Accelerator (alkali) substitution problem

[claudiosz]

Chemistry is hard to learn, but so intresting.
Each substance have different behaviour in aqueous solutions. In tha case of Sodium Sulfite, the cationic part of the molecule -sodium- tends to form small quantities of sodium hidroxide, and tends to go up pH slightly (this is the hydrolysis mechanism), and 100 gr/litre and generate a pH 8 approx. solution. In chemistry we think the anionic part of a molecule as less active than the cationic part in water solution.
In Sodium Carbonate, hydrolysis is so strong: may be 1 gr/litre can grouth pH 10 to 11.5. Why? In chemistry we think Carbonate anion as a lot less active in water than the activity of sodium (alkaline hydrolysis so strong).
Here we have a problem to give you the right answer to the original question. In theoretical therms, we can do a few calculations to give the theoretical amounts of PURE sodium carbonate, may be a few milligrams/litre. But, because of the capacity of sodium carbonate to absorb water, you never know exactly how much sodium carbonate and how much water are you weighting. And, may be you don´t use analytical grade of reagents, and you don´t know about purity of your sodium carbonate... I suppose you are not weighting with a 0,0001 g scale and the purity of your Soda, may be is less than ultra pure reagent.
This is not the correct way to correct with some degree of precision the pH of an alkaline solution.
If you want to grow up pH slightly, may be boron compounds may be more accurate to weight and manipulate (Borax or Metaborate, or Boric Acid and Sodium or Potassium Hidroxide).

 

[gainer]

You may learn by photographic experiment that a good pH meter is more useful than a very expensive weight balance. The pH of an accurately measured weight of a substance in an acurately measured quantity of water depends upon the purity of the substance. Water of hydration of sodium carbonate can vary so that the weighing of a few grams to the milligram is not worth the trouble, but adjusting the pH of the solution by adding small amounts of water or concentrate alkali solution might be worthwhile. A pH meter or a test strip or an indicator solution will be very useful.

 

[John Wiegerink]

PH tester

Does anyone use or know of a cheap PH meter/tester that would be accurate enough for photo chemical mixing? JohnW:confused:

 

[hrst]

I'm now using the cheapest one from Ebay / Hong Kong. One like this:
http://cgi.ebay.com/Digital-pH-Mete...ultDomain_0?hash=item53dfd27373#ht_1467wt_940

It has a +/- 0.1 reading accuracy and temperature compensation. So far it seems to give quite good results. I mean, when I put it in 4.0 buffer solution and adjust the calibration screw so that it shows 4.0, and then put it in 10.0 buffer solution, it shows 10.0. But it's still unclear to me how long this will last.

In developers, pH usually plays quite a significant role. So for exact work, a cheap Chinese meter with +/- 0.1 precision is probably not enough. Much better than nothing, still.

 

[John Wiegerink]

hrst
That's kind of what I'm after in the way of a PH meter. Not as good as the lab units, but better than test paper and much faster. As far as lasting? Who cares at that price you can buy three or four and have some back-up when one pukes. For me it's also worth trying at that price. I don't indulge in alcohol anymore and used to spend that much at the local pub in one hour. I guess that justifies the purchase, just like it has many purchases before it and probably many after it. JohnW

 

[lensmagic]

If you want to grow up pH slightly, may be boron compounds may be more accurate to weight and manipulate (Borax or Metaborate, or Boric Acid and Sodium or Potassium Hidroxide).[/QUOTE]

I don't understand why one would use boric acid when wanting to make graduated increases in pH. I should add, though this may be apparent, that I have not made a study of chemistry.

 

[Ian Grant]

Boric acid is used alongside Borax as it creates a much more stable buffer, and controls pH better than just Borax on its own.

Ian

 

[eclarke]

Boric acid is used alongside Borax as it creates a much more stable buffer, and controls pH better than just Borax on its own.

Ian

Hi Ian, Can you give an example??..Thanks..Evan

 

[Ian Grant]

Hi Ian, Can you give an example??..Thanks..Evan

Here's three

D76d /W37 Fomadon P

Metol 2g
Sodium Sulphite 100g
Hyroquinone 5g
Borax 8g
Boric Acid 8g
Water to 1 litre


ID-68 (Micropheen)

Phenidone 0.13 g
Sodium Sulphite (anh) 85 g
Hydroquinone 8 g
Potassium Bromide 1 g
Borax 7 g
Boric Acid 2 g
Water to 1 litre

To use: FS, 1+1, 1+3


Axford-Kendall Phenidone-Hydroquinone Fine Grain Developer
a PQ variant of ID-11/D76 - Autophen - Photofinishing developer

Sodium sulphite (anhydrous) 100 g
Hydroquinone 5 g
Borax 3 g
Boric acid 3.5 g
Potassium bromide 1 g
Phenidone 0.2 g
Water to make 1 litre

The two Ilford developers are designed to be used Replenished, there was also a replenisher for D76d, it's under these conditions that good buffering is most critical. The Axford-Kendall developer ws designed for large scale photofinishing use and there were two different replenishers, one for bleed, the other topping up.

Ian

 

[lensmagic]

Boric acid is used alongside Borax as it creates a much more stable buffer, and controls pH better than just Borax on its own.

Ian

My ignorance is here compounded. And revealed to all.

1. What is being buffered? Is boric acid the buffering agent?

2. How does boric acid control pH better than just borax?

 

[Kirk Keyes]

See: http://en.wikipedia.org/wiki/PH_Buffer

Boric acid with borax is a buffer. You need both to make the buffer.

 

[lensmagic]

See: http://en.wikipedia.org/wiki/PH_Buffer

Thank you for providing the link to Wikipedia. If I understand the explanation in Wikipedia, adding the buffer (boric acid) will make the pH stable within a narrow range during the operation of the working solution of the developer. Is the stable pH necessarily an advantage to a film developer? What differences in effect might one expect to see in negatives produced using an unbuffered accelerator and a buffered accelerator??

 

[neelin]

I'd like to add one more question to lensmagic's. Is buffering primarily required for re-used developers and of much lower significance (to negligible) in one-shot developers?

 

[claudiosz]

We need to think something: reducing reactions of halogen halide substances are happening y molecular scale. Byproducts of reaction are going to the solution in millions of places in the contacting surface between film and developer. If for a developing agent the by products are alkaline, we have some time of increased alkalinity inside million of emulsion developing points; in contact with more fresh developing agent, we have million of increased activity points, and secons later this local alkalinity is diluted in the total mass of developer. At this point the buffer action may be important:as an example, when we have some borax in water, sodium ions are a lot more active than borate anions (because of this we have a salt with alkaline behaviour in water), and may be we have some borate without disociation. As alkaline byproducts of reaction are going from the film to the body of solution, some borate without disociation reacts with it and neutralize this additional alkalinity. Because of this we call buffer this kind of solutions.
In the other side, if byproducts of reaction are acidic, some disociated borate becomes without disociation, and the pH remains constant.
At the same time, air contains carbon dioxide who is dissolving in our developer solution by action of agitation, and sum some acidic environment. Again, the buffer works estabilizing this pH variation.
When some person design a developer, think hardly in the better environment to work, sometimes a developer works with a large alkalinity reservoir (as in D72, 40 gr/l of sodium carbonate) and some times the designer have a choice for a little buffer alkalinity as in D76 (2 gr/l).
The choice for a strong buffer as Borax 8 gr/l +boric acid 8 gr/l may be have a relationship with some kind of protection from carbon dioxide disolution, may be.

As you can see, english is not my easy language. Sorry...

 

[Anon Ymous]

Thank you for providing the link to Wikipedia. If I understand the explanation in Wikipedia, adding the buffer (boric acid) will make the pH stable within a narrow range during the operation of the working solution of the developer. Is the stable pH necessarily an advantage to a film developer? What differences in effect might one expect to see in negatives produced using an unbuffered accelerator and a buffered accelerator??

Yes, buffering is something good in developers. It provides a (more) stable pH in the developer and obviously gives more stable performance, with minimal fluctuations. D76 for example is a developer whose pH (in it's original formulation) wasn't very stable. The buffered D76d variant is much better in that respect. If you search the MSDS information provided by Kodak, you'll notice that the current formulation of D76 is closer to D76d, than the original D76. To be more specific, the current D76 has some boric anhydride, which is boric acid once the powder is solved. The original D76 had no boric acid in it.

 

[gainer]

By definition, an anhydride is an oxide which in water gives an acid OR base. The mechanism by which the anhydride stabilizes pH in D-76 may be somewhat different from that of a combination of borax and boric acid. OTOH, the MSDS is not always a good indicator of the active composition of a solution. For example, one might choose to mix NaOH and HCl in proportions that will result an a solution of NaCl in water. You could swim in the solution, but not in either of the original ingredients.

 

[lensmagic]

It looks like regardless of the weight of borax (in a solvent state) in a developer, if an equal weight of boric acid is added, one would have a nicely buffered developer solution whose alkalinity (pH) would rise and fall in tandem with the weight of the borax in the solution. Does this make sense?

 

[gainer]

The pH of borax solutions of varying concentration will be found in a table in the CRC handbook. A rather large change in concentration results in a rather small change in pH.

The simplest of widely used developers, D-23, is composed only of Metol and sodium sulfite in water.

If you concoct a developer using only rules of thumb, you may think you are proving those rules. Don't be afraid of rebelling against them. It may cost you some failures, but at least you will have learned that rules of thumb are not always optimum.