Fixer and Water

[wfe]

During print washing does the fixer float on the water or sink to the bottom?

 

[Anon Ymous]

None of the above, but diffused in the wash water? I mean really, when making your fixer working solution, does the fixer sink to the bottom, or float at the surface?

 

[Mainecoonmaniac]

APUGer straightened me out about fixer being heaver than water. It's a myth I lived with for over 20 years. I wasn't as crushed as when I learned about the Easter Bunny and Santa Claus

 

[rmolson]

He is probably confused by the thinking that in washing a hypo laden print the hypo might have a greater specific gravity and sink to the bottom of the print washer. A few print washers were designed on this idea . However the theory didn’t work The hypo is removed by diffusion not gravity.

 

[RalphLambrecht]

During print washing does the fixer float on the water or sink to the bottom?

What the others said, but if you're interested in a bit more detail:

The process of print washing is a combination of displacement and diffusion. Just prior to the wash, a relatively large amount of excess fixer is gently clinging to the print through surface adhesion. An initial, brief but rapid, rinse in water quickly displaces this excess fixer, simply washing it off the surface. However, there is still plenty of fixer left in the print, and this is a bit harder to get rid of. It has been deeply absorbed by the emulsion and saturates the print fibers. The remaining fixer can only be removed by the process of diffusion.

As long as there is a difference in fixer concentration between the print and the wash water, fixer will diffuse from the print into the water. This gradually reduces the fixer concentration in the print and increases it in the wash water. Diffusion continues until both are of the same concentration and equilibrium is reached, at which point no further diffusion takes place.

Replacing the saturated wash water entirely with fresh water repeats the process, and a new equilibrium at a lower residual fixer level is obtained. However, diffusion is an exponential process that decreases geometrically with time. This means that the rate of diffusion slows down rapidly towards the equilibrium. Print washing is quicker if the wash water is not entirely replaced in certain intervals, but slowly displaced with a constant flow of fresh water across the print surfaces, keeping the concentration difference, and therefore the rate of diffusion, at a maximum during the entire wash.

 

[ic-racer]

If you mix some coloring with fixer concentrate and pour it into some water you can see how it heads right to the bottom like a lead weight.

However, to make valued decisions about how to fix prints, you need to know the "rest of the story" which Ralph has clearly presented.

 

[Rich Ullsmith]

That seems to go against anything I learned in chem concerning solubility and diffusion. Are you sure about this?

 

[RalphLambrecht]

That seems to go against anything I learned in chem concerning solubility and diffusion. Are you sure about this?

Rich

Who are you asking?

 

[fotch]

If you mix some coloring with fixer concentrate and pour it into some water you can see how it heads right to the bottom like a lead weight.

However, to make valued decisions about how to fix prints, you need to know the "rest of the story" which Ralph has clearly presented.

Could the dye added weight make a difference? Not disagreeing, just wondering if this would prove anything.

 

[wfe]

Thanks to all, just wanted to dispel the myth....

 

[Steve Smith]

I wasn't as crushed as when I learned about the Easter Bunny and Santa Claus

What's that you say about Santa?


Steve.

 

[Photo Engineer]

Ralph is exactly correct and this also describes the Ilford wash method of several successive washes vs continuously washing with fresh water. It is a very contentious subject and has generated a lot of heat in another thread on wash methods.

The equation is a simple calculus statement of dC/dt = wash conditions. Where dC = change of concentration of fixer in the photomaterial and dT = change in time. Mason gives a full treatment of this with graphs in this textbook and Ralph has reproduced it here in plain English for us all.

The Ilford method produces a stepped curve of hypo vs time which is matched by a smooth curve if you use running water. The stepped curve does not reach the lowest level in retained hypo and salts that the continuous method does, BUT - here is the source of contention:

1. If the level of retained salts tests ok, then it really is ok regardless of method.
2. The amount of effluent is identical in both cases.
3. The distribution of effluent is different.
4. The continuous wash method can use more water.

And, last but not least, any solution with higher density than water will sink in water. However, if agitated, it will mix, and if the tank is deep enough it will mix gradually while sinking due to outward diffusion. This is why you should agitate prints as they wash.

PE