder's and glass blower's goggles.

Misch metal, used in making cigarette lighters, contains about 5% praseodymium metal. The rare-earth oxides, including Pr2O3 are among the most refractory substances known. Along with other rare earths, it is widely used as a core material for carbon arcs used by the motion picture industry for studio lighting and projection. Salts of praseodymium are used to color glasses and enamels; when mixed with certain other materials, praseodymium produces an intense and unusually clean yellow color in glass. Didymium glass, of which praseodymium is a component, is a colorant for welders goggles.

• Permanent magnets: praseodymium in alloys for permanent magnets, in practice as neodymium-praseodymium oxide feeding neodymium-iron-boron magnets neodymium-praseodymium oxide and neodymium-iron-boron magnet block are listed potential United States stockpile acquisitions for fiscal years 2024 and 2025 (usgs-mcs2025, PDF p149, printed p145)
• Aerospace alloys: high-strength praseodymium-magnesium alloy for aircraft engines
• Glass, ceramics and pigments: yellow colourant for glass, enamel and glazes; didymium glass (with neodymium) for welding and glassmaking goggles
• Steel and metallurgy: about 5 percent of mischmetal for lighter flints; carbon arc electrodes for studio lighting and projection
• Rare earth mining and separation: mined as bastnaesite (a rare-earth fluorocarbonate) at Mountain Pass, California, and as monazite (a phosphate) in heavy-mineral-sand concentrates; separated from the other rare earths by ion exchange and solvent extraction world mine production of rare earths 390,000 t of rare-earth-oxide equivalent in 2024 (USGS estimate), of which China 270,000 t (production quota), the United States 45,000 t, Burma 31,000 t, Australia, Nigeria and Thailand 13,000 t each; reserves over 90 million t, China 44 million t, Brazil 21 million t (usgs-mcs2025, PDF p149, printed p145)

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
• H251 Self-heating; may catch fire Self-heating substances and mixtures
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Carl Auer von Welsbach

Discovered

1885

First isolated

not in sources

Named by

not in sources

Origin of the name

after Greek πρασιος, "leek green" (for the color of its salts) and δίδυμος, "twin" (of lanthanum)

Praseodymium is soft, silvery, malleable, and ductile. It is somewhat more resistant to corrosion in air than europium, lanthanum, cerium, or neodymium, but it does develop a green oxide coating that falls off when exposed to air. As with other rare-earth metals, it should be kept under a light mineral oil or sealed in plastic.