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what waS your last photography related purchase?

koraks said:
It's possible I somehow forgot to account for this; I really don't know. Kind of hard to imagine since it's a pretty silly thing to overlook, but we all make mistakes.
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I did the math just to satisfy my curiosity...

Parodinal calls for 20g of sodium hyroxide which has a molar weight of 40g/mol

So that works out to be 0.5mol of NaOH.

Potassium hydroxide as a molar weight of 56.1g/mol.

0.5mol of KOH is thus 28.05g.

This assumes they work identically in the developer and all you have to do is achieve an equivalent reactive amount.

I have now exhausted my 50+ year old chemistry knowledge and am more than open to being refuted or corrected
 
koraks said:
It's possible I somehow forgot to account for this; I really don't know. Kind of hard to imagine since it's a pretty silly thing to overlook, but we all make mistakes.
Click to expand...

Not accounting for this difference would certainly account for a weak developer.

The pH of the final solution made with KOH instead of NaOH and not taking into account the difference in molar mass would be significantly lower than expected. Thus, any phenolic developing agent would be much less effective.


This is correct! You did well!!!!
 

Thanks! Even the elderly can still do the maffs

Now then, riddle me this Reaction Man. If we only added 20g of KOH instead of the prescribed 28.05g, is there a direct way to compute what effect it would have on the resulting developer pH assuming the correct amount would have yielded a pH of 10?

IOW, is there a way to determine whether this alone would have accounted for @koraks outcomes?
 

Okay, not a chemist but I did spend a while reading up on parodinal. The hydroxide isn't just responsible for the the pH. It's partially consumed converting the paracetamol to 4-aminophenol. The typical recipe that calls for 15g of paracetamol is approximately 0.1 mole of paracetamol. IIRC the stoichiometry of the hydrolysis of paracetamol consumes an equal molar amount of the hydroxide, and then the acetic acid resulting from that hydrolysis consumes an additional equivalent.

In the recipe as written you end up with a solution of 250ml of ~1.2m hydroxide which the internet tells me has a pH of around 14. If you used 20g of potassium hydroxide instead you end up with a solution that's around 0.6m hydroxide that's allegedly around 13.8. I'm not sure how big the difference ends up being post dilution
 

Yes, but it is not straightforward.

One would need to know all of the other components and the pKa of each titratable functional group. You could then derive a set of simultaneous equations that would need to be solved.

chuckroast said:
IOW, is there a way to determine whether this alone would have accounted for @koraks outcomes?
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Again, this would not be straightforward and the result of any calculation would be an approximation with an unknown uncertainty. Doing the experiment would yield a better answer.
 

That's kind of what I figured, but I do love solving simultaneous eqns... unless they're differentials.
 

Your pH calculations are certainly not accurate. They ignore all of the other components of the solution that contain titratable groups.

We can agree that the solution made with KOH will have a lower pH than that made with NaOH. Exactly how much is difficult to say... see my other post above.

Furthermore, titration curves are very steep when the pH of the solution is near (roughly +/-1 pH unit) the pKa of the important functional group. Thus, small changes in pH can have large effects on reactivity.
 
Good to see this discussion, I simply considered a 1:1 ratio for both.

I'll speak up if I have any problems with my mixes, but I don't believe that'll be the case.

Eli
 
fgorga said:
Your pH calculations are certainly not accurate. They ignore all of the other components of the solution that contain titratable groups.
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Almost certainly. Given that standard rodinal has a pH of 14 (according to the MSDS) I figured they're probably fairly close though.
 
thinkbrown said:
The hydroxide isn't just responsible for the the pH. It's partially consumed converting the paracetamol to 4-aminophenol.
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Exactly. If this was indeed the error I made (and I'm afraid I can't confirm or deny it; no notes and it's been a while), then I expect partial hydrolysis of the paracetamol to be a potentially bigger issue than the pH as such. But there will be an effect to both, undoubtedly.
 
A Leica R3 which lived in my house for exactly 1 hour...then I returned it:-((((
 
I just bought another Nikkormat FT2 to replace my favourite FT2 that developed fungus in the prism and has been consigned to the box of shame.

It came with (another) Nikkor 50mm f/2 - this time the Pre-AI "K" version. This means I now have all the versions of the 50mm f/2 - the Nikkor-H, the Nikkor-H.C, the K version, and the AI version. It's still one of my favourite 50s.
 
Tamron make some great lenses. The only autofocus lens I ever bought was a Tamron, and it takes some very good pictures at about half the cost of a first-party equivalent.
 
What About Bob said:
Just came in: Tamron 60mm f/2 macro lens.
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I've used Tamron lenses for many years, usually SP. I've just worn pour my EOS mount 28mm-75mm f2.8, it is still a great performer but hunts at the 70-75m end, and there is wear, it's wobbly at those FLs. However, to be fair it has had very heavy use particularly in its early years, it's about 22 years d and paid for itself in well under a month.

I decided to replace it with a 24mm-70mm f2.8 Canon lens, should last me quite a few years.

Ian
 
The 75mm for my VH-R. I already have the cam. This completes the set of lenses from 65mm to 180mm. It’s a fun camera and since it uses the same focusing mechanism for all the lenses, it’s actually lighter than my Fuji GL690 if you’re carrying three lenses.
 
Technically not a "purchase", but printed out a couple "zone system scales" to put my Soligor spot meters. Not a 100% match (since the scale was made for the Pentax digital spot meter), but its close/good enough.

And from my local library for 50 cents, 3d printed a "camera hotshoe extension" to put the Hugo Meyer Rangefinder ontop of my Kodak Monitor 620 so I dont have to always take the rangefinder in & out my pocket all the time when using it. Had to do a a minor mod (cut/round out the edges to it) for it to fit & used some tape on it to make sure it doesnt fall off when in use, but it works.
 

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This Plaubel Makina 111 with 10 and 7.3cm lenses, hoods, lens caps, filters, roll film, and sheet film back. No other bidders and feels like o got a decent buy on it. Condition looks really clean.
 
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