Unexpected behavior during preparation of copper hypophosphite
I am using copper hypophosphite for the activation of through holes in printed circuit boards. The circuit board is dipped into activator solution and upon heating, metallic copper is deposited in the holes, rendering them conductive and allowing me to electroplate copper to the desired thickness.
The solution I have prepared works for this purpose, but during its preparation I observe behavior that does not match what others report for the same process. I hope that somebody can suggest what I might be doing wrong.
Here is the recipe:
* 140 ml distilled H2O 30 gm CuSO4 pentahydrate MW = 249.677 [30/249.7 = 0.12 mol]
Stir until dissolved, then add:
* 22 gm Ca hypophosphite MW = 168.038 [22/<PHONE_NUMBER> = 0.13 mol]
The Calcium hypophosphite is a small excess, so all the CU will become CU.H2O4P2 +2 and all the SO4 will precipitate as Ca sulfate == gypsum
* Filter solution (Buchner vacuum filtration)
* Rinse with 100 ml distilled water and discard sediment. Add
* 40 ml NH4OH 25% (25% is a 13.4M solution) [.040 * 13.4 = 0.536 mol]
“Solution may thicken and develop white flakes, but as the reaction
proceeds the flakes will dissolve and the solution will become a deep blue color.”
And this is where my problem occurs: when I add 40 ml of 25% ammonia, I get a flash of dark blue followed by the development of a green slurry. Adding more ammonia creates a local area of dark blue solution, but mixing causes it to vanish into the green slush. Instead of 40 ml Ammonia, I've had to use 110 ml of Ammonia to dissolve the slurry and convert it from a green slush into a dark blue solution. The solution does work as expected, but I can't understand why my results are (repeatedly) different from those reported by others.
I have four sources for this preparation. One of them is the original Russian patent, one is a Russian electronics enthusiast who shared the technique with the hobbyist community, one is a Russian-speaking American electronics blogger, and one is an engineer at a high-end fabrication company. All of these sources agree on the molar ratios of the chemicals, and three of them have images showing that the addition of 40 ml ammonia will lead to the immediate formation of a dark blue solution. One of them has shared a video of the entire process, including all weighing and measuring, and it's clear that something very different is happening on my bench.
I searched the literature and found suggestions that the green precipitates are likely insoluble basic salts of copper (mostly hydroxides). 979
I found sources indicating that copper hydroxide dihydrate precipitates when stoichiometric amounts of ammonia are added, and dissolves in an excess of ammonia. However, 40 ml of 25% w/w ammonia should provide more than double the stoichiometric amount needed for the formation of copper hydroxide.
I've verified that all the sources agree on the molar ratios and that the reagents they are using are labeled similarly to mine. I've checked my weights and measurements carefully through multiple trials. I've verified my scale. The ammonium hydroxide is a new bottle that was full upon opening. The calcium hypophosphite is a reagent grade. My copper sulfate pentahydrate is a technical grade intended for home use, but I prepared a saturated solution at two temperatures and verified that the solubility at each temperature matches the published solubility for copper sulfate to the limits of my scale. Apparently there is some likelihood that industrially prepared copper sulfate could be contaminated with small amounts of iron or phosphorus, but I haven’t attempted to analyze for contaminants because I found no suggestion that they could double the requirement for ammonia. Although I rinse my glassware with distilled water, I did not dry it and re-wash, so it’s possible that some small amount of tap water could have consistently contaminated my process.
The temperature in my lab area is 25 – 26 C. I found no mention of temperature in the published process, nor did I find any suggestion in any online source that the solubility of metal hypophosphites in ammonia is strongly temperature dependent. 979