# Measuring the pH of sorbic acid (powder form)

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### #1 Prerna

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Posted 25 February 2021 - 08:17 AM

Dear all, I have been trying to measure the pH of 81% phosphoric acid.  pH meter is calibrated and working fine. The result came to be -0.6 which i think is not correct. Also, I wanted to check the pH of sorbic acid (powder form). Can someone please provide solutions? Thanks.

### #2 beautiophile

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Posted 25 February 2021 - 08:33 AM

What pH range does its calibration cover? from pH=0 to pH=14? Measurement values out of range are invalid.

pH values can be negative, in case of high concentration of acid.

pH depends on temperature.

The pH notation only applies to aqueous solutions, not to solid stuff.

### #3 Prerna

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Posted 25 February 2021 - 08:43 AM

pH meter calibration range is 0-14.

For solid stuff, is there a way to calculate the pH ?

For eg. mixing it with a water and making a solution to measure pH ? If yes, what calculations can be used ?

Thanks.

### #4 Charles.C

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Posted 25 February 2021 - 09:24 AM

t's definitely possible to calculate a negative pH value. ...

In practice, any acid that yields a concentration of hydrogen ions with a molarity greater than 1 will be calculated to have a negative pH. For example, the pH of 12M HCl (hydrochloric acid) is calculated to be -log(12) = -1.08.

and, regarding solids, it's usually a question of interpretation.

Kind Regards,

Charles.C

### #5 beautiophile

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Posted 26 February 2021 - 01:16 AM

pH meter calibration range is 0-14.

For solid stuff, is there a way to calculate the pH ?

For eg. mixing it with a water and making a solution to measure pH ? If yes, what calculations can be used ?

Thanks.

Assuming it's a pure solid acid, it has a disassociation constant which can be found in a chemistry handbook. The pH of its dissolved form is calculated from that constant.

But what's the point of pH measuring? To ensure the consistency of material quality?

and, regarding solids, it's usually a question of interpretation.

pH has a fixed and clear definition. If it remain a question of interpretation, that's the problem of people interpreting it.

### #6 Prerna

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Posted 26 February 2021 - 01:47 AM

Yes, its just to check if the pH of the final product is getting affected by the raw material.

### #7 beautiophile

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Posted 26 February 2021 - 03:04 AM

Yes, its just to check if the pH of the final product is getting affected by the raw material.

Then you can mix (e.g 10g) good materials with pure/distilled water (100mL) and measure the pH of the solution under a controlled temperature (e.g. 23°C).

Do that 3 times to get an average pH.

You can experiment bad materials (low impurity) in the same manner and check the pH variation. Finally, set your desired pH range for good materials (after mixing with water).

### #8 Prerna

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Posted 26 February 2021 - 03:14 AM

Thank you very much for the information :)

### #9 Charles.C

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Posted 26 February 2021 - 08:36 AM

Assuming it's a pure solid acid, it has a disassociation constant which can be found in a chemistry handbook. The pH of its dissolved form is calculated from that constant.

But what's the point of pH measuring? To ensure the consistency of material quality?

pH has a fixed and clear definition. If it remain a question of interpretation, that's the problem of people interpreting it.

You cannot measure the pH of a solid since there is no pH. Somewhere in the specification it should state how the pH of a solution off the powder in water was measured
For example "pH 2 to 3 in 5% solution"

Two methods for the measurement of surface pH of pharmaceutical solids, namely, the dye-sorption method and the slurry pH method, were compared. High purity drug substances, instead of excipients, were used as model solids, because acidic or basic impurities present in excipients could influence slurry pH. Solid test samples were prepared by sorption of methanol-water solutions of several indicator dyes, and their diffuse reflectance UV-visible spectra were measured. The solid surface pH values were estimated by comparing base-to-acid peak ratios of the diffuse reflectance UV-visible spectra of solid samples to the calibration plots of dye solutions in aqueous standard buffers of known pH.

In the slurry pH method, pH values of concentrated slurries of the compounds in water were considered to represent solid surface pH. The agreement between the two methods was mixed and depended on the compound or the indicator used.
It was concluded that in many cases calibration plots of indicator dye spectra in aqueous buffers were not applicable to the solid state, and, as a result, the reliability of the method was low.
The slurry method provided a simple and reliable measurement of surface pH indicating that concentrated slurry may closely represent solid surface pH.

Clandestine Lab instructor - chemistry/energetic (2006-present)

Use neutral deionized water pH near 7. Dampen the test strip using a spray bottle (mist) and test substance immediately. Check that the material is not water reactive and if the substance is miscible in water it will indicate a near accurate result. Some will argue that this does not work but I have done it for the better part of 20 years. It works.

Kind Regards,

Charles.C

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### #10 beautiophile

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Posted 26 February 2021 - 09:15 AM

Hi Charles,

Those are a bunch of misconceptions invented by non-chemistry people.

Everything you quote and attach needs water. Any pH measurements in those examples also represent the water (-dissolved) components inside treated samples, drug surface, meat, fruit, etc.

Solid test samples were prepared by sorption of methanol-water solutions of several indicator dyes, and their diffuse reflectance UV-visible spectra were measured. The solid surface pH values were estimated by comparing base-to-acid peak ratios of the diffuse reflectance UV-visible spectra of solid samples to the calibration plots of dye solutions in aqueous standard buffers of known pH. In the slurry pH method, pH values of concentrated slurries of the compounds in water were considered to represent solid surface pH.

Edited by beautiophile, 26 February 2021 - 09:15 AM.

### #11 Charles.C

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Posted 26 February 2021 - 11:01 AM

Hi Charles,

Those are a bunch of misconceptions invented by non-chemistry people.

Everything you quote and attach needs water. Any pH measurements in those examples also represent the water (-dissolved) components inside treated samples, drug surface, meat, fruit, etc.

Hi beautiophile,

Somewhat sweeping perhaps.

Maybe  try the third attachment also.(eg Pg 20).

Kind Regards,

Charles.C

### #12 Spidey

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Posted 26 February 2021 - 04:09 PM

Yes, its just to check if the pH of the final product is getting affected by the raw material.

In that case, why don't you make 2 batches, one with the raw material in question and one without, then measure the pH's of both products.  Measuring the pH of a raw material only gives you so much information, you need to account for any chemical interactions it may have with other raw materials in the finished product.

### #13 Prerna

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Posted 28 February 2021 - 10:09 PM

Thank you all for taking time and replying to my topic. All the ideas are really helpful for me.

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