Colloid titration was first proposed by Hiroshi Terayama in 1948 and has been developed primarily in — Organic Chemistry Chemistry Question
Colloid titration: titration of a cationic surfactant with polyanion
Colloid titration was first proposed by Hiroshi Terayama in 1948 and has been developed primarily in Japan. Colloid titration is based on the interactions of charged colloids (charged polymers, polycations, and polyanions) with inversely-charged colloids (charged polymers and molecules capable of forming colloids). The reactions employed for colloid titration entail rapid precipitation, which leads to a cloudy suspension. Precipitation is completed when the positive and negative charges are equal. The end point of colloid titration is detected by changes in the color of dyes upon interaction with colloids—typically the adsorption and desorption of dyes on colloids as well as the association of dyes with colloids. Colloid titration can be employed for the quantitative analysis of various colloidal species, such as surfactants, as well as for the determination of the charges of natural colloids, such as clay particles and humic acid.
In this experiment, you will determine the amount of a cationic surfactant, benzyldimethyltetradecylammonium chloride, using standard potassium poly(vinylsulfate) (PVSK). The end point is detected using bromochlorophenol blue (BCPB). BCPB is initially associated with the cationic surfactant, and its color changes upon release from the cationic surfactant, which completely interacts with PVSK at the end point.
Chemicals and reagents
* acetic acid (c = 0.25 mol dm-3)
* benzyldimethyltetradecylammonium chloride (Zephiramine, Zeph; preferably with a concentration ranging from 5×10-5 to 8×10-5 mol dm-3)
* bromochlorophenol blue (BCPB; indicator)*)
* potassium poly(vinylsulfate) solution (PVSK; titrant)) (c = 0.0025 mol dm-3)
* sodium hydroxide (NaOH) solution, c = 0.01 mol dm-3
________
*) Prepare by dissolving the required amount of BCBP in 5 cm3 of NaOH solution (c = 0.01 mol dm–3 ) and diluting it with 200 cm3 of water.
) The concentration is defined as being that of its monomer unit, C2H3SO4K. Its equivalent weight is 162.2, provided that the purity (degree of esterification) of PVSK is 100%.
Apparatus and glassware
* beaker (300 cm3)
* burette (25 cm3)
* conical beaker (200 cm3)
* volumetric pipette (10 cm3)
Procedures
1. Dissolve 0.048 g of BCPB in 5 cm3 of a NaOH solution (c = 0.01 mol dm–3) in a 300 cm3 beaker, and dilute it with 200 cm3 of water. The final concentration of BCPB should be ~ 4×10-4 mol dm–3.
2. Take precisely an aliquot (10 cm3) of the Zeph sample solution with a concentration at about 1.0×10–4 mol dm–3 and add it to a 200 cm3 conical beaker using a volumetric pipette; then add 1 cm3 of 0.25 mol dm–3 acetic acid and 1 cm3 of the BCPB solution. Check the pH of the solution using pH test paper. If the pH is not around 3.3, add acetic acid or NaOH solution to achieve the desired pH of 3.3.
3. The initial colour is blue. Carefully drop the standard PVSK solution into the sample solution while stirring continually. Before the end point is reached, the solution may become somewhat cloudy. If the solution becomes cloudy, drop the standard PVSK solution in slowly. At the end point, the colour should change to yellow.
4. Read the volume of the PVSK solution added.
Calculate the Zeph concentration in mg dm–3. The reaction ratio of the monomer unit of PVSK to Zeph is 1 : 1.
Why does the solution become cloudy, as the end point is approached? Why does the solution become clear again after the end point is reached?
Model Answer
When the PVSK poly anion aqueous solution is added to the Zeph cation aqueous solution, both ions associate quantitatively through electrostatic interaction and yield non-polar ion pair species. In the early stage of titration with a large amount of free Zeph cations (known as a surfactant), the ion pairs are surrounded by Zeph cations to form cationic micelles. Near the equivalence point, the solution becomes cloudy due to generation of a precipitate of aggregates of the ion pairs, since the amount of free Zeph cations is not sufficient to maintain the micelles. Beyond the equivalence point, when an excessively large number of PVSK anions is present in the solution, the solution becomes clear again, since the ion pairs are surrounded by PVSK anions and can be dispersed into the solution once more as anionic colloids.