Sunday, March 27, 2011

Citric Acid as a replacement for KBr - a look at the pH variation

I will consider Reinholds CC-H and CC-L, since these are the best documented and at separate ends as far as concentration and pH goes.


Coffenol CC-H

Soda 54 gram
Vit C 16 gram
Inst. Coffe 40 gram
1 gram Kbr  (no influence on pH)

Mix in half the volume 
Water to a total volume of 1 litre.

we find the molarity of each ingredient :

Soda 54/106  =  0.50943M  ~= 0.51M
Vit C 16/176  =  0.090909M  ~=  0.09M
Coffe (40/10)/180 = 0.02222M ~= 0.02M

The pH calculates out at pH 10.81, KBr is not active in the ph-calculation.

CC-H (citric)

Coffenol CC-H
Soda 54 gram
Vit C 16 gram
Inst. Coffe 40 gram
2 gram Citric acid

Mix in half the volume 
Water to a total volume of 1 litre.

Soda 54/106  =  0.50943M             ~= 0.51M        pKa = 3.67
Vit C 16/176  =  0.090909M          ~= 0.09M         pKa1 = 4.10 pKa2 = 11.60
Coffe (40/10)/180 = 0.02222M      ~= 0.02M         pKa = 6.7
Citric acid 2/210.14 = +.00951M   ~= 0.001M       pKa1 = 3.09  pKa2 = 4.75 pKa3= 6.41

Citric acid is a weak acid, but is has 3 pKa-points, in other words looses 3 atoms, this could get very complex!

The pH for this mix calculates out to ph 10.70

Conclusion :
2 gram of citric acid only varies the pH in this with ca pH 0.10.

This is insignificant, any reduced activity in this developer from this small amount of citric acid is not dependent on pH, if there is a marked difference between KBr and Citric acid, it probably depends on other interactions between these organic substances, and since nnone can say anything specifica about the particular brand of coffe at hand, it reains a mystery, if its noticeable.

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Coffenol CC-L  (low)

Soda 16 gram
Vit C 10 gram
Inst. Coffe 40 gram
1 gram Kbr  (no influence on pH)

Mix in half the volume
Water to a total volume of 1 litre.

we find the molarity of each ingredient :
Soda 16/106  =  0.1509M            ~= 0.15M       pKa = 3.67
Vit C 10/176  =  0.05681M         ~=  0.057M     pKa1 = 4.10 pKa2 = 11.60
Coffe (40/10)/180 = 0.02222M   ~= 0.02M       pKa = 6.7

The ph calculates out to pH 10.25  KBr is not active in this ph-calculation

Coffenol CC-L  (low)

Soda 16 gram
Vit C 10 gram
Inst. Coffe 40 gram
2 gram Citric acid 


Mix in half the volume
Water to a total volume of 1 litre.
we find the molarity of each ingredient :

Soda 16/106  =  0.1509M            ~= 0.15M       pKa = 3.67
Vit C 10/176  =  0.05681M         ~=  0.057M     pKa1 = 4.10 pKa2 = 11.60
Coffe (40/10)/180 = 0.02222M   ~= 0.02M       pKa = 6.7
Citric acid 2/210.14 = +.00951M   ~= 0.001M       pKa1 = 3.09  pKa2 = 4.75 pKa3= 6.41

The ph calculates out to pH 9.90 

Citric acid adjusts the pH somewhat more in this mix, that means the buffer capacity of the Soda is less (not surprizing!)

Now since these are theoretical calculations, and we need a lot of datapoints in the actual mixes  (they will be forthcoming, from now on mixes will be measured by a calibrated ph_measure...) and IF the actual pH in the low-soda mix is lower before citric acid is added, we COULD reach a point where CC-L don't work.

Conclusion high- and low-soda mixes of caffenol :

Since the pH-number for ascorbate activity remains unknown, its my conlusion so far that it is better to avoid citric acid as a fog controllant i LOW-SODA mixes. In high-soda mixes its influence is insignificant.

2 comments:

  1. You can simplify the calculations somewhat by neutralizing the ascorbic acid. Either use sodium bicarbonate + ascorbic acid or use sodium ascorbate directly and the pH will be close enough to 7 to not make any significant impact.

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  2. No problem I can calculate any mix, with as many chemicals as anyone can imagine - if I can be convinced there is any rationale!

    The trouble is with your method:

    1 Its very hard in practice to balance sodium bicarbonate with ascorbic acid, because they do not prescisely how this works.....

    2 Its been a rather lengthy discussion around sodium ascorbate, its way, way harder to get this than ascorbic acid, and IF you try to get it, you can buy Metol and Phenidone instead, forget about ascorbates and be done!

    3 Im not convinced I'll spend time on aclculating a neutral or slightly acidic developer, so you have the ball!

    :-)

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