How to test pH of skin?

May 11, 2012 5:21 PM

How would one test the pH of skin? And other questions related to the pH of skin, shampoo, and conditioner.

One of my dogs is having skin issues that I think are related to shampoo/conditioner. I've repeatedly read that skin pH varies by breed, so my thought is to try to learn what his natural pH level is and then find a very simple, unscented shampoo & conditioner and adjust its pH to match his. But first I want to test the pH of his skin. I know someone with a pH meter at the university so will have no trouble testing the shampoo/conditioner; it's the skin part I don't understand. Also, I'm just plain interested in tinkering around with this. I'd love to test various commercial dog shampoos and see if they're really any different from human shampoo.

On a related note, can anyone explain to me why the pH level of shampoo should matter if you are thoroughly rinsing afterward? Conditioner is designed to leave residue so I can see why it would matter there . . . although I have read that only water-based substances have a pH, and not oils, and wouldn't it only be the oils/other nonaqueous crap left once the hair dries?

Obviously I have no science background, but I'm very interested in learning about this, so feel free to tell me just as much as you can. Thanks!

posted by HotToddy to science & nature (9 answers total)

On a related note, can anyone explain to me why the pH level of shampoo should matter if you are thoroughly rinsing afterward? Conditioner is designed to leave residue so I can see why it would matter there . . . although I have read that only water-based substances have a pH, and not oils, and wouldn't it only be the oils/other nonaqueous crap left once the hair dries?

Some shampoos will leave a residue as well. Also, exposure to irritants, even for a short time, can have effects long after the exposure.

For example, I am allergic to most grasses. If I go roll around in my lawn, I'll be fine for a few hours and then I'll break out in hives. Sometimes, even showering doesn't help.

Which brings me to my point - I get rashes from some soaps, shampoos, lotions, etc. I have to be careful what my clothes get washed with. It may be that your dog is experiencing an allergic reaction rather than a simple chemical one.

If that is the case, you'll need to do some experimentation to isolate the brands or chemicals.
posted by Pogo_Fuzzybutt at 5:27 PM on May 11, 2012 [1 favorite]

pH indicator strips
posted by Blazecock Pileon at 5:29 PM on May 11, 2012 [1 favorite]

I do know about pH indicator strips, but how would you use them to test skin?
posted by HotToddy at 5:39 PM on May 11, 2012

What kind of dog is this and what type of coat does the dog have? Is the dog flea free? Some dogs are super super sensitive to fleas.

Have you considered that it might be a food allergy? Tons of dogs have skin issues because of food issues.

How often are you bathing this dog and with what products?

I'm a groomer. I use hypoallergenic shampoos about 90% of the time. Most dog skin issues I have seen are food related or flea related. Ringworm makes an appearance maybe 2-3 times a year.
posted by fluffy battle kitten at 5:50 PM on May 11, 2012

You can't test the pH of a solid, at least in my experience. Wikipedia agrees. The food industry purees solids, and adds water if necessary. The EPA adds water to soil and other solids before testing.

I assume they wash the poor animal with deionised water and then test that. Or maybe they actually mean the pH of sweat.
posted by kjs4 at 6:06 PM on May 11, 2012 [1 favorite]

I know nothing about shampoo or dogs, but I know a little about pH. pH usually refers to aqueous solutions and represents the concentration of hydrogen ions (H+) in the solution. An acid has lots of H+ (pH 1-7) a base has very little H+ (pH 7-14) - pure water will have a pH of 7 under standard conditions. The H+ can react with other molecules (or your skin) - this will potentially change the pH of the solution.

I'm not sure I completely understand the concept of skin having a pH, but I can guess that when someone says skin has a pH of 5.5 it means that if I expose skin to an aqueous solution of pH 5.5 no reaction will occur between the H+ of the solution and the skin. If I have more H+ (pH < 5.5) I expect the skin cells will be impacted by the difference. It's possible this effect is not reversible, thus washing off the solution won't return the cells to their original state. Similarly If I have less H+ (which - this is slightly complicated - for water it means I actually have more OH-) and thus have a basic solution it will impact the cells on the surface of the skin. In this case I do know what happens, if you get a moderately strong base on your skin it will feel very slippery - a similar reaction happens in the reduction of animal fat (cells) with Lye (strong base) to produce soap. Washing again will not return the skin to normal since you have caused a chemical reaction to take place at the surface of the skin.

So to answer your question, I don't believe you can use pH strips to measure the pH of skin. You might be able to wet the skin and try, but I don't believe this is the right measurement. Instead you would need to apply a variety of different solutions to the skin and observe for a reaction at the cellular level. To get this right I would probably try skin cells under a microscope and adjust the pH of an applied solution to see if I can see a structural change in the cells...

One more suggestion, a pH meter (depending on the type) may not produce a reliable measurement of pH for a complex liquid like a shampoo. pH paper might be a better bet in this case...
posted by NoDef at 6:25 PM on May 11, 2012

From Wikipedia: Citric acid is used to adjust the pH down to approximately 5.5. It is a fairly weak acid which makes the adjustment easier. Shampoos usually are at pH 5.5 because at slightly acidic pH the scales on a hair follicle lay flat making the hair feel smooth and look shiny. It also has a small amount of preservative action. Citric acid as opposed to any other acid will prevent bacterial growth.
posted by cosmic.osmo at 6:28 PM on May 11, 2012

Get some 18 M-ohm water and put a drop on the skin of the dog. Wait a minute and the absorb the water on a pH test strip. You would need a super precise one because the pH isn't going to very much from 7.

NoDef is almost correct above, pH isn't necessarily dependent on aqueous solutions (you can get a pH in DMSO for example)

In reality pH = -log([H+]) (where in chemistry terms [] means the concentration of whatever is inside the brackets) In most cases this runs from 0 to 14 (or one molar concentration of Hydronium to 10^-14 molar concentration of H+ ions)

Every acid will also have a Ka (Acid disassociation constant) (or pKa where pKa = -log(Ka) (see the trend)) Strong acids will have a negative pKa (sulfuric acid, hydrochloric acid, etc) meaning that the H is always loose from the conjugate base.

so for a generic acid HA which will disassociate into H+ and A- you have the following equilibrium equations:

HA -> H+ + A-

Ka = [H+]*[A-]/[HA]

So knowing the pKa and the pH you can get the concentration of the acids present in the solution.

I don't know why you would need to get the concentration of the acid on a dogs skin, or if that was even constant but the way to test it would be to soak a sample of skin with ultrapure water and measure the pH very carefully.
posted by koolkat at 8:41 AM on May 12, 2012

Talking about the pH of a non-solution isn't really correct - as koolkat says, it's all about hydronium concentration.

The thing to remember, though is that pH is a logarithmic scale and people don't think well in logs. For example, I know scientists who scoffed at this notion, confident that they could think logarithmically, but then balked when I asked first digit of the log of their phone number (which is an absolute freebie if you just stop and think about it).

What this means is that if you add enough strong acid (such as HCl, H₂SO₄ HNO₃ to very pure water to go from pH 7 to pH 5 and then add that much again, you'll go from pH 5 to pH 4.7. This is why people trying to adjust the pH of a solution to something near 7 will routinely overshoot and end up way too high or too low and why the pH of tap water can vary wildly from place to place and day to day.

pH also won't tell you anything about how aggressive the shampoo is about removing oils and causing dry skin.
posted by Kid Charlemagne at 6:52 PM on May 12, 2012 [1 favorite]