# Measuring alkali's purity

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#### ngian

##### Well-Known Member
Hello, I saw a few days ago a very interesting video that Kevin Dunn shows how someone can measure the purity of NaOH. You can watch this video here:

http://fyi101.com/the-balancing-act-part-ii-presented-by-dr-kevin-dunn/

I might try it sometime but I have a few questions:

1) Has anyone done the calculations with the molecular weight of KOH in order to get the formula and find the purity of KOH we have in hand with the CA titration?

2) Why/how phenol p works as an end point when the procedure stops at the time the solution turns pink (8,2PH) when the method starts with distilled water (PH 7), then we add ~10gr of anhydrous CA (causing the PH to drop less than 7), and then we add ~ 6.5gr of NaOH until PH 8,2 (just about when the solution turns pink).
I just don't see the logic with the endpoint of 8,2PH and why this is the PH spot that will show the valid purity of NaOH. If anyone can explain it to me better I would be grateful. DeeAnna has written something here but I didn't understand it.

3) I guess that Caustic Soda will grab moisture only at the top most granules inside the bottle/bag that is stored -the ones that are in contact with air-, rather than the ones that are hidden under them. So Caustic Soda would be partially contaminated with moisture / carbon dioxide. Is this thinking true?

Lastly the video has two interesting points:
a) Caustic soda that was exposed to air for about 6-7 hours (with ~50% humidity) (based on the program of 3rd Weekend Retreat – March 27th to 29th 2015) lost its purity from 98% to 88%, and b) the fact that NaOH can lost its purity by two ingredients: 1) moisture that NaOH only gains more weight and 2) carbon dioxide that transforms part of NaOH to soda ash.

Thank you in advance for any input.

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Hello, I saw a few days ago a very interesting video that Kevin Dunn shows how someone can measure the purity of NaOH. You can watch this video here:

http://fyi101.com/the-balancing-act-part-ii-presented-by-dr-kevin-dunn/

I might try it sometime but I have a few questions:

1) Has anyone done the calculations with the molecular weight of KOH in order to get the formula and find the purity of KOH we have in hand with the CA titration?

2) Why/how phenol p works as an end point when the procedure stops at the time the solution turns pink (8,2PH) when the method starts with distilled water (PH 7), then we add ~10gr of anhydrous CA (causing the PH to drop less than 7), and then we add ~ 6.5gr of NaOH until PH 8,2 (just about when the solution turns pink).
I just don't see the logic with the endpoint of 8,2PH and why this is the PH spot that will show the valid purity of NaOH. If anyone can explain it to me better I would be grateful. DeeAnna has written something here but I didn't understand it.

3) I guess that Caustic Soda will grab moisture only at the top most granules inside the bottle/bag that is stored -the ones that are in contact with air-, rather than the ones that are hidden under them. So Caustic Soda would be partially contaminated with moisture / carbon dioxide. Is this thinking true?

Lastly the video has two interesting points:
a) Caustic soda that was exposed to air for about 6-7 hours (with ~50% humidity) (based on the program of 3rd Weekend Retreat – March 27th to 29th 2015) lost its purity from 98% to 88%, and b) the fact that NaOH can lost its purity by two ingredients: 1) moisture that NaOH only gains more weight and 2) carbon dioxide that transforms part of NaOH to soda ash.

Thank you in advance for any input.

In this technique, the numeric pH value drops gradually until you get near the endpoint, at which point it drops dramatically as you add more acid. The difference between 8.2 and 7 might just be just a drop. That makes phenolphthalein a good indicator.

Ideally this titration is done with hydrochloric acid. With a strong acid and a strong base, the pH at the equivalence point will be very near 7. With citric acid it will be a little higher but the indicator still works well (and the procedure is safer).

If I recall, the example of testing KOH is in Scientific Soapmaking.

Thank you for your answer TOMH, and after searching at Dunn's book he describes how to titrate a 5‰ KOH solution, and he is doing it vice versa. He first inserts KOH solution turning the water pink because of phenolphthalein and titrates with 5‰ citric acid until the solution is colorless.

It seems not very difficult, but the base and the acid must be in an aqueous solution and not just right out of their vessel in granules form.

Watching also the video, I can understand it better, and I hope someone can convert the formula in order to calculate KOH's purity with the video's method above.

Well after trying Kevin's Dunn method for the alkali's purity check, I have to say that once someone understands the whole procedure and try it a few times, then it is very easy and can be done in a sort time.

I want to thank DeeAnna very much for the nice pdf she created at her site. I have edited her text for my own preferences and commodity and I have written something like this:

After weighting 10.0 to 10.5 grams of anhydrous citric acid and mixing it in the water we add at least:

NaOH 6.2 to 6.5 gr ( for 100% purity) [0,1gr = 1,6%]

KOH 8.7 to 9.2 gr (for 100% purity) - 9.7 to 10.2 gr (for 90% purity) - 10.3 to 10.8 gr (for 85% purity) [0,1gr = 1,15%]
where this means that if we have an alkali that the manufacturer states it is 85% pure (KOH), if we add 10.5gr of CA we should at least add 10,6-10,7gr of KOH to be close to the specific purity target (10.8gr for 85%). Because if we add 8,7gr then to get close to the real purity of 85% we must add 0,1gr for about 15 times... Also if we add 0,1gr NaOH each time that changes the purity by -1,6%. For KOH 0,1gr means -1,15% in purity change.

I have tested my 3 alkalis [NaOH pellets chemically cleaned (10E /kgr), NaOH flakes (2E / kgr) and KOH pellets chemically cleaned (10E/kgr)] for 8 times all of them and I had for both NaOHs 98% purity and the KOH 85% purity.

The chemically cleaned NaOH is supposed to be 99% pure but it is ending (I have only ~250gr out of 1000gr in its vessel), so I guess that it has some drop in purity because of the times I have opened it so far.

The test can be done within 10-12 minutes max if someone has done it a few times (as Dunn states in the video).

I have three remarks:
I have a cheap scale from ebay (~6E) that can weight a total of 500gr with 0,01gr readability that turns automatically off within ~50 sec. I have to remember to press with my thumb the scale's plate from time to time in order not for the scale to turn off, while I'm mixing the alkali in the water seeking for purple colors. Although Dunn has a way to solve the problem once your scale turns off in the middle of the method, I still find it more easy my way.

My alkalies were not in granule form and thus each pellet of NaOH/KOH weigh 0,08 to 0,1gr and thus if I dropped one granule and the color of the solution turned pink before the full dilution of it then I would calculate that the weight would be less than 0,1gr

When we dilute the alkali in the citric acid solution, then there are little pink areas around the alkali inside the water when we mix it and after they are fully diluted these areas are gone. As we are close to the point that the entire solution will turn pink then these areas are much bigger while they disappear again by their fully dilution. So this phenomenon help us to understand that we are close to the final weight of the alkali that we will add for our purity test.

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