Aqueous solutions and precipitates of praseodymium
Praseodymium, as most of the lanthanides, only exists in the +3 oxidation state or in the metallic state. Praseodymium ions are very pale green. The color can only be seen in very concentrated solutions. However, compared to the color of manganese (II) ions, the color of praseodymium (III) ions still is quite strong.
Oxidation state +3
Below follows a picture of an acidified solution of praseodymium (III) chloride, which is clear and pale green. The color is caused by the Pr3+(aq) ions, present in this solution. The concentration of the praseodymium ion is high, approximately 10% mass percents of praseodymium content.
The right picture shows a precipitate of praseodymium (III) hydroxide, made by adding some of the pale green liquid to an excess amount of dilute solution of NaOH. The picture nicely shows that a precipitate of praseodymium hydroxide is very light green, almost white. The green color, however, can nicely be seen in contrast to a white sheet of paper.
When the light green liquid is heated and evaporated to just a very small volume, then the color intensifies remarkably at a certain point of concentration. Apparently, the structure of the Pr3+(aq) ion changes at very high concentration. It might be that Pr becomes coordinated to less molecules of water or to chloride ions. The color intensifies and shifts towards yellow.
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