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Preparing a buffer solution
Hi guys,
Hope you are doing well.Recently I have been working on baby shampoos and I want to make a buffer with citric acid 20% and NaOH(powder) for pH 5.
Could you tell me how to calculate the concenteration of the acid and NaOH in 100cc of my baby shampoo to make the buffer(making the salt of the weak acid indirectly)for pH 5? -
7 Replies
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Given that you don’t use plain water but rather large amounts of soaps which may not be pH inert, calculating will only give you a hint (apart from the fact that calculating with polyprotic acids is a PITA unless you’re really good with maths or use an app). In the end it’s easier to just add citric acid and then start adding NaOH until you hit pH 5. That would also be a safer way than adding dry NaOH to the formulation and titrating downwards with diluted citric acid as you intend doing (unless I misunderstand your procedure).As a rough estimate which in practical regards is usually more than good enough, 1.5 mole equivalents of NaOH will bring a citric acid solution to a pH slightly below 5 (you could also mix equimolar amounts of citric acid and trisodium citrate to get the same result). -
Pharma said:Given that you don’t use plain water but rather large amounts of soaps which may not be pH inert, calculating will only give you a hint (apart from the fact that calculating with polyprotic acids is a PITA unless you’re really good with maths or use an app). In the end it’s easier to just add citric acid and then start adding NaOH until you hit pH 5. That would also be a safer way than adding dry NaOH to the formulation and titrating downwards with diluted citric acid as you intend doing (unless I misunderstand your procedure).As a rough estimate which in practical regards is usually more than good enough, 1.5 mole equivalents of NaOH will bring a citric acid solution to a pH slightly below 5 (you could also mix equimolar amounts of citric acid and trisodium citrate to get the same result).Thank you for your answer. I just need to know what you mean by “it’s easier to just add citric acid and then start adding NaOH until you hit pH 5″. Would you mean I need to add for example 1 cc citric acid 20% to 100cc of my shampoo to reach the pH below 5 and the and a solution of NaOH to reach the pH 5?
Can it make a buffer? since as far as I know to have a solution of citric acid without its salt and using a strong base instead for pH 5, citric acid should lose its two hydrogens out of three for the pKa ~5, but the mole strong base must be less than 2 to react with the acid and produce the conjugated weak acid salt.
So, if it is right, could you please tell me how much citric acid is required to go below pH 5 and adjust it with what molarity of NaOH? -
Now you have me a bit confused…It is absolutely impossible to tell you how much you’d have to add, even if we knew the starting pH of your product. Many detergents have a pH above or below 7 and even then, they tend to act as buffers themselves. Sugar based surfactants as an example tend to have an alkaline pH (9-11) for stability reasons. Alas, we don’t know how much and what kind of alkali has been added to achieve said pH.The first and foremost question is: Why do you need a buffer? What and why do you want to buffer?From a scientific point of view, it is rather ridiculous to buffer a shampoo because it would require either too much buffer solution or the stuff is self-buffering and all you’d need to do is adjust pH.
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Thank you so much for dedicating your time. Yes, you are absolutely right. Some surfactants dochange the pH of the solution. The reason I want to buffer my shampoo is to avoid the shampoo’s pH rising during rinsing with water whose pH is 7.
So your advice would be add some citric acid until the pH has become a little below 5 and then add alittle amount of NaOH (solution 0.1M for example) to reach the pH 5?Pharma said:Now you have me a bit confused…It is absolutely impossible to tell you how much you’d have to add, even if we knew the starting pH of your product. Many detergents have a pH above or below 7 and even then, they tend to act as buffers themselves. Sugar based surfactants as an example tend to have an alkaline pH (9-11) for stability reasons. Alas, we don’t know how much and what kind of alkali has been added to achieve said pH.The first and foremost question is: Why do you need a buffer? What and why do you want to buffer?From a scientific point of view, it is rather ridiculous to buffer a shampoo because it would require either too much buffer solution or the stuff is self-buffering and all you’d need to do is adjust pH. -
I see.Now, in that case you’d need about 4 grams of citrate buffer. That is for medium hard tap water and for EVERY LITRE of water coming out of your shower and for every ‘scoop’ of shampoo you use during that particular shower session.As you can see, you don’t stand a snowball’s chance in hell maintaining a pH other than tap water. All you can do is dropping the pH in the bottle and for that, simply adding enough citric acid is all that’s required.
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Ok thank you for your kind advice.
Pharma said:I see.Now, in that case you’d need about 4 grams of citrate buffer. That is for medium hard tap water and for EVERY LITRE of water coming out of your shower and for every ‘scoop’ of shampoo you use during that particular shower session.As you can see, you don’t stand a snowball’s chance in hell maintaining a pH other than tap water. All you can do is dropping the pH in the bottle and for that, simply adding enough citric acid is all that’s required. -
Heheheh. I am confused as well!
What @Pharma meant by “it’s easier to just add citric acid and then start adding NaOH until you hit pH 5″ is like this….
Step 1 : Add random amount (or a set amount) of Citric Acid.Step 2 : Add ‘some’ alkali (of your choice) to Citric Acid bit by bit until you get the desired pH.
Even expert chemists need to do that manually. Yes, manually. Mathematics is dead. LOL! I joke but I ‘suffered’ that way when I tried to get the purrrrfect pH/buffer for my lovely Urea, my Obsessive Compulsive Disorder of accuracy must get that 99.999999% spot on pH number. I thought maths was infallible because it is…maths! I was restless battling with maths demanding it to tell me as to why it betrays humanity. I ended up adjusting the pH manually and sleep soundly. Not spot on pH but way less work punching in the numbers and wondering why they are not matching in reality.
Ah I see what you are trying to achieve. You want pH from start to finish!
You would need an MLM rubbish shower attachment which claims to constantly makes water either alkaline or acidic. (Sounds like an ion exchange resin but not that simple and straightforward! It is MLM!)
I am not trying to insult you but in where I live, few MLM companies really do sell thing like that. They even have infrared and magnetic shower attachments!
In all seriousness, I do not think you need buffer!
All you need is a pool of water, make that water pH 5 with lots of Citric Acid (or other acids), et voila!
But it must be a pool of water, because running water from the main is not buffered at low pH nor naturally low pH. This means if the (water) main is pH 8 then it will keep changing the pH of anything it touches to alkaline.
You have control over a pool of water but not the main.If you are selling baby shampoo to other people then you have no control over how they use the product, let alone ask them to collect a pool of water and make it pH 5. Not many people are willing to go to that length if their babies do not have skin issues.
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