Re: Why mix chemicals twice?


"David Nebenzahl" <nobody@xxxxxxxxxxxxxxx> wrote in message
news:1126250884.3363b522fc7c381cc0822c38b2df3e87@xxxxxxxxxxx
> On 9/8/2005 7:40 PM Francis A. Miniter spake thus:
>
>> David Nebenzahl wrote:
> >
>>> On 9/8/2005 6:43 AM UC spake thus:
>>>
>>>> David Nebenzahl wrote:
>>>
>>>>> Just another of those "wonder why that is?" things.
>>>>> Mixing some developer
>>>>> (D-76) today, it struck me: why am I being instructed
>>>>> to first mix the
>>>>> contents of the package with 800 ml of hot water, and
>>>>> then told to mix it with
>>>>> water to bring the total to 1 liter?
>>>>
>>>> So you have EXACTLY one litre. Otherwise, you have more
>>>> than one litre.
>>>
>>> OK, so as I suspected, it's a matter of ending up with a
>>> certain amount of solution, not a chemistry thing.
>>>
>>> So if one wanted to save a little time, one could figure
>>> out exactly how much water it takes to make 1L and mix
>>> the powder in this quantity, right?
>>
>> Then, of course, you could not rely on the manufacturer's
>> (or any one else's) time recommendations. You would have
>> to reestablish all your times. You may get some contrast
>> changes as well. It will be a different formula after
>> all, and some formulas will vary from recipe more than
>> others because of solubility issues.
>>
>> Remember those chemistry class experiments about the
>> differing amounts of salt and sugar that you can dissolve
>> in a specified quantity of water and how temperature
>> affects solubility.
>>
>> Do you know enough chemistry to compensate for all of
>> these factors?
>
> What? I think you misunderstood what I meant. All I
> proposed was mixing the stuff *all at once*, rather than
> in two stages as currently instructed to do. In other
> words, figure out how much water *total* is needed to make
> the 1L of solution (something a little less than 1L) and
> mix it once. The resulting solution would be exactly the
> same as if one had followed the instructions.
>
> As was pointed out, it's simply done this way so that you
> end up with 1L of solution without having to measure, say,
> 0.95L of water, which isn't convenient. (Kodak tells me to
> mix the powder in 800ml.)
>
>
Well, it really isn't two stages since the water is just
another ingredient. The idea, as has been pointed out by
others, is to achieve a known concentration in the mixed
developer. Generally (but not always) developers, and other
solutions, are mixed using moderately hot water to speed up
the solution. One of the factors in the amount is the
expansion of the water when hot. Probably not a big deal.
Another factor is the amount the various chemicals are going
to affect the volume. Should be predictable but may not be
due to variations. Another reason is to cool off the mixture
once everything is in solution. Note the instructions
usually say to add cool water.
I don't see that there is any time saving involved since
you can mix the solutions in a vessel which is marked for
the final volume. For large volumes, for instance Xtol,
which comes in a five liter package, I mix in the specified
amount of water and then measure it out in a one liter
graduate as I pour it into the final container. The last one
will not be full so I add water to that to bring it up to a
liter and that's that.
I do have a lot of older formulas and non-Kodak formulas
that give the volume of water first. They do not specify
what the final volume is to be.


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Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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