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Chem Science

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posted on 23-2-2023 at 15:31

Budget-friendly strong acid titration

INTRODUCTION

Titrations, sometimes annoying and other times so useful and interesting. We are all familiar with titrations, is the most common procedure in a laboratory and i think there’s no lab in the world ever that doesn’t do at least one titration at some point in time.
The classic apparatus used to do titrations is the following.

[Titration Apparatus]

In this report I will inform you about a novel method for strong acid titration that is completely amateur, and requires no scale, no burette, and uses ultra-common substances as indicators.

REAGENTS

_Sodium Bicarbonate [NaHCO3]
_Copper Sulfate [CuSO4, 5H2O]
_Zinc Sulfate [ZnSO4]
_Water [H2O]

PROCEDURE

I] Preparation of titrant.

In a capable beaker, a generous excess of sodium bicarbonate [NaHCO3] is added to an arbitrary amount of water.

The beaker is then submerged in a water-Ice bath. Make sure enough ice is in the water for the procedure. The solution is left to stir for at least 1 hour. The objective is to prepare a saturated solution of sodium bicarbonate at 0ºC. The Water-Ice bath will have a temperature of 0ºC so long as ice is present in the water.

After an hour or more time has passed, the solution is rapidly filtered and stored.

Once the filtration is done the solution can be used at any temperature above 0ºC. These is the titrant for the procedure.

II]Titration

A] First measure a random, but known volume of titrant. In these case I used 20 mL of the saturated bicarbonate, measured with a syringe.

B] Now the Pre-Indicator is added. In these case I used a small amount of solid purified copper sulfate. A better way to do these is to have a solution of pre-indicator made and filtered so no particulates are present and the solution is perfectly clear.

The copper sulfate reacts with the bicarbonate to produce basic copper carbonate.

The basic copper carbonate dissolves in the bicarbonate giving a strong blue color. Possibly unknown complex formed here.

C] To the solution of Titrant with indicator we add the acid we want to titrate, very good stirring is important, in my case I used impure hydrochloric acid.

Titrant [right] Acid [X] [Left].jpg - 79kB

The acid is added drop by drop until the solution turns from opaque to crystal clear.

The transition is very sensible and 1 drop will be the difference between opaque and crystal clear.

D] Once the transition is identified, the volume of acid added is recorded and proceed to do the concentration calculation. In these case the volume used was 4,4 mL.

TITRATION FUNDAMENTS AND CALCULATIONS

To do a titration we perform a chemical reaction which is very well stablished and whose stoichiometry is very well known.
In these case the reaction of titration is the following:

H_nX + n NaHCO₃ = Na_nX + n CO₂ + n H₂O (n is an integer)

A titrant of known concentration is used, in these case we use sodium bicarbonate as the titrant, and the concentration will be that of a saturated solution at 0ºC.
Sodium bicarbonate at 0ºC forms a solution of concentration equal to 0,823M
A known volume of titrant is used, we used 20 mL so the N#°of moles of NaHCO3

N# moles [NaHCO₃] = M x V (liters) = 0,823 x 0,02 L = 1,65x10^-2 moles.

In the reaction of bicarbonate with acids, there is no indication of when the reaction is stoichiometric, as the reaction is colorless. So we need an indicator to detect the equivalent point, the point when equal mols of acid were added as mols of titrant were present. The indicators work by being insoluble in water an forming a suspension which disappears at the equivalent point. The indicators that work best are basic copper carbonate and zinc carbonate. These are formed in-situ by adding drops of solutions of coper or zinc sulfate, the reactions that occur are the following:

2CuSO₄ + 4NaHCO₃ = Cu₂CO₃(OH)₂ + 2Na₂SO₄ + 3CO₂ + 2H₂O

ZnSO₄ + 2NaHCO₃ = ZnCO₃ + Na₂SO₄ + CO₂ + H₂O

Although the reaction for the formation of the indicator gives off carbon dioxide, there is no loss of perceive moles of titrant, in fact it’s possible to use all the bicarbonate to make indicator and use that instead, and the result will be equal.
At the equivalent point the indicator reacts with the acid to form a soluble copper or zinc salt, and the solution turns crystal clear.
To calculate the concentration of acid, we solve the following equation:

[Acid] = [Molarity of NaHCO₃ x Volume of NaHCO₃] / Volume of acid

In the case of these example

[HCl] = [0,823M x 20 mL] / 4,4 mL = 3,74M

RECOMMENDATIONS

The purity of the sodium bicarbonate is important for these titrations to be accurate, its recommended to recrystallize the sodium bicarbonate before preparing the titrant.
The purity and clearness of the pre-indicators is also important. The best way is to make a solution of copper or zinc sulfate, thoroughly filter the solution and stored. 1 or 2 drops of the solution is added to the titrant to form the indicator.
A theoretical analysis of the reaction is recommended as the chemistry is relatively easy. Why the perceived moles of titrant don’t change with the formation of the indicator?
Why zinc and copper are better than calcium or magnesium as indicators?
These are an interesting questions to answer.
In practice the use of a pre-indicator is better than adding solid zinc or copper carbonates, the reason is that particle size is important for the mechanism of the titration to work well.

CONCLUSIONS

A video of these procedure was made in Spanish in my YouTube Channel If you wish to watch. LINK HERE
The method was extensively validated comparing to conventional titrations and the results are within less than a 1% margin of error in my experiments. The method is good to perform quick titrations with low cost as well as very amateur friendly.

[Edited on 24-2-2023 by Chem Science]

Edit by Texium: changed title for clarity

[Edited on 2-25-2023 by Texium]

blogfast25

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posted on 24-2-2023 at 04:17

Quote: Originally posted by Chem Science

In this report I will inform you about a novel method for strong acid titration that is completely amateur, and requires no scale, no burette, and uses ultra-common substances as indicators.

Good effort but no one serious would use a syringe for a pipette and burette. Syringes are hopelessly inaccurate compared to the latter.

I'd also be very hesitant to use a titration that uses visual opaque to clear as an end-point. Potentiometry would be better.

The basic copper carbonate dissolves in the bicarbonate giving a strong blue color. Possibly unknown complex formed here. Yes. Carbonato complexes are quite rare. Be²⁺ forms one of them.

Great write-up!

[Edited on 24-2-2023 by blogfast25]

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Chem Science

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posted on 24-2-2023 at 04:57

Hi blogfast25.

Yes it’s true that these method is not so Trust-worthy as the common methods of colorimetric titrations, and it does have some potential problems with complex formations, and yes Potentiometry is powerful.
however, the purpose of these method is not to replace established accurate titration methodology, but to give the amateur, the beginner chemist a way to titrate their acids and bases.
And despite the opaque to crystalline transition seems not so trust wordy, in my experience the precision is very close.
If you can’t find your pH indicator a burette is usually useless for the beginner. These is not the case anymore because here’s a method with an indicator that can be extremely easily found.
Oh and yes … i don’t use these method in my lab regularly … I love my colorimetric titrations 8)

blogfast25

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posted on 24-2-2023 at 07:30

Quote: Originally posted by Chem Science

Hi blogfast25.

If you can’t find your pH indicator a burette is usually useless for the beginner. These is not the case anymore because here’s a method with an indicator that can be extremely easily found.

Pen-type pH meters are cheap as chips nowadays, as well as beginner burettes and pipettes. Several acid-base indicators are also easily obtainable.

woelen

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posted on 24-2-2023 at 13:37

Good piece of work! This is a creative piece of thinking.

Of course, things can be more accurate, but for an amateur with little resources, this is a nice method of assessing the concentration of stuff like dilute solutions of HCl, H2SO4, HNO3, but also of NaHSO4.

An improvement is possible if a more accurate method of volume measurement is used (as stated by blogfast25), but the basic principle behind this method is nice

One remark: Purity of OTC NaHCO3 can be very low. Sometimes it contains NaCl, it also can contain Na2CO3. I have had perfectly soluble NaHCO3 in the past, but it had almost one quarter by weight of impurities. On eBay, however, you can buy good quality NaHCO3. Maybe your procedure also can be adjusted to make use of Na2CO3.10H2O. The latter can be obtained at very high purity at supermarkets in the form of so-called crystal soda (at least, where I live this is available in that way), in the form of big glassy crystals. These must be stored in a tightly sealed container, otherwise they lose water (effloresce) and the hydration state becomes lower than 10.

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Bedlasky

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posted on 24-2-2023 at 14:43

Quote: Originally posted by woelen

Food grade NaHCO3 is pretty much pure (or at least in my country it is).

blogfast25

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posted on 25-2-2023 at 10:40

Quote: Originally posted by woelen

Sometimes it contains an anti-caking agent like rice powder.

Always best bought from a chemsupplier.

SplendidAcylation

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posted on 25-2-2023 at 14:01

Quote: Originally posted by woelen

Good piece of work! This is a creative piece of thinking.

Exactly! Whether or not this is a method one would use frequently, it is a brilliant write-up, well done!

Chem Science

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posted on 26-2-2023 at 16:48

Thanks SplendidAcylation and Woelen

For some reason, despite the low applications for these method, researching and being capable to solve the problem i wanted, brought me joy and a couple of smiles.
It was a fun Project. A simple and not so important problem, but none the less I think is a noble solution

And also Tanks Texium for the more accurate Title hehe xD Sorry for my Caps. ^.^

Sulaiman

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posted on 26-2-2023 at 18:19

Nice write up.

With the very low cost of digital scales,
Working and calculating in mass rather than volume has a few advantages.
Choosing a suitable indicator can be critical.

I find the cheap hand held pen type of pH meter unusable for small liquid volumes.

CAUTION : Hobby Chemist, not Professional or even Amateur

blogfast25

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posted on 27-2-2023 at 02:55

Quote: Originally posted by Sulaiman

I find the cheap hand held pen type of pH meter unusable for small liquid volumes.

Depends on what you call 'small liquid volumes'. I've used a wide neck 100 ml CF with a pen-type pH mete and mag stirring, w/o problems.

But mostly I still prefer indicators.

Potentiometry is more for fully automated robo-titrators, I feel.

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