HOMEFAQsBOOKS JOBS: Help WantedSuggestions      you are here: Hotline/Forum => Letter 25859

---

What chemical is used to lower pH of copper pyrophosphate bath

+++

What chemicals can we use to lower the pH of the copper pyrophosphate plating bath? Is Polyphosphoric acid ok?

Steven Ang
Student - Singapore

---

+++

We don't use the term polyphosphoric. If it means pyrophosphoric, is O.K.

Guillermo Marrufo
Monterrey, NL, Mexico

---

---

---

November 4, 2011

Hi!

I would like to ask how to maintain pH on a PyroCopper Bath? Our pH trend is increasing everyday and right now we are using Sulfuric Acid to decrease its pH. Experts said that pH of this bath should be decreasing in the long run...But why is it that everyday pH still increasing on our side? What are the possible causes?

Our pH parameter is 8.2 - 9.2...usual pH everyday reach 9.3 to 9.5.

Thanks ahead.

Diana Fe Espinosa
Philippines

Diana Fe Espinosa
plating service employee - Philippines

---

First of two simultaneous responses -- November 10, 2011

Hi Diana,

pH increase could be due to hydrolysis of water at cathode instead of copper ions reduce to copper metal:

2e-
2H2O ------> H2 (hydrogen gas) + 2OH-

You may verify cathode efficiency by:

1. Observe if significant gassing around cathode;
2. Observe if bath solution copper concentration is climbing up (anode dissolution more than cathode plate out);
3. Perform weight gain test to compare actual copper deposit weight vs theoretical weight under Faraday Law.

Other than that, drag in of alkaline species may be a reason as well.

Regards,
David

David Shiu
- Singapore

---

Second of two simultaneous responses -- November 10, 2011

Hi,

In the early days of Copper Pyrophosphate plating, we used and recommended "Tetraphosphoric acid". It was made by mixing water and phosphorus pentoxide to give a polyphosphoric containing 4 P atoms - H6 P4 O13. In reality it contained a mixture of polyphosphates with an average P of 4. It worked very well and rapidly became pyrophosphate (P2) in the plating bath.
We soon abandoned it in favour of of the cheaper and readily available sulphuric acid. Sulphuric acid In moderate quantities sulphate has no adverse effects. Ultimately it begins to lower the maximum current density as it adds to viscosity of the bath.

Your rate of pH rise is too high and the question you should have asked how to prevent it. pH rises in pyro baths because of excessive anode area. There are several drawbacks, the need to add acid being one of them. Others are:
1) lowering of the optimum P207:Cu ratio
2) If your are using brightener, it will precipitate on the sludge which forms on the anodes at LCD.

Controlling anode area is the is key to running a copper pyro bath. The pH should slowly rise, say 0.2 - 0.3 units per week. There are also disadvantages with falling pH.

My advice is to get the anode area down to meet the above condition. When the anode current density is correct, the anodes should have a clean, pink or slightly dusty appearance. Anode CD too low gives a brown sludge and too high, a bright electropolished appearance.

Well, there's some advice - whether you want it or not!

Harry

Harry Parkes
- Birmingham, UK

---

November 10, 2011

Is the volume of your acid solution staying constant?

Scott Hope
- Chicago, Illinois, USA

---

December 28, 2011

I run a Copper Pyrophosphate Plating Bath. It's small, about 150 gallons. My additives (with each load) is a brightner and ammonium hydroxide. Both of these items have a higher pH than my bath. So with each load, my bath pH increases. However my DI water is a bit of a lower pH to my bath, and so this tends to even out. Once a week I usually add a solution of 10% Sulfuric Acid to 90% DI Water. This solution is generally (for my bath) made up in a 3L bottle and poured into our sump tank. This generally drops the pH from 8.9 (which is the highest I will allow it to go), to 8.6. I sometimes have to add 2 x 3L bottles, depending on my concentration amounts.

Hope this helps.

Dante Ellis
- Camarillo, California

---

December 31, 2011

TO LOWER pH USE POLYPHOSPHORIC ACID (THE TERM POLY PHOSPHORIC ACID REF. TO CONC. GRADES OF PHOSPHORIC ACID ABOVE 95%). TO RAISE pH USE 10% POTASSIUM HYDROXIDE.

Ajay Raina Punjab, India