# Digital pH meter: is it of any help?



## Romane (Feb 12, 2012)

Hello all, 

My pH stripes are running out and I'm considering investing in a digital pH meter. I need it for soap, lotion, cream, shampoo, etc.

After some research, I'm still not sure if it worths the money. 

Anyone using one? If so, what do you think?

Thanks for your answer!


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## judymoody (Feb 12, 2012)

How important is your personal safety and that of the people that use your product?

Strips are generally not that accurate.

I have just branched into making lotions and am getting a meter this week.  As certain preservatives are effective only within a particular pH range, I think it's important to confirm with greater accuracy.

Swift has a good post on using a digital meter on her blog, point of interest.

You can get a decent one on Amazon from between $ 50-100.


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## Romane (Feb 13, 2012)

How can we test the pH of a solid soap bar with it? If i soak a peic of soap in water, is the water going to change the pH?


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## Fragola (Feb 14, 2012)

Yes, any excess lye will dissolve in water much quicker than the soap (the soap doesn't need to actually dissolve).

I usually cut some small shreds of soap, like with a potato peeler.


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## scrubbie (Feb 14, 2012)

*PH meter Amazon.*

http://www.ebay.com/itm/NEW-MINI-PH009- ... 33697ec72c

Is this one any good? I am looking at this one to buy


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## scrubbie (Feb 14, 2012)

Or is one for testing soil, where you stick in the probe into a cube of your tester soap


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## Fragola (Feb 15, 2012)

Yes, this one is for testing solutions and can be used for soap. 

It has no probe that you can stick into soil. Never crossed my mind that you can use a soil pH meter for testing soap.


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## agriffin (Feb 15, 2012)

Romane said:
			
		

> How can we test the pH of a solid soap bar with it? If i soak a peic of soap in water, is the water going to change the pH?



I wouldn't use a digital meter to test the pH of solid soap.  If you dissolve the soap in water then it's going to be quite diluted and your reading will not be accurate.


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## Fragola (Feb 15, 2012)

Always when you test for pH, you're going to have dilution. That includes liquid soap. 

Also, if you increase the dilution, the pH will change much less. Any resulting measurement error will be quite small.

Keep in mind that you're not checking soap pH, because that is already known - instead, you are trying to determine if your soap contains free alkali.


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## agriffin (Feb 15, 2012)

Fragola said:
			
		

> Always when you test for pH, you're going to have dilution. That includes liquid soap.
> 
> Also, if you increase the dilution, the pH will change much less. Any resulting measurement error will be quite small.
> 
> Keep in mind that you're not checking soap pH, because that is already known - instead, you are trying to determine if your soap contains free alkali.



I test my diluted liquid soap because that is the finished product.  The product that goes on the skin.  And yes, I am checking the pH of it to see if I need to buffer a bit or neutralize.  

If you take a piece of bar soap and dissolve it in water and then stick your pH meter into it you are not getting an accurate reading as to the pH of the bar soap.  

Dissolve a 1 oz piece of soap in 4 oz of water.  Now add some more water.  The pH reading on the meter will be lower.  This is not accurate.


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## Fragola (Feb 15, 2012)

You seem to be focusing on the "undiluted", bone dry soap bar which lays on the shelf.

Soap isn't used dry onto the skin, but heavily diluted (a bar of soap lasts for thousands of gallons of washing water). 

Dry soap kind of behaves with a lower pH than diluted soap. This is difficult to explain, but think of lye pellets and lye solution: the pellets are much less reactive and don't burn your skin as much as lye solution does. 

That being said, there would be no point in testing the pH of dry soap, (if that would be even possible).



> Dissolve a 1 oz piece of soap in 4 oz of water. Now add some more water. The pH reading on the meter will be lower.


Or maybe *you* could do that. You might be surprised 



> This is not accurate.


1. It is close enough for our purpose, which is to detect free lye.
2. Accuracy can be achieved, since this is a systematic error. That is, if you really must.
3. To my knowledge, it's the best available alternative.


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