April 03, 2020, 07:45:55 PM
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Topic: How mg of sodium hydroxide to make pH solution of 12, citric acid for pH of 4  (Read 294 times)

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Offline born2dive00

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Hello everyone

I am trying to make a pH calibration  solution for my phone tester.

I have taken 3g of sodium hydroxide
I have taken 3 g of citric acid, I have put both in the oven at a temp of 150'F for 2 hours to make sure there is no moisture in the powder.

I also have 200ml of distilled water.

So I am trying to find out how many mg of sodium hydroxide I have to add to 100 ml of water to get a solution with a ph of exactly 12.52 @21'c

I am also trying to find out how many mg of citric acid I need to add 100 ml of water to get a solution with a pH of exactly 4.73 @ 21'c

Please let me know and thank you

Offline chenbeier

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How is pH defined? So do you know relationship between pH and pOH?
This you need for die sodiumhydroxide.
The citric acid is more complicated. It's a three carbonic acid.

https://www.humblebeeandme.com/hive/topic/how-much-citric-acid-you-should-weight-to-get-a-specific-ph/

Offline born2dive00

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Hello Chen I teach english I hate maths and I most certainly am not a chemist so I don't know, I don't the jargon nor how to deal with moles either. I only know that if I add this much of X to this much of Y i should get Z

Please keep it simple for this non scientist, #of mg of citric acid to 100 ml of water to get a ph of 4.73 @21'c
And the number of mg of sodium hydroxide to 100 ml to get a solution with the phone of 12.52 @21'f

Offline Borek

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No way to make the solution with such precise pH using NaOH, no matter how long you will try to dry it.

Measuring pH with 0.01 accuracy is difficult even with a bench class pH meter, with phone (whatever you mean by that)  it is out of the question.
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Offline born2dive00

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Hey Borek

The pH meter that bought off eBay (non working, had a blown micro capacitor) for $120 was the Fischer VERSA STAR™ pH Benchtop Meters which sells for well over $1700. The Versa star can read down an accuracy of .001+/- .002

However I agree that MOST cheap pH meters only read down to .01 +/- .01 accuracy

I have tried the versa star since I fixed it and it seems to do very well on the pH measurements.

The only thing is I live in Cairo at the moment and can not get access to high quality pH calibration solutions.
Or if I can, they are like $150 for 500ml shipped from over seas.

Hence I am trying to take what dried acids and bases I have, measure them out on my mg scale to make the necessary testing solutions.

What dried base would you suggest in place of pharmaceutical grade NAOH I should use?


Offline born2dive00

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Borek

Would something like mono sodium phosphate be better for creating a base testing solution?

Offline Borek

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Just because the pH meter displays 0.01 accuracy (or is advertised to do so) doesn't mean that's the real pH of the solution. Calibration errors, electrode nonlinearity, alkaline error, electromagnetic noise and kaboom, your pH is off. For a real 0.001 accuracy you need not just an expensive pH meter, but an earthed and screened measuring stand. Otherwise I wouldn't trust the results.

Preparing your own buffers is tricky. First, a lot depends on purity of the substances - you need analytical grade reagents. Second, strong bases are unreliable - they tend to absorb atmospheric CO2 and humidity, you never really know what their composition is and purifying them is a nightmare. Third - even if you do your best and follow the best standard procedures, the only practical way to make sure what the pH is is to measure it and compare against a proven standard, which you can't.

That being said, check these calibration solutions: http://www.ph-meter.info/pH-electrode-calibration-buffers - they are proven to be relatively reliable (as in not prone to common errors and made of substances which can be easily obtained in a pure, stable form). Not sure if they will be cheaper to prepare though.
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Offline born2dive00

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Borek
Correct me if I am wrong but food grade is the highest purity isn't it?

These are the food grade chemicals I have on hand.

Italic acid
L ascorbic acid
Tartaric acid
Magic acid
Citric acid
Magnesium chloride
Potassium chloride
Calcium chloride
Calcium di sodium EDTA
Sodium sorbate
Sodium Benzoate
TRI Sodium Citrate
Tri sodium phosphate (99.9%pure)
Di sodium hydrogen phosphate (98.5% pure)
Potassium phosphate (mono,di,tri is unknown)
Sodium hydroxide 99.9% pure
Phosphoric acid 70%
And Mrs.Wages pickling lime (calcium hydroxide?)

What calibration solution can we make from this?

Offline born2dive00

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Borek from what I understand Mrs. Wages pickling lime is food grade pure calcium hydroxide.

Would this work? What does it mean on the other link you gave me "saturated at 25'C"?

If when you say a shielded and grounded stand, a faraday cage, I actually have one of these for my electronics component and rf isolation testing, and it is large enough to put in a chemistry stand and related equipment.

When this is done in chemistry are you putting the whole bench tester inside (seems like the rf given off by the bench would negate any benefit of the cage) or are you putting the bench outside with shielded leads, and the probes inside(which would isolate any em/rf interference from the probe caused by the bench tester).

Offline Borek

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Correct me if I am wrong but food grade is the highest purity isn't it?

No, food grade just means no pathogens and safe to consumption, it doesn't say anything about the chemical purity - they may contain several percent of inert stuff (which will mean you have no idea what is the concentration of solutions you make), or even worse - stuff that is perfectly safe, but is chemically completely off when it comes to controlling pH.

High purity (as in over 99% of the main component) is analytical grade (these don't have to be safe for consumption, so they can contain traces of toxic contaminants - not that they do, just nobody cares).

Unfortunately that means that most of the things you have are useless. Citric acid and trisodium phosphate can be used to prepare buffers covering very wide pH range, but if your citric acid is just food grade I wouldn't trust it too much. You can try to recrystallize it, that should help, but it is still not guaranteed to OK.

Saturated at 25°C means you have a solution that contains undissolved solid at the bottom (that guarantees nothing more can dissolve - which is definition of "saturation" here).

Are you sure you need very high precision measurements?

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Borek from what I understand Mrs. Wages pickling lime is food grade pure calcium hydroxide.

See above - food grade and pure are different things. Plus, calcium hydroxide is a base and - as I wrote earlier - it will easily absorb CO2 from the atmosphere, so it is contaminated with CaCO3. Doesn't matter much for pickling, substantially changes the pH of the saturated solution.
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