Sciencemadness Discussion Board

Bad pH paper, or?

Mateo_swe - 4-2-2023 at 15:02

So i was making some glutamic acid from MSG, monosodium glutamate.
I dissolved 93.5 grams (0.5mol) of MSG in 500ml of 70°C water and began adding 30% HCL.
When checking the pH with pH-paper it gave same color (about pH6-7) no matter how much HCL i added.
Luckily i have an electronic pH meter so i started using that and it showed that the pH indeed had gone down.
The pH papers i had used was the color of pH6-7 where it had been wetted but the dry part above did turn pink after a while.
What made the pH paper fail to indicate the pH?
Because the solution was hot or something else?
Or is my pH paper bad?
Probably a newbie error as i have no lab work experience.

The experiment seem to have been a success though.
I aimed at pH3 but added a little too much HCL and got to 2.5.
The solution turned whitish with precipitated glutamic acid after i had come down enough in pH and now there is a layer of glutamic acid in the bottom of the beaker.
I will filter it and dry the glutamic acid tomorrow.
I don't know why the pH paper failed though, anyone know?

Sulaiman - 4-2-2023 at 16:39

Possibilities,
1 the pH papers are not working
2 you have papers intended for indicating only higher pH values
3 the solution inhibits the pH papers from working,
due to temperature and/or the chemistry of the solution.

1 and 2 can be quickly checked with a drop of acid and a drop of base.

If not 1 or 2 then you need a more experienced member to contribute.

j_sum1 - 4-2-2023 at 20:44

Sounds to me like you have some narrow range papers.

They do get manufactured to cover different parts of the pH scale.
I remember the first time I encountered these I was thoroughly confused. Turns out I had a mid-range paper that presented a range of colours between pH 5 to 8.
It is also possible that you have something that is mislabelled.

Since you have a pH meter, it would be sensible to do some investigating and calibrate the papers you have. They could still be useful.

Here are some examples of narrow range:
https://www.ebay.com.au/itm/404112265297
https://www.ebay.com.au/itm/262681260855
https://www.ebay.com.au/itm/265905468323

SplendidAcylation - 5-2-2023 at 03:30

The first thing that came to mind was that what you have is essentially a buffer solution...

When you add acid to a solution of glutamate ions, you wouldn't expect the pH to decrease much, until you have added the majority of the stoichometric quantity of acid.

When I did this reaction, the pH was about 5.3 even after I had added 75% of the required acid.

You mentioned that the dry part of the paper turned pink, this is also consistent with my theory, because HCl is volatile, so the vapours tend to hang around in the air for a while.
Often if you add HCl to a solution, and then measure the pH, the dry(ish) part of the paper will turn red due to the HCl vapours while the solution may be neutral or even alkaline.


The only thing that goes against this is that you obtained an acid pH reading on your pH meter, are you sure it is accurate?

As the others have said, maybe check the pH paper with various acids and bases, or see what it says on the packet, if available.

Mateo_swe - 5-2-2023 at 07:58

Yeah it quite strange.
The pH papers are full spectrum 1-14pH but maybe it was mislabeled. They are also quite old, about 5 to 6 years at least since i bought them.
I should have tested them with some HCL and a base to see if they show correct colors.
I will do it to verify the papers.
Can the NaCl salt produced from releasing the sodium in MSG interfere with pH paper readings?
The electronic pH meter is new and the probe tip have been stored in its dedicated store solution.
It showed almost exactly 7 for tap water.
I have calibration solutions and can do a calibration of the electronic pH meter if needed but it seem to measure correctly.
I also experienced not much happening at pH around 5 then a sudden a jump down in pH, maybe its just a natural part of this reaction.

Maybe i should try with sulfuric acid next time, only half molar amount needed and NaSO4 produced as side product instead of NaCl when HCL are used.
But maybe the NaSO4 will also precipitate or does it stay in solution at around 2.5 - 3 pH?
My guess is that it stays in solution but im not sure.
Sulfuric is quite hard to get where i live but HCL is at hardware store so i try save the sulfuric if other acids are usable.

woelen - 5-2-2023 at 11:14

I also would say that your observations are due to buffering capabilities of the weak acid/weak base system of glutamic acid and glutamate. If you have N moles of glutamate in your solution, then only after adding more than N moles of HCl you will observe a sharp drop of pH (assuming that glutamic acid is a monobasic acid). Until you add that amount, you only will observe a gradual and weak decrease of pH.

Mateo_swe - 6-2-2023 at 02:59

Aha, that would explain the fast drop in pH and maybe thats what affected the pH papers too.
According to my electronic pH meter the pH slowly went down to 5 something and stayed at that level until enough HCL had been added.
The pH papers didnt seem to indicate any drop at all after beginning adding the HCL but maybe the colors at 7 to 5 are so similar it didnt show.
As these pH papers are 1-14pH there are just slight color change at 7 to 5 pH.
I have put the glutamic acid in a dessicator with some MgSO4 and will dry it at 100-110°C right before use.

SplendidAcylation - 6-2-2023 at 14:12

Quote: Originally posted by Mateo_swe  
Aha, that would explain the fast drop in pH and maybe thats what affected the pH papers too.
According to my electronic pH meter the pH slowly went down to 5 something and stayed at that level until enough HCL had been added.
The pH papers didnt seem to indicate any drop at all after beginning adding the HCL but maybe the colors at 7 to 5 are so similar it didnt show.
As these pH papers are 1-14pH there are just slight color change at 7 to 5 pH.
I have put the glutamic acid in a dessicator with some MgSO4 and will dry it at 100-110°C right before use.


Yep. Full-range pH papers are only really good for qualatitive testing IME
I.E. they tell you whether it is alkaline, acid, or neutral.

Sounds like you did fine, please post your percentage yield! Think I got about 85%.

Mateo_swe - 7-2-2023 at 14:08

So i did put my prepared L-Glutamic acid in an crystallizing dish and heated it on a hotplate to about 110°C for a while to get it absolutely dry.
The final weight is 65.3grams.
I started with 93.5grams (0.5 mol) of monosodium glutamate monohydrate (187.127 g/mol) and got 65.3g (0.4438 moles) of glutamic acid (147.13g/mol).
0.4438 divided by 0.5 is 0.8876 = 88.76% yield based on starting amount of MSG.
Much better yield than expected.

I will use this glutamic acid and some phthalic anhydride in the solventless preparation of N,N-phthaloylglutamic acid.
It will be interesting to see how that turns out.
If successful i try my luck with the preparation of Thalidomide.