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The colloidal carbon material, carbon black, is created via the incomplete combustion of heavy petroleum products. Carbon black typically exists in a spherical form with aggregate sizes less than 1000 nm.

The iodine adsorption number (in mg/g of carbon) represents the quantity of iodine that may be adsorbed on the surface of a specific mass of carbon black.

This number is dependent on surface porosity, meaning that it is beneficial in the characterization of carbon black’s surface area. This value is determined in accordance with ASTM D 1510-16.1

ASTM D 1510-16 dictates that a specified and known amount of iodine should be added to the carbon black before the solution mixture is centrifuged and shaken.

Next, excess iodine is titrated with standard sodium thiosulfate solution in order to ascertain the amount of adsorbed iodine.

Image Credit: Mettler-Toledo - Titration

1. Sodium thiosulfate, Na2S2O3, c(Na2S2O3) = 0.0394 mol/L:

Using a 1000 mL volumetric flask, transfer a total of 9.78125 g of Na2S2O3 in 250 mL of deionized water. Next, add 5 mL of Amyl alcohol to the flask before mixing this well and diluting it up to mark using additional deionized water.

2. Iodine solution, 1/2 I2, c(I2) = 0.04728 mol/L:

Add a total of 57.0 g of KI in 1000 mL to an amber-colored volumetric flask which contains 100 mL of deionized water. Next, weigh and transfer 6.0 g of I2 to this flask before mixing this well and diluting it up to mark with additional deionized water.

3. Potassium iodate/iodide solution, 1/6 KIO3/KI, c(KIO3/KI) = 0.0394 mol/L:

Transfer a total of 45 g of KI in 200 mL deionized water into a 1000 mL volumetric flask. Next, add 1.4054 g of KIO3 – that had been previously dried at 125 °C for 1 hour - to the flask before mixing this well and diluting it up to mark with additional deionized water.

Slowly and carefully add a total of 7.0 mL of concentrated H2SO4 to a 250 mL volumetric flask containing 90.0 mL of deionized water. Next, swirl the flask and mix this well before diluting it up to mark with deionized water. Allow this solution to cool to room temperature.

The carbon black sample should be dried for a total of 1 hour in a gravity convection oven. This should be done at 125 °C using an open petri dish. Finally, this should be cooled to room temperature in a desiccator.

6. Standardization of Sodium Thiosulfate, 0.0394 mol/L:

The titer should be performed using a standard solution of 0.0394 mol/L 1/6 KIO3/KI.

7. Standardization of Iodine solution, 1/2 c(I2), 0.04728 mol/L:

A total of 0.04728 mol/L 1/2 I2 solution should be pipetted into the titration beaker before adding 40 mL of deionized water. This should be titrated against the standardized 0.0394 mol/L Na2S2O3 solution.

It is important to store the titer of I2 solution in the titrator to function as the auxiliary value H[Normality of Iodine].

A total of 5 mL of 0.04728 mol/L 1/2 I2 solution should be pipetted into the titration beaker before 40 mL of deionized water is added. This should then be titrated against the standardized solution of 0.0394 mol/L Na2S2O3.

The dried sample should be weighed in a 15 mL centrifuge tube according to the table below:

A total of 10 mL of 0.04728 mol/L 1/2 I2 solution should be pipetted into the centrifuge tube containing the sample prior to capping this. The tube should be kept in the mechanical shaker and shook for a total of 1 minute at a minimum of 240 strokes per minute.

The solution should be centrifuged immediately at 8000 rpm for 3 minutes before decanting and capping the supernatant solution into a clean and dry flask.

A total of 5 mL of this sample solution should be pipetted into a titration beaker before adding 40 mL of deionized water. This should then be titrated with a standardized solution of 0.0394 mol/L Na2S2O3.

2 Na2S2O3 + I2 → Na2S4O6 + 2 NaI

A. Standardization of c(Na2S2O3)  = 0.0394 mol/L

Titer value obtained=0.99819, n=6, s=0.00195; srel=0.195 %

B. Standardization of Iodine solution, 1/2 c(I2), 0.04728 mol/L:

C. Determination of Blank Iodine:

It is important to note that this application does not replace ASTM D1510-16.

ASTM D 1510-16 states that varying volumes of iodine solution and sample mass are only permitted in instances where a constant iodine solution to sample mass ratio is maintained according to the sample size table.

It is important that clean and dry glassware is used and that the iodine (I2) solution is protected from direct exposure to light, ideally stored in an amber-colored bottle. Standardization of I2 solution should be performed daily.

The solution should be immediately decanted after centrifugation because failure to do so may change the iodine adsorption number. Should KI solution be oxidized by air, this may appear yellow due to the release of iodine. It is therefore important to consistently use freshly prepared KI solution.

Iodine adsorption number, I (mg/g of carbon black) as per ASTM D1510-16:

The formula employed in the titrator is:

R= ((B[Blank Iodine in mL]-VEQ)/B[Blank Iodine in mL])*sf1*H[Normality of Iodine]*C/m  Constant, C = M/z

The sample factor (sf1) should be added to the formula to illustrate the volume of I2 solution added to the sample (V). Normality of I2 solution (N) is then added from the standardization of iodine solution – the latter being saved as an auxiliary value H.

It is also important to note that this application method has been developed for the specifically mentioned sample, and it may be appropriate to further optimize the method for different samples. It is possible to automate the application using the InMotion autosampler.

The solution should be neutralized prior to disposal.

This information has been sourced, reviewed and adapted from materials provided by Mettler-Toledo - Titration.

For more information on this source, please visit Mettler-Toledo - Titration.

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