1. I've synthesized the compound potassium trioxalatocobaltate(III) trihydrate and for it I have two reaction equations. One of them is:

    $$\ce{CoCO3 + 3K2C2O4.2H2O -> K2[Co(C2O4)3] + 2CO2}$$

    and the other:

    $$\ce{K2[Co(C2O4)3] + PbO2 -> K3[Co(C2O4)3].3H2O}$$

    I see that the lead dioxide oxidizes $\ce{Co^2+}$ to $\ce{Co^3+}$ in the second step but I can't figure out what happens in the first. Does cobalt(II) make a ligand complex with the oxalate group?

  2. One of the beginning steps was a also to mix oxalic acid and potassium oxalate. I assume (but I am not sure) that it was done to dissolve the potassium oxalate as other acids might have led to an unwanted reaction that would hinder the reaction following reaction with $\ce{CoCO3}$. I wanted to know that this was in fact the case.

  • 3
    $\begingroup$ Your first equation is not balanced. Your second equation is not correct? How do you explain $\ce{Pb^4+ -> K+}$ conversion? $\endgroup$ Aug 1, 2019 at 15:55
  • $\begingroup$ @MathewMahindaratne for the first reaction you could also write it as $\ce{Co^2+ + 3C2O4^2- -> [Co(C2O4)3]^4- }$. I took the information from bit.ly/2OxsNi1. The second one might be incorrect because of the number of potassium on the product it would have to be K4 meaning the oxidation of Co goes from 2+ to 3+. I'm not 100% sure either so that;s why I asked the question. $\endgroup$
    – bobsburger
    Aug 1, 2019 at 17:35
  • 1
    $\begingroup$ Is the iupac naming correct? Shouldn't it be tris(oxalato)? $\endgroup$
    – Mithun M R
    Apr 21, 2023 at 6:59

2 Answers 2


It appears that Sørensen was the first to publish the synthesis of trioxalatocobaltate(III) trihydrate [1]. A summary written in English (and that should answer both of your questions) including chemical reactions can be found in the very first volume of Inorganic Syntheses1 [2, p. 37]:



$$\ce{H2C2O4 + CoCO3 → CoC2O4 + H2O + CO2↑}$$ $$\ce{2 CoC2O4 + 4 K2C2O4 + PbO2 + 4 HC2H3O2 → 2 K3[Co(C2O4)3] + 2 KC2H3O2 + Pb(C2H3O2)2 + 2 H2O}$$

Twenty-three and eight-tenths grams (0.2 mol) of cobalt carbonate is dissolved in a solution of 25.2 g. of oxalic acid $(\ce{H2C2O4·2H2O})$ and 73.7 g. of potassium oxalate $(\ce{K2CrO4·H2O})$ in 500 cc. of hot water. When the solution has cooled to 40°C., while it is vigorously stirred, 23.9 g. of lead dioxide (see synthesis 16) is added slowly, followed by 25 ml. of glacial acetic acid added a drop at a time. The stirring is continued for an hour, during which time the color changes from red to deep green. After the unused lead dioxide is filtered out, the trioxalatocobaltiate is precipitated by the addition of 500 ml. of alcohol. The material appears as emerald-green needles, which are sensitive to both light and heat; The yield is 70 g. (71 per cent).


  1. Please disregard obsolete typography and notations used in this textbook such as full stops after unit symbols, as well as the lack of white space between the numerical value and a degree symbol (as seen in “40°C.”) and use of notations such as “cc.” (cubic centimeter).


  1. Sörensen, S. P. L. Kritische Präparatenstudien. III. Darstellung von Einigen Kobaltidverbindungen. Z. Anorg. Chem. 1896, 11 (1), 1–5. https://doi.org/10/fh47nq. (in German)
  2. Inorganic Syntheses; Booth, H. S., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1939; Vol. 1. ISBN:9780470131602, https://doi.org/10.1002/9780470132326.

In the first step, one makes an oxalate complex by decomposing the carbonate salt of cobalt (II) with oxalic acid. I am not sure how an oxalate salt will decompose the carbonate to carbon dioxide. Oxalate ion makes a complex. The second step is lead dioxide oxidation.

See this book chapter for proper synthesis equations of cobalt (III) oxalate complex, search PbO2 in the file.


  • 2
    $\begingroup$ OP mentioned using oxalic acid too. $\endgroup$
    – Mithoron
    Aug 1, 2019 at 23:00

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