Dear David:
1. The developing reaction is straight forward;
Na(2)CO(3) + RCO-OH ----> NaHCO(3) + RCO-ONa
Where RCO-OH = photopolymer
Na(2)CO(3) = sodium carbonate
NaHCO(3) = sodium bicarbonate
RCO-ONa = "developed" photopolymer
Primary variables that affect the bicarbonate concentration are pH and
"total" carbonate concentration (the sum of carbonate and bicarbonate).
In developing a photopolymer with a 1% solution of sodium carbonate
(approximately 10 g/L sodium carbonate), the pH of the bath initially is
about 11.5. As you proceed to develop the pH drops due to the
consumption of carbonate (the weak base) and the formation of
bicarbonate (the conjugate weak acid). The proton (H+) comes from the
carboxylic acid of the photopolymer.
2. The effect of bicarbonate on developing is straightforward also, but
dependent upon whether the system is static (batch dump) or dynamic
(feed and bleed). In a static system, the carbonate concentration
decreases proportionally to the increase of bicarbonate, until the pH of
the solution is reduced to a value where the developing reaction
ceases. Typically the bath is dumped prior to this value, or at a pH of
10.2 - 10.4. You will also note the speed of the "developing" reaction
slows as
the pH decreases.
In a dynamic system, a carbonate/water mixture is added to maintain the
pH (and consequently the speed). The volume of the sump is maintained
by gravity overflow, or in some cases a level control and pump.
In both cases, there is a direct relationship between the "photopolymer
loading factor" (mil-square foot/gal) and concentration of bicarbonate;
both of which can be approximately calculated based upon the
relationship between pH and the carbonate/bicarbonate ratio.
3. While it is possible to convert bicarbonate to carbonate chemically
(employing sodium/potassion hydroxide is one method), it is not
practical or necessary to remove the bicarbonate, at least in its
entirety, from a working developer bath. It is only necessary to
maintain the pH at a fixed value and the concentration of "total"
carbonate at or near 10 g/L. Because the relationship between pH and the
carbonate/bicarbonate concentration is known, any one variable can be
mathematically determined if the other two are known. Additionally, the
photopolymer concentration can be inferred based upon the concentration
of bicarbonate. Hence if the pH is controlled, the photopolymer loading
factor is controlled. This is the basis of pH controlled developer feed
and bleed system.
4. Out of curiosity, if you would like to measure the estimated
bicarbonate concentration in a working developer bath, use the following
equation. (Note: you must know the sodium carbonate concentration of
the bath; either determined by bath make-up or titration):
{[(11.5 - pH)/0.05] x (0.00147) + 0.00351}(A/10)(B) = sodium bicarbonate
Where pH = measured pH of working bath
A = total sodium carbonate of system in g/L
B = F.W. of sodium carbonate (105.99)
If you have additional questions regarding feed/bleed developing,
bicarbonate relationship to "developed" photopolymer, or detailed
determination of the above equation, please call me directly.
Regards,
Ted Stern
612-479-6525
David Albin wrote:
>
> Gents
> I have been asked by my company to investigate a method of
> monitoring the level of bicarbonate in (sodium carbonate) developer
> solutions. I have a couple of questions I hope someone can answer:
>
> * How is the bicarbonate formed in the first place and what variables
> affect its' concentration?
> * What effects does this bicarbonate have on developmeny (if any).
> * Is there any way of subsequently removing the bicarbonate?
> * Finally, is anyone already measuring bicarbonate level and what
> system works best for you?
>
> Thanks for your time.
>
> David Albin
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