pH-meter

Dear all,

In order to further optimize the dithionite-cleaning method (http://www.strahlen.org/smf/index.php/topic,3087.msg38782.html#msg38782) it is important to have the right pH of the solution (I hypothesized.. ).
I am now playing with the pH of the solution to evaluate how it affects the cleaning process.
For that I ordered a simple but decent pH-meter with an accuracy of 0.1 units. Unlike laboratory
applications, I felt that for this application an accuracy of 0.1 would do the job (http://www.hannainstruments.nl/product/ph-orp-ise/testers/HI-98108). What I did find important is that the device should have an automatic temperature compensation, to correct for fluctuating temperatures (pH changes with temperature).

Is anyone else using a digital pH-meter at home? Or did someone alter the pH of the "Waller-solution"?
What are your experiences?
Tim

Hi there,

a pH-meter is a quite difficult system. Calibration, aging of electrodes etc.

What about: http://de.vwr.com/app/catalog/Product?article_number=1.09557.0003
fine tuned indikatpr-paper? Resolution 0.2 just for cross-cheking?

I have to use PH-meter in the job...

BR

Frank

I tried to order something from VWR last week, but they did not want to deliver to me, and not even when I said I ordered from the Strahlen Foundation as a legal entity in the Netherlands with tax number ; they were scared we had something to do with terrorism the man told me on the phone!! So I told him to read our website and our philosophy. Then he told me that he was afraid we would do terrorist things (incredible...) with chemicals etc that they are delivering ; I told him I only needed a Y-stage/Labjack, not chemicals, and that I would pay in advance if that was their real fear... Then after a second phonecall one day later (!) he told me the law in the Netherlands did not allow him to deliver to me or to the Strahlen Foundation. What sounds like an excuse, they probably only want big clients. Arrogant assholes...
I think only professional labs can order stuff there, so just ignore that company if you're a normal human being like us....... They do not want us as clients. They seem to have bigger and more important things to do than sell things to us....

Hi there,

a pH-meter is a quite difficult system. Calibration, aging of electrodes etc.

What about: http://de.vwr.com/app/catalog/Product?article_number=1.09557.0003
fine tuned indikatpr-paper? Resolution 0.2 just for cross-cheking?

I have to use PH-meter in the job...

BR

Frank

I know Frank-1, I am using pH-meters (glas-AgCl-sytem) on my lab on a professionally basis daily and if there is one thing I don't like it is indicator-paper... . inaccurate stuff.

Frank-2, that was what I already was afraid of, since VWR only delivers material to labs. Nevertheless, a simple labjack should not be a problem anyway.

Hi,

this is a general problem in Germany. Here is a part of the terms of trade from another company:

Chemikalien werden an Privatpersonen, laut Chemikalien-Verbotsverordnung, nur gegen Vorlage eine Kopie vom Personalausweis und dem Nachweis der erforderlichen Sachkunde geliefert.

Sachkenntnis (durch berufsqualifizierenden Ausbildungsabschluss) besitzen:
- Apotheker ( Approbation )
- Apothekerassistent
- Pharmazieingenieur
- Pharmazeutisch-technischer Assistent
- Drogist
- Geprüfter Schädlingsbekämpfer
- Hochschulbesucher, der im Rahmen des Studiums eine entsprechende Lehrveranstaltung besucht und eine entsprechende Prüfung bestanden hat.

Bei Bedarf fordern wir von Ihnen zu bestimmten Chemikalien eine Endverbleibserklärung an.

T- und T+ Stoffe sind generell von der Abgabe an Privatpersonen ausgeschlossen.

Hinweise zu Überwachungspflichtigen Chemikalien:
Aufgrund gesetzlicher Vorgaben (Grundstoffüberwachungsgesetz vom 7.10.1994), dem Verhaltenskodex des Verbandes der Chemischen Industrie (VCI) sind wir verpflichtet, den Absatzweg der Produkte zu verfolgen, welche als Vorläufer für chemische Kampfstoffe oder Drogen dienen können.

Darunter entfallen:

A Chemikalien (Drogen-Vorprodukte), die für die unerlaubte Herstellung von Betäubungsmitteln missbraucht werden können, und nach dem Grundstoffüberwachungsgesetz (GÜG) bzw. Verordnung (EWG) Nr. 3677/90 sowie aufgrund des Verhaltenskodex des Verbandes der Chemischen Industrie (VCI) nationalen und internationalen Überwachungen unterliegen, sowie

B Chemikalien (Chemiewaffen-Vorprodukte), die für die unerlaubte Herstellung von Chemiewaffen (C-Waffen) missbraucht werden können, und - u.a. gestützt auf die Verordnung (EG) Nr. 3381/94 ("Dual use" Verordnung) - sowie aufgrund von Übereinkommen der chemischen Industrie (in Deutschland: Verband der Chemischen Industrie VCI) internationalen Überwachungen unterliegen.

Günter

I was ordering an Y-stage/Labjack, not chemicals........ for chemicals I can (partially) understand
(but let's get back to Tim's question ; sorry Tim !)

The Waller-solution is buffered.
Adding sour (old?) dithionite will produce some bubbles, but the pH of the solution stays >7.

Greetings
Milo

I know, weakly, by the NaHCO₃
But the question is whether changing the buffer-pH would influence the cleaning process.
Or increasing the buffer capacity would increase the half-life of the Waller solution? Hence --> more intense cleaning over time. I guess that too low pH will cause the formation of SO₂ from the dithionite and screw up the Waller solution, but more alkaline solutions might yield better results in terms of reduction of the Fe³⁺ to Fe²⁺??...
T

I am fiddling around with pH measurements the last 10 yrs. I do not know any really reliable system....but I doubt the 0.1 accuracy. To reach this you need a lot of calibration and pretty expensive pH meters/electrodes. And then it still depends at which range you are measuring. pH 7 is never the problem, pH 1.5 definitely is (production here is troubled by this). We will switch to conductivity. If your solution just contains salt, deionized water and some acid, conductivity is a better solution. At least you can drop the electrode and step on it....

I would not use pH paper with dithionite...it might just bleach your paper without indicating something.

Cheers,
Stefan

I know Frank-1, I am using pH-meters (glas-AgCl-sytem) on my lab on a professionally basis daily and if there is one thing I don't like it is indicator-paper... . inaccurate stuff.

Hi Tim,

I have to work sometimes in ultra-ion pure water This makes a paper sometimes better than a pH-meter
The other sometimes is pH>10 or other ugly stuff.

Frank

Hi Tim & everybody else interested,

the recipe (Waller Solution)  you are referring to is fine. I am not sure to understand what you are aiming at when you want to control/change the pH of the dithionite solution. A buffered aqueous solution with a pH-value equal to or above 9 is mandatory to avoid acid-induced decomposition of sodium dithionite. This can be a problem e.g. with decaying sulphides which may contain appreciable amounts of acidic impurities.

Sodium dithionite dissolved in water will react spontaneously with dissolved oxygen (and is therefore lost for reducing Fe3+) - that' s why containers used for treatment of minerals  with Waller solution should have  a tightly closing lid (best are plastic buckets with a plastic lid - these are not a problem, when pressure builds up inside).

Hence, in order to increase the efficiency of the cleaning process, you should degass the water being used and carry out the whole process in protective atmosphere (i.e. in nitrogen or Ar atmosphere) - this is the only feasible approach to avoid loosing dithionite as consequence of undesired side-reactions.

Forget about the pH-meter (too expensive and too difiicult to handle if you have no preceding experience), forget about pH-paper (the indicator will react instantaneously with dithionite and make any reading useless) ... better spent time and money on repeating dithionite cleaning until you are satisfied with the result.

Best

Harald

 

... forget about pH-paper (the indicator will react instantaneously with dithionite and make any reading useless) ...

Yes, you might be right!

Frank