The purpose of the paraformaldehyde is a reaction with the (bi)sulphite in what is known as a "clock reaction" to increase the PH. There is anecdotal evidence on the web that suggests acetone may be substituted at a ratio of 2.2ml per gramme of Paraformaldehyde - Most lith formulas call for 7.5g which equates to 16.5ml Acetone per litre, so on the face of it, Tim Rudman is out by a long way...
Originally Posted by WHof
Just carried out a little experiment with a solution containing 30g Sodium Sulphite, 2.5g Sodium Metabisulphite in 1000ml water. The PH was measured at 7.4, a fairly neutral value. Dividing the solution in to two equal parts, one had ~3.75g of Paraformaldehyde added. The other half had increasing amounts of Acetone added, starting with ~8ml.
On adding the Paraformaldehyde, the PH climbed to 11.5 within seconds indicating a reaction had taken place.
With 8ml Acetone, the PH climbed to 9.1 within seconds and further additions of Acetone saw little increase. With ~40ml, the PH only went up to 9.5. Some 30 minutes later, this had risen to ~PH10.
Lith developers are typically around PH11, so on the face of it, whilst the Acetone substition looks plausible, additional chemicals would need to be added to get the PH up - This would typically be Sodium Hydroxide as used in the Ansco 70 formula. The figure of 82.5ml is certainly wrong if the purpose is to raise the PH over 11. If a PH of 9 is the target, then around 16ml (based on 7.5g Paraformaldehyde per litre) is sufficient.
If you have Paraformaldehyde (or a Formalin solution) to hand, I would suggest using it and thus avoid any need to increase the PH with Sodium Hydroxide.
Note: I am not a chemist and can not comment on "clock reactions", only report on my (limited) observations using cheap Chinese scales & PH meter.
Last edited by paul_c5x4; 01-20-2013 at 09:18 AM. Click to view previous post history.