pH changes with concentration of base

[swmcl]

Hi,

This should be a fairly easy one for our chemists ...

I'm using a concentrate of 200g sodium carbonate per litre of distilled water as my stock solution.

When I add 0.25ml of this stock to 300ml of distilled water I get a pH of 10.75. Please see the following results:

0.25ml --> 10.75
0.5ml --> 10.87
0.75ml --> 10.94
1ml --> 10.99
1.25ml --> 11.00
1.5ml --> 11.03
1.75ml --> 11.06
2.0ml --> 11.06
3.0ml --> 11.06

My pH meter is a Chinese meter that I am fairly sure is drifting with ambient temperature. The gain is very variable. I do have buffer solutions to calibrate the meter just before I do the testing.

The distilled water started with a pH of around 7.2 and it dropped to around 5.6 pr 5.7 or so as I stirred the pH tester probe in the water. This is expected as the water is dissolving CO2 into carbonic acid.

My question is whether these results are representative to the process. Do they look right ? Is the graph that would be created atypical of this test ?

My subsidiary question would be, is there a carbonate that would produce a lower pH at a given concentration that would be commonly used in photography ? For example, say at 1ml per 100ml I'm getting a pH of 11.06 here, what carbonate would given me a pH of say 10.3 ?? I'm assuming potassium carbonate would give an even higher pH than sodium carbonate and am thinking there might be another product again to give me a lower pH.

Cheers,

Steve

 

[Gerald C Koch]

Actually there is and it is commonly called sodium sesquicarbonate (trisodium hydrogen dicarbonate). It can be formed by adding equal molar quantities of sodium carbonate and sodium bicarbonate. A 1% solution has a pH = 9.9. You can also purchase it from www.chemistrystore.com/

 

[swmcl]

Well there you go !

 

[swmcl]

I think I should've said 'is there an alkali ...' instead of 'is there a carbonate ...'

 

[swmcl]

I think the sodium sesquicarbonate would give me a similar result in my tests as the 99% TEA that I have as it is a similar pH ?

Hey I found this site ... http://www.aqion.de/site/191

In it I see a product called magnesium hydroxide which seems to like sitting at a pH of 10.4 for various concentrations ...

I wonder ...

You must be giggling at all this Gerald.

 

[Gerald C Koch]

I think the sodium sesquicarbonate would give me a similar result in my tests as the 99% TEA that I have as it is a similar pH ?

Hey I found this site ... http://www.aqion.de/site/191

In it I see a product called magnesium hydroxide which seems to like sitting at a pH of 10.4 for various concentrations ...

I wonder ...

You must be giggling at all this Gerald.

You will note that the columns are in millemoles which is a low concentration. Magnesium hydroxide is milk of magnesia which is insoluble in water except in minute amounts. It would not be suitable for developers. It gives the same pH because only a very small amount dissolves regardless of how much is added to water.

In the same way calcium hydroxide is a strong base but its limited solubility makes it safe.

 

[Rudeofus]

One thing I find curious in the tables of this aquion site is, that they claim you can substitute NaOH and KOH on a mol per mol basis: the pH of various equimolar dilutions of KOH and NaOH matches to the final digit. Strangely, this does not really lineup with the different pKb of NaOH and KOH ...

 

[sfaber17]

One thing I find curious in the tables of this aquion site is, that they claim you can substitute NaOH and KOH on a mol per mol basis: the pH of various equimolar dilutions of KOH and NaOH matches to the final digit. Strangely, this does not really lineup with the different pKb of NaOH and KOH ...

They have such a low pKb that they are fully ionized for all practical purposes so you would expect that behavior like you would with strong acids like HCl or HNO3. The relation between pKa and pH shows up more when you have a buffer solution situation with equal concentrations of the acid and base ion forms.

 

[Xmas]

Use a boric acid borax buffer for film as many films cannot tolerate carbonate alkalis.

If you have to ask pH questions suggest you need to go to college instead.

 

[mrred]

+1. One of the reasons this site exists.

 

[eng1er]

If you have to ask pH questions suggest you need to go to college instead.

Sounds like you need to go to charm school.