3
$\begingroup$

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:

  • Mix 140 ml distilled $\ce{H2O}$ and $30\ \pu{g}\ \ce{CuSO4.5H2O}\\ \pu{MW} = 249.677\ \pu{ g/mol}\ \dfrac{30\ \pu{g}}{249.7\ \pu{g/mol}} = 0.12\ \pu{mol}$
  • Stir until dissolved, then add $22\ \pu{g}\ \ce{Ca(H2PO2)2}\ \text{(Ca hypophosphite)}\\ \pu{MW} = 168.038\ \pu{g/mol}\ \dfrac{22\ \pu{g}}{168.04\ \pu{g/mol}} = 0.13\ \pu{mol}$
    The Calcium hypophosphite is a small excess, so all the $\ce{Cu^2+}$ will become $\ce{Cu(HPO2)2}$ and all the $\ce{SO4^2-}$ will precipitate as $\ce{CaSO4}$ Ca sulfate (gypsum)
  • Filter solution (Buchner vacuum filtration)
  • Rinse with 100 ml distilled water and discard sediment.
  • Add $40\ \pu{ml}\ 25\% \ \ce{NH4OH}\ (13.4)\ \pu{M}\\ 0.040\ \pu{L} \times 13.4\ \pu{M} = 0.536\ \pu{mol}$

“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). 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.

The Russian blogger says this:

After a lot of experiments, I came to this recipe, in 80 ml of water we dissolve 15 grams of copper sulfate (copper sulfate), add dry calcium hypophosphite with stirring, take it in excess, it's 11 grams, stir for 40 minutes, to form calcium sulphate precipitate, then filter out the precipitate, washed with 20 ml of water is obtained bluish liquid copper hypophosphite and calcium hypophosphite solution was then poured slowly with stirring ammonia water (ammonium hydroxide) to 25%, often it will be 16-25 ml, should get a homogeneous dark solution.

enter image description here

Here are some of the sources for this process:

An instructable on the topic

A video queued up to this point in the process

The improved patent

So what am I doing wrong???

$\endgroup$
  • 1
    $\begingroup$ How old is your ammonium hydroxide solution? $\endgroup$ – Ben Norris Oct 13 '17 at 10:42
  • $\begingroup$ "Recipe??". You are not making food. Are you? $\endgroup$ – Nilay Ghosh Oct 13 '17 at 12:33
  • $\begingroup$ As @BenNorris implies, 25% ammonium hydroxide outgasses so much you need to open it under a hood, so titrate a sample to check its strength. $\endgroup$ – DrMoishe Pippik Oct 15 '17 at 15:38
  • $\begingroup$ As indicated in my notes, the ammonium hydroxide was a new bottle just received from a reputable supplier, sealed and completely full when opened. I'll do a titration anyway, but the discrepancy wouldn't be explained unless it was down to 8%, which isn't entirely consistent with what I experienced when opening the bottle :) $\endgroup$ – Craig.Feied Oct 16 '17 at 10:33
  • $\begingroup$ @NilayGhosh Not food, exactly. But calcium hypophosphite is schedule 1 because it's used to "cook" meth, so I felt "recipe" was appropriate... $\endgroup$ – Craig.Feied Oct 16 '17 at 10:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.