POTENTIOMETRIC DETERMINATION OF ANIONIC AND CATIONIC SURFACTANTS WITH IONODE SOLID-STATE SURFACTANT ELECTRODE

POTENTIOMETRIC DETERMINATION OF ANIONIC AND CATIONIC SURFACTANTS WITH IONODE SOLID-STATE SURFACTANT ELECTRODE

INTRODUCTION

The Ionode solid-state Surfactant Electrode indicates the potentiometric endpoint when titrating anionic or cationic surfactants in solution. The specially designed solid-state surfactant electrodes will allow the end user to re-generate the active surface.

Anionic surfactants can be titrated with cationic surfactants and vice versa. This manual describes a wide range of substances that can be determined and lists the relevant working conditions and parameters.

In contrast to the classical “two-phase titration” according to Epton, the titration with the anionic and cationic surfactants electrodes can be performed without chloroform.

The surfactant ISE provides help here in many cases and also benefits the environment. It has been specially developed for surfactant determinations using potentiometric indication.

REQUIRED EQUIPMENT

A Titrator or Ion Meter.
Surfactant Electrode
Deionized water for standard and titrant preparation.
Pipet or burette for solution delivery.

REQUIRED SOLUTIONS

For Anionic Surfactant –
Titrant for anionic surfactants, hyamine 1622 (Benzethonium chloride), 0.05 M
pH adjuster solution – 0.1M HCL

For Cationic Surfactant –
Titrant for cationic surfactants, sodium lauryl sulphate, SLS, 0.05
pH adjuster solution – 0.1M HCL

For Polyacrylates –
Select titrant based on the requirement.
pH adjuster solution – 0.1M NaOH

For All solutions –
Sample Additive is diluted Triton X-100 to keep electrodes clean when added to all samples.
Use dilute HCl for acidic rinsing (0.01 M or less).
Use dilute NaOH for alkaline rinse (0.01 M or less).

GENERAL PREPARATION

ELECTRODE PREPARATION

This is a solid-state electrode, there is no need for filling internal filling solution etc. Prior to first usage, or after long term storage, just remove the protection cap from the bottom of the electrode and immerse the electrode in deionized water for thirty minutes. Connect the electrode to the meter.

PREPARATION, MAINTENANCE AND STORAGE OF THE SURFACTANT ELECTRODE

For long term storage, clean the electrode tip with de-ionised water and store the electrode dry with protection cap on. For short term storage (less than a week), it is fine to store the electrode in de-ionised water.

The electrode is best conditioned by two to three titrations whose results should be ignored.
Adherent deposits are removed with a lint-free tissue paper, moistened in methanol. In sample changer operation, the electrodes are dipped briefly in methanol, while stirring.

The electrode (PVC membrane) is not resistant towards almost any organic solvents. Chloroform, hydrocarbons, acetone, MIBK, tetrahydrofuran, etc. destroys the electrode.
High proportions of methanol (30 – 40%) or ethanol (20%) in the solvent shorten the lifetime of the electrode.

Several thousand titrations can be performed with the electrode under normal conditions. Evidence of a decrease in the responding behaviour of the electrode is shown in flatter titration curves and a shortened potential range. For a short time, such an electrode can be regenerated by submerging it for 30 min. in a sodium dodecyl sulphate solution (0.005 mol/L). If this does not help, the electrode must be re-generated (refer to re-generation section) or replaced.

TITRANT PREPARATION

The titrant concentration may need to be adjusted depending on the concentration of the sample and the method of titration in use.

For example, for the titration of anionic surfactants of around 0.5mM concentration, dilute the 0.05M Hyamine 1622 10-fold with De-ionised water solution, so the titrant concentration is 5mM.

Titrate against a known concentration of SLS to standardize the titrant. Calculate the exact concentration of the titrant using the same formula given above.

MEASURING HINTS

1. The sensing membrane is based on PVC, it can uptake water thus might appear milky or white. This has no effect on performance.

2. Constant, but not violent stirring is necessary for accurate measurement. To avoid foaming, slow stirring is recommended.

3. Check the electrode for air bubbles trapped to the membrane surface after immersion in solution. Agitate the electrode gently to remove any air bubbles.

4. A slow or sluggish electrode response may indicate surface contamination of the electrode membrane. Soak the electrode tip in deionized water for about five minutes to clean
the membrane.

SAMPLE REQUIREMENTS

1. To help keep the electrode clean and working properly, add sample additive, diluted Triton X-
100, to all samples. For every 50 ml of sample, use 1 ml of sample additive.

2. Samples should be diluted to help preserve electrode life, avoid foaming during the titration, and help improve long term results.

3. Anionic surfactants, as well as sulphated and sulfonated surfactants, may be titrated with Hyamine 1622. Adjustment to pH 2.5-4.5 should be done by addition of 0.01M HCl.

4. Polyacrylates should be adjusted to pH 10-11 with 0.1M NaOH before analysis.

5. Cationic surfactants should be titrated with an anionic reagent, such as Sodium Lauryl Sulfate,
after acidification to pH 3 with 0.01M HCl.

6. All samples and standards must be aqueous. They must not contain organic solvents.

ANALYTICAL PRCEDURES
Sample Analysis by Manual Titration (For an Automatic Titrator, follow the Titrator manual recommendations and settings)

The surfactant electrode is used as an endpoint indicator during titration. An example of the titration procedure is illustrated using the analysis of an anionic surfactant as an example.

1. Using the acid rinse solution, rinse the surfactant electrode and blot dry with a soft, lint-free
tissue before the titration.

2. Add 50 ml of the unknown sample to a 150-ml beaker. Add 3 ml of 0.01M HCl and 1 ml of the sample additive (diluted Triton X-100). Place the beaker on a magnetic stirrer, and start stirring at a constant and slow rate. Lower the electrode into the solution so that the tip is completely immersed and wait until the mV reading is stable, drift is +1 to 2 mV/minute, before adding any titrant. Remove any bubbles by re-dipping electrode.

3. Add 0.05M Hyamine 1622 titrant to a 10-ml buret until filled. Once mV stability has been
reached, add the titrant in 0.5-1.0 ml increments at the beginning of the titration, and in
increments of 0.1-0.25 ml in the region of the endpoint. The endpoint is at that volume of titrant
where the potential changes dramatically with the slightest addition of titrant. The electrode
potential should be recorded after each addition of titrant. Continue titrating until 1 or 2 ml past
the endpoint. On standard coordinate graph paper, plot milliliters of titrant added versus mV
reading. The endpoint is the point of greatest inflection.

Typical titration curve for cationic and anionic surfactants with Ionode solid-state surfactant electrode

Depending on the sample concentration and on the method used, this basic procedure may need to be modified.

1. The sensing membrane is normally subject to water uptake and might appear milky or white.
This has no effect on performance.

2. Constant, but not violent, stirring is necessary for accurate measurement. Slow stirring is
recommended to avoid foaming.

3. Check the electrode for air bubbles adhering to the membrane surface after immersion in
solution. Agitate the electrode gently to remove any air bubbles.

4. A slow or sluggish electrode response may indicate surface contamination of the electrode
membrane. Soak the electrode tip in deionized water for about five minutes to clean the
membrane.

5. To help keep the electrode clean and working properly, add sample additive (diluted Triton X-100) to all samples. For every 50 ml of sample, use 1 ml of sample additive.

6. Samples should be diluted to help preserve electrode life, help avoid foaming during the titration, and help improve long term results.

7. Anionic surfactants, as well as sulphated and sulphonated surfactants, may be titrated with Hyamine 1622. Adjustment to pH 2.5-4.5 should be done by addition of 0.01M HCl.

8. Polyacrylates should be adjusted to pH 10-11 with 0.1M NaOH before analysis.

9. Cationic surfactants should be titrated with an anionic reagent, such as Sodium Lauryl Sulfate,
after acidification to pH 3 with 0.01M HCl.

10. All samples and standards must be aqueous. No organic solvents are allowed.

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