Tuesday, September 12, 2017

How to test pH of our products and more (updated for 2017)

In this post, Weekend Wonderings: Have a question?, Valerie asked: Are there guidelines on to dilute (with distilled water) a sample of a product before testing to get ph test accurate results - like shampoo, lotion etc. I have pH tested my liquid soaps in the past and was instructed to dilute appropriately before testing to get accurate results. Any advice is appreciated! I have made a powdered foaming exfoliating cleanser that I would like to pH test. I have a digital reader and a know how to calibrate it using the solutions. Also, I would love to post this recipe for feedback and suggestions on how to improve it.

I've been using the method put forth by Kenna of Modern Soapmaking. "I’m aiming for a 1% soap solution, meaning 1% of the solution is soap and 99% of the solution is distilled water. To make it easy, I weighed approximately 1 gram of soap and 99 grams of distilled water..."

If you're using distilled water - which should have a pH of 7 or neutral - then the only change to the water is the thing you want to measure. So using as little as 1% should be enough to alter the pH enough to be measured. I generally use 10% as I usually have my 1 gram scale handy versus my tinier scale, so I use 10 grams product to 90 grams distilled water. 

Why do we have to dilute our products? It's obvious in the case of a shampoo bar or solid soap, but this is important for anything that's thicker than water as the electrodes aren't equipped to measure pH in viscous materials, like lotions. It won't give you an accurate measurement if you just dip it into the lotion or shampoo. Dilute it and you're good! 

How to calibrate your machine? You'll have to read the specifics from the manual, but the ones I've used suggest we should have liquids for pH 4 and 7 and, if possible, 10. I push the button, and place it in the pH 7 liquid until it notes it's done there, hit save, then put it in the pH 4 liquid, and do the same thing. It's calibrated! 

Where do I get my calibration fluids? I get mine from either Lotioncrafter or at my local hydroponics store. 

Those of you who have been here for a while may have noticed that I'm using a different pH meter. My old one, a Jenco Vision Plus 630, which I loved so much, needed a new electrode, and I couldn't figure out where to buy it. I bought a new one from Amazon last month - Hydrofarm HM Digital HMDPH200 Waterproof PH and Temperature Meter - and I'm really happy with it so far. (Not an affiliate link, just sharing information.)

As a quick note, if you're a soap maker or want to be one, consider supporting Kenna of Modern Soapmaking on Patreon. She's the one who turned me onto it, and I can never thank her enough. I'm embarassed to realize that I haven't been supporting her, and I made that right today! I met her earlier this year at the conference, and she's such a great person! 

Related posts:
Let's take a look at what pH means!
pH of our skin's acid mantle
pH of our bodies

pH OF OUR PRODUCTS
What pH should lotions be?
Another look at the pH of our lotions
pH of shampoo
pH of conditioners

ADJUSTING pH
Adjusting pH of our products
An example of me adjusting pH in our products

pH METERS
Calibrating my pH meter
What pH meters are good?
Equipment for measuring pH

2 comments:

Eija said...

I have to say that I'm highly suspicious about correctly measuring pH if you do 1% dilution of your product. It may represent the using conditions and so the pH in the time of using (like washing your hands with the soap) of the product but not the pH of the product itself.

A properly made buffer system can easily withstand 50% dilution without any notable change in the pH and you can even add mild acids or bases to it without any changes in the pH for some time. But mild acids and mild bases definitely will change pH when you add water to them. Even if it's distilled water, it still has it's own pH and that pH will change the pH of the product.

You will get a approximation of the pH with diluting the product but the more you dilute the more inaccurate it will be.


The other thing I want to comment is about calibrating a pH-meter. First of all, always follow your devices instructions. They vary from model to model so if you want to get accurate results, you have to follow the instructions to _your_ device and nothing else.

Most common buffer solutions for calibrating are indeed pH 4, 7 and 10 but again it can differ. Choose your buffer solutions so that they are on the side of the scale you are going to measure. So if you think your product is acidic, use 4 and 7. If alkaline, then 7 and 10. Two point calibrations should be plenty enough for any measurements.

You can store small volume of the buffer solutions in a tightly closed bottles and use those for the calibrations multiple times. Just rinse your electrode with distilled water between the buffers and they will keep for several weeks. When you see things growing in them then it's time to pour them to the drain. Never pour used solutions back to the original bottle. The original bottle will keep years if you only take solution out of it and never put anything back.

And one very important thing. Your pH-meter will keep years if you store it properly. Again, follow your device's instructions but usually you need to keep the electrode wet in a potassium chloride (KCl) solution. Most often saturated or 3 mol/l solution of KCl in distilled water. And if you aren't using it for weeks or months, check it regularly! If the electrode dries out it can ruin it permanently.


I'm a laboratory professional so this comes from professional point of view. Which can be little over-zealous for making cosmetics at home as a hobby. =P

I do have some conditioner that doesn't work for my hair properly so I think I'll do some experiments on pH in the lab next week when I have time at work. I'm highly suspicious but I'm not a chemist so I'm shaky on the theoretical side of how pH work, but good on practise so I will experiment.

Susan Barclay-Nichols said...

Thanks for your thoughts! As I mention in the post, I'm using a 10% - 90% solution for my products. I've written about calibrating your meters quite a few times, and will have another, more in-depth post next week about it.