Hi Arthur,
To reduce diammine-palladium(II)chloride (yellow palladium salt) to the metal, principally there exist several different methods, the most simple one being calcination at temperatures up to at least 800oC, using a porcelain dish or crucible, covered with a loosely sitting lid. Heating has to be done not too quickly, because decomposition begins at much lower temperatures (ca. 300oC), under emission of white fumes of HCl and NH4Cl. The according reaction is: 3 (NH3)2PdCl2 => 3 Pd(metal) + N2 + 4 NH4Cl + 2 HCl. Holding the decomposed mixture for about 2 hours at 800oC assures complete reaction to palladium-metal as a greenish-blueish-gray metal sponge.
A different method involves reduction with formic acid, which has to be done very carefully, the reaction being quite difficult to control. Formally reaction occurs according to the following equation: (NH3)2PdCl2 + HCOOH => Pd-metal + CO2 + 2 NH4Cl. The active reduction reagent is not formic acid (HCOOH) itself, but it's anion, HCOO(-), which is obtained by deprotonation of the acid, which can be achieved, but only in small quantities at the beginning, through the complexed ammonia-molecules in the yellow palladium complex-salt. The formiate anion formed slowly in the mixture is oxidized to CO2 and a second proton, H(+), which is neutralized by the second NH3-molecule contained in the yellow complex salt. The actual reduction reaction in a simplified form thus can be written as follows: Pd(II) + HCOOH + 2 NH3 => 2 NH4(+) + Pd-metal + CO2. Mixtures of yellow palladium salt and aqueous formic acid solution do not react at room temperature. Heating to at least 80oC, up to nearly boiling some times, is necessary to onset reaction, but once begun, reaction can become very violent, such, that even a quite large reaction vessel can become too small for containing the reacting mixture, resulting in a mess of spilled palladium-black, starting material and salt-solution. Therefor I propose to you the following procedure:
1.) Suspend the yellow palladium salt in sufficient water, to obtain a theoretical concentration of 20 to maximum 50 grams Pd in 1 liter. Heat the suspension to nearly boiling under constant stirring (mechanical stirrer or magnetic stirring bar).
2.) Under constant stirring and heating, add no more than ca. one tenth of the required amount of formic acid to the hot suspension. Wait until reaction has onset, clearly visible on sudden upfoaming and formation of palladium-black (stay patient, sometimes much time is needed).
3.) Only after subsiding of this first reaction, add the rest of formic acid in small portions, waiting after every addition on subsiding of the newly ongoing reaction. Usually an excess of formic acid over the stoichiometrically calculated amount has to be used, because formic acid, together with water vapour, is volatile, and secondly because a Pd-catalyzed side-reaction, producing hydrogen-gas and hydrogencarbonate uses up some formic acid (formiate). At the end of the total reaction, addition of more formic acid does not cause more upfoaming of the now totally black mixture.
4.) If the palladium black does not settle down quickly after having stopped the stirrer and heating, add a few milliliters of a strong mineral acid (HCl; H2SO4) until the mixture shows a clearly visible strong acidic reaction (pH-paper), stir and heat again and let settle again after a few minutes boiling. If necessary repeat the acid-treating. Usually a well coagulating Pd-black is obtained, suspended in a colourless, waterclear solution. To be sure of complete reaction, test the clear solution by addition of a few drops of DMG-solution.
5.) Work up by decantation/filtration, washing with plenty of water, drying and/or calcining.
Regards, freechemist
