Dear Sir… At the beginning i would like to inform you about my
self. My name is Raiya AL-Esry. I’am working in the Assay Laboratory
as a chemist.I would like to have your helps in the following problem
with my solution. We prepare solution of Nitric Acid ( HNO3) 33% with
concentration of 24Bums( density = 1.2) whichused to paration
silver(Ag) from Gold(Au) as mention In ISO11426, Also we used water(
PH=7.4) but after 5minute -1 days we found the concentration of
solution( HNO3) higher than before around 34Bums( 46%) . SO why did
you thing this happen is it effect with the following :-
PH of water is higher?
Room Temperature?
At the end thanks a lot , Sorry to disturb you
Best Regards
Raiya AL-Esry.
Head of precious metal Laboratory
Greetings Raiya Al-Esry What is the intent of your undertakings?
Without understanding this, how can I be of avail? As you have
inspired my curiosity, I hope you do not mind my asking about your
lab - where is it located and what is it that you endeavour to do
there? Very Respectfully, JM Kekahbah
We prepare solution of Nitric Acid ( HNO3) 33% with concentration
of 24Bums( density = 1.2) whichused to paration silver(Ag) from
Gold(Au) as mention In ISO11426, Also we used water( PH=7.4) but
after 5minute -1 days we found the concentration of solution( HNO3)
higher than before around 34Bums( 46%)
Well, let’s be clear, nitric acid doesn’t form spontaneously out of
the air, so on the basis of the little you’ve given, you
have not got more HNO3 there than you started with.
So, what’s going on? I can think of two possibilities:
-
Maybe when you measured it initially the liquid was hot, and the
next time it was cold. The change in temperature would lead to an
increase in concentration, but a smaller volume. However, this
doesn’t sound very plausible, as it wouldn’t give the size of change
that you mention.
-
Hate to say this, but, maybe an error in your measurements? How
did you determine the concentrations? A variation in temperature
could well give large errors if you are measuring density as a means
of estimating concentration. Also note that if you use a pH meter
then these too are very sensitive to temperature, as well as needing
careful calibration.
Pure water (a rather rare comodity chemically) has a pH (note, lower
case “p”) of 7.0. Water with a pH of 7.4 is slightly alkaline, not
acid… However, a few decimal points worth of variation either
side of neutral isn’t going to make any difference at all in the
context of 33percent nitric acid.
It’s an intriguing problem that we won’t be able to solve without
more I suspect.
Now, I have to say this carefully, as I don’t want to cause offence.
If I did then I wouldn’t have taken the trouble to write the
comments above, OK? But, I am a bit puzzled by all this. You asked
some questions here almost exactly a year ago, which I did my best to
answer, both on and off list. But your further offlist questions and
indeed you initial enquiry, and this one too, don’t sound exactly
like something that would be puzzling a “Head of precious metal
Laboratory”. I wonder also about that “hotmail” address…
Anyway, I’ve given you my thoughts on your current problem. No
doubt there are other approaches to it that I haven’t thought of, and
others here will come up with those. No question is too stupid to
ask, provided it’s a genuine enquiry. But I still wonder who you
are…
Kevin (NW England, UK)
We prepare solution of Nitric Acid ( HNO3) 33% with concentration
of 24Bums( density = 1.2) whichused to paration silver(Ag) from
Gold(Au) as mention In ISO11426, Also we used water( PH=7.4) but
after 5minute -1 days we found the concentration of solution( HNO3)
higher than before around 34Bums( 46%) .
The only mechanism which would cause the solution to concentrate
like this over a short period of time would be the evaporation of
water by heat. I suspect that this may be due to the way you are
preparing the solution and that you may be adding the concentrated
acid to the water (you are doing it this way round aren’t you??) too
quickly. If you just dump the acid into the water quickly, this will
cause the temperature of the water to rise, sometimes even to boiling
point, and so the water will evaporate as it cools down. Try adding
the acid very slowly and put it into a stoppered bottle until you
need it.
Best wishes,
Ian
Ian W. Wright
Sheffield, UK
Reading between the lines of your post of April 8, I gather you are
quartering a gold alloy. That is, you are either adding sufficient
silver or copper to the alloy to reduce the quality to 6K or less,
or the sample is already 6K or less. At that quality a 1:1 by
volume of conc. nitric acid : distilled water solution will react
with the alloy dissolving the Ag, Cu, Zn, etc. and leave a very fine
brown powder which is fairly pure gold. Concentrated reagent or CP
grade nitric acid is 68 - 70% (g/100 cc) acid with a density of 1.42
g/cc. Specific Gravity and density, for all practical purposes, are
the same to the second decimal point. Diluting with an equal volume
of water would give 34 - 35% acid with a Sp. Gr. at 60 degrees F of
1.21 which is equivalent to 24 degrees Baume (your “24 Bums”).
Please note that Baume should have an accent mark over the e.
If you checked the acid with the hydrometer immediately after
diluting/mixing and obtained a reading of 24 degrees Baume, and then
checked again five minutes later and got 34 degrees, then you have
an inexplicable situation. A case of black magic or the first
reading was in error. If, however, you took the gravity reading
about five minutes after making the solution and got 34 degrees
instead of the expected 24 degrees, then the only answer is that 50
ml of water per 100 ml of conc. acid was added instead of 100 ml of
water. The only way to increase the acid concentration by 16% is to
remove water by distillation. There is the tacit assumption in all
this that the conc. acid used was at full strength and was of normal
purity.
The 7.4 pH of the water used indicates distilled water was not used,
or the distilled water was contaminated, or the pH reading was not
accurate. If the pH of 7.4 is correct, the small amount of base
present would decrease the amount of acid, not increase it. It is
necessary to use distilled water in the process to eliminate
precipitation of AgCl which would contaminate the Au product. The
way to check the pH meter is to use standard reference buffer
solutions on a regular basis. This will assure you that the acidity
measurements are correct and that the glass electrode is functioning
properly. I don’t know what the ambient temperature is where you
work, but it would be advisable to check the temperature
compensation of the meter from time to time also.
If the situation you describe had happened to me, I would have
titrated a sample against standard base to confirm the hydrometer
reading. You would then know that insufficient water was added.
I wish you good fortune in your future endeavors.
Captain Blood
"Marlinespike Seamanship in Precious Metals"
@Alden_Glenda_Blood