Sodium is a very, very difficult thing to determine through titration. However, I believe there are some chemicals that complex sodium well enough to be used in a complexometric titration. I think one of these is CyDTA. Endpoint determination may be a bit tough, though. You might need some sort of ion-specific electrode to find that.

I am wondering if you can't somehow back-titrate with 12-crown-4? Add a slight access, then use another method (or ISE, if you have one) to determine how much 12-crown-4 is not complexed? Just a thought.

Titrimetrically not possible , either sulphate or sodium...

Most of the techniques for the titration of Na are based on the formation of NaZn(UO2)3(CH3COO)9. This salt is insoluble in water, so it can be separated and titrated by any ox-red technique suitable for uranyl ions.

Also, in presence of sulfate ions up to 7:1 ratio it is possible to precipitate Na with 1-naphthylamine-8-sulfonic acid and titrate the obtained precipitate with KBrO3.

Unfortunately, I only have the detailed procedures in Russian, but I hope that you will be able to find something in English too.

Lets adopt those methods of analysis which are widely accepted and provide reproducible results across different soils...

Thanks very much for all answers, I will work on the suggestion.

The concentration of the sulfate anion in the sodium sulfate solution can possibly be assayed by titration with BaCl2 aq. sol., or with other Ba(II) water-soluble salt; by means of BaSO4 precipitation. A few reference papers are signalled:

https://pubs.acs.org/doi/abs/10.1021/acs.jchemed.5b00941?journalCode=jceda8

Article Barium titration of sulfate with chlorophosphonazo III as indicator

Article Direct Titration Method for Determination of Barium

Article Direct Titration of Sulfate

Article Direct Titration of Sulfates

For a RG discussion on the pH of pure sodium sulfate (Na2SO4) aq. sol.:  https://www.researchgate.net/post/What_do_you_call_the_electrolyte_of_sodium_sulfate_a_base_an_acid_or_neutral

Dear Esteemed Colleague,

Thank you for your inquiry regarding the determination of sodium sulfate by titration. The quantitative analysis of sodium sulfate, a common inorganic salt, can indeed be approached through titrimetric methods, although direct titration of sodium sulfate is not commonly practiced due to the absence of a suitable direct titration reaction. However, an indirect method can be employed to quantify sulfate ions in a solution, which involves the precipitation of sulfate ions as barium sulfate and subsequent determination. Here, I provide a detailed, logical approach to this analytical challenge:

Gravimetric Analysis via Barium Chloride Precipitation

While not a titration in the traditional sense, gravimetric analysis is a widely accepted method for determining sulfate concentration, which can be adapted for sodium sulfate solutions. This method involves the precipitation of sulfate ions as barium sulfate (BaSO4) by adding a barium chloride (BaCl2) solution. The precipitate is then filtered, washed, dried, and weighed. The mass of the precipitate allows for the calculation of the original sulfate concentration.

Indirect Titration Method

An indirect titration method for the determination of sodium sulfate involves the following steps:

Considerations

• Accuracy: It's important to conduct the titration under conditions that prevent the precipitation of barium salts other than BaSO4, as this could affect the accuracy of the titration.
• Sample Preparation: Proper sample preparation, including pH adjustment and filtration to remove the BaSO4 precipitate before titration, is crucial for accurate results.
• Stoichiometry: Understanding the stoichiometry of the reactions involved is essential for calculating the concentration of sodium sulfate from the titration data.

In summary, while a direct titration method for sodium sulfate determination may not be readily available, the indirect approach through the precipitation of sulfate ions and subsequent titration of excess barium provides a viable alternative. This method requires careful attention to experimental conditions and precise execution of analytical techniques to ensure accurate quantification.

I trust this explanation offers a clear and comprehensive guide to determining sodium sulfate concentration through an indirect titration method and aids in your analytical endeavors.

Best regards.

Perhaps this protocol list can give us more information to help solve the problem.