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Offline maxvortex

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30% Koh solution mixture
« on: December 02, 2012, 03:40:51 PM »
I hate to open new topic but this has nothing to do with the other questions so i will start new thread and maybe someone else then me would find this useful.

The problem:
After reading some posts and some patents from Edison on his battery design i came to conclusion that there are few KOH solution and not all of then are using 30% solution.

The one that i need should use 1.32 g / mL of 85% or higher KOH powder.
So this should be 132g /dcl or 1320g / L. Is this correct ? If it is then this is not 30% solution or I'm missing something.

If i would like to make 30% solution then the powder value would be much greater then this. Using some basic formula - 0.85*100/30 = 2.8g / mL --> 2800g / L.

Which calculation should i use to get 30% solution ?
Max
 
 

Offline Babcock_Hall

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Re: 30% Koh solution mixture
« Reply #1 on: December 02, 2012, 04:39:29 PM »
I must say at the outset that I did not entirely follow your question, but I will make a few comments in the hope that they are helpful.  One is that commercial KOH pellets contain roughly 86-87% KOH and the rest is water (you probably know this already).  Two is that percentage methods of describing how to prepare a solution are often ambiguous and/or frustrating.  I would interpret a 30% KOH solution to mean that it was prepared by adding 30 grams of KOH pellets to a final volume of 100 mL of solution, unless I were told otherwise.  However, I can begin to appreciate a little bit better the problem you may be having here: my suggested method of preparing 30% KOH actually contains a lower concentration of KOH than a hypothetical preparation of 30% KOH with anhydrous KOH would.

Offline discodermolide

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Re: 30% Koh solution mixture
« Reply #2 on: December 02, 2012, 07:51:48 PM »
If you want a 30% KOH solution and are only using KOH at 85% purity then you must compensate for this by taking 15% more KOH!
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Offline maxvortex

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Re: 30% Koh solution mixture
« Reply #3 on: December 03, 2012, 04:11:05 AM »
In both versions is stated that this is 30% koh solution but how can 1,34g / ml be 30% KOH solution. This is the part that i dont understand.
 
Even if i have to add 15% more KOH pellets -- 1.32 *15% =  1,51 -- it is still not 30% KOH solution.

Max

Offline discodermolide

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Re: 30% Koh solution mixture
« Reply #4 on: December 03, 2012, 05:01:43 AM »
30g pure KOH in 100 mL water = 30% solution.
If the purity is 85% then you must take 34.5g and dissolve it in 100 mL water.
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Offline Borek

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Re: 30% Koh solution mixture
« Reply #5 on: December 03, 2012, 06:10:49 AM »
30g pure KOH in 100 mL water = 30% solution.

Actually 30/130, so 23%.

If you meant 100 ml of solution, not water, that's still wrong - that would be 24.4%, as density of the KOH solution containing 30 g of KOH in 100 ml of solution is 1.232 g/mL.

Quote
If the purity is 85% then you must take 34.5g and dissolve it in 100 mL water.

Then you will get 29.3 g of KOH in 100+(34.5-29.3) g of water, so 27.9% (assuming the impurity is water). If it is not water, the result would be slightly different, but still not 30%.

Density of the 30% KOH solution is 1.2905 g/mL. Such solution contains 38.7 g KOH in 100 mL.

But I doubt 30% needs to be that precise.
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Offline DrCMS

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Re: 30% Koh solution mixture
« Reply #6 on: December 03, 2012, 12:07:02 PM »
I guess the OP was actually looking at the density quoted for the 30% solution they were supposed to be using and not as they thought the recipe to produce it.

Offline curiouscat

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Re: 30% Koh solution mixture
« Reply #7 on: December 03, 2012, 02:37:39 PM »
When someone says "30 % solution"; to a chemist does that mean w/w or v/v or some such by convention?

Just curious.

Personally I prefer the w/w method every time.

Offline Borek

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Re: 30% Koh solution mixture
« Reply #8 on: December 03, 2012, 02:47:20 PM »
1.32 g/mL means 33% KOH solution. Close enough. But 1.30 is 31%, even closer.

All the time I mean w/w.
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Offline Babcock_Hall

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Re: 30% Koh solution mixture
« Reply #9 on: December 03, 2012, 03:54:32 PM »
curiouscat,

In Quantitative Analysis, 4th ed., 1958, Pierce, Haenisch, and Sawyer (page 101) list five physical methods of specifying the concentration of a solution:
1.   grams of solute per liter (or 100 mL) of solution
2.   grams of solute per liter (or 100 mL) of solvent
3.   grams of solute per mass of solution (often 100 or 1000 grams)
4.   grams of solute per unit mass of solvent
5.   Percentage methods.  The concentration of solutions is frequently stated in percentage, particularly when only an approximate value is indicated.  Unless further defined, the term might mean either the weight [mass] or volume of solute per 100 g or 100 mL of solution or of solvent, but by general usage the term has come to mean the weight [mass] of solid or the volume of liquid in 100 mL of the final solution.  Thus, a 10 percent salt solution contains 10 g salt per 100 mL solution, and a 10 per cent alcohol solution contains 10 mL liquid alcohol per 100 mL solution.

Beginning with this quote, I wrote up a handout to give students on this general subject, because it is often an area of confusion.

Offline maxvortex

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Re: 30% Koh solution mixture
« Reply #10 on: December 05, 2012, 03:10:10 PM »
In my situation KOH solution has very big influence.
If its to strong it will destroy the cells faster and capacity will be lower then expected.

I mixed the 30% version and charge the cell with 1.5V / 1A battery.
The situation is as follows:
--> charge time = 3 hours
--> current on the cell after I disconnected the charge = 800mA
-----------------------------------------

And now comes the real problem :-)
--> first 10 seconds current is 0.8A
--> next 10 seconds current drops to 0.5
--> in next 2 seconds current drops very fast to 250mA
--> this 250mA will stabilize and hold for cca 6 days.

My question would be:
How is it possible that current drops so fast. ?
Is this because air influence, hydrogen production ( its very low ) or something else ?
Maybe the charge current is to small so the chemical reaction can not start ?

I was thinking that the cell can not hold more then 250mA but this could not true because then it would not hold 0.8A first 10 seconds, and on the other side, cell are quite big so it should hold much more then 1A. 

Btw. Instead of nickel am using copper and copper oxide as my positive plate.

Max

Offline curiouscat

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Re: 30% Koh solution mixture
« Reply #11 on: December 05, 2012, 03:13:08 PM »
Polarization?

Passive layer formation?

Offline maxvortex

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Re: 30% Koh solution mixture
« Reply #12 on: December 07, 2012, 01:52:13 AM »
Quote
Polarization?
Passive layer formation?
- More info, please :-) ?

My opinion would be that the copper plate is partly destroyed in the charge process so there is no way that the cell can hold such current. Additional proof would be the impurity of koh solution after charging. The solution becomes brown but when i pull out the plates there are no visible damage on them. I will try to make fresh koh solution and new plates glued with epoxy. Maybe this could help. On the other side this could be some logical explanation.. or ?   

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