Determination of Carbonyl Functional Groups in Bio-oils by Potentiometric Titration: The Faix Method

Here we present a potentiometric titration technique for accurately quantifying carbonyl compounds in pyrolysis bio-oils.

The overall goal of this procedure is to measure the complete carbonyl content of various biomass-derived materials by titrimetric methods. This method gives insight into the reactive carbonyl compounds contained within biomass-derived products such as pyrolysis oils and the effect that those compounds have on the aging of bio-oils. This method uses smaller sample sizes, less toxic materials, is more precise and more accurate than previously established methods.

To begin this procedure, prepare the reagent solutions as outlined in the text protocol. Dry a sodium carbonate primary standard in an oven at 105 degrees Celsius overnight. After this, allow the sodium carbonate to cool to room temperature.

Weigh 100 to 150 milligrams of the dried sodium carbonate into a metal pan. Transfer it to a titration vessel and record the actual weight. Then add a stir bar and enough water to cover the pH electrode bulb and the junction.

Using an automatic titrator, titrate with the hydrochloric acid solution to the endpoint which is the inflection point on the titration curve. Repeat this titration twice using a new sodium carbonate sample each time to obtain a total of three endpoint values. To begin preparing blank A, add 5 millimeters of DMSO to a five milliliter vial with a spin vane.

Next, add two milliliters of the prepared hydroxylamine hydrochloride solution and two milliliters of the prepared triethanolamine solution. Cap the vial tightly then place the vial in a block heater preheated to 80 degrees Celsius. Stir for two hours.

After stirring is complete, transfer the sample to a titration vessel. Wash the reaction vial several times separately with ethanol and water such that the final solution is 80%ethanol. Then use an automatic titrator to titrate with the acid solution to the endpoint.

Weigh out approximately 100 milligrams of 4-BBA to a five milliliter vial. Record the actual weight then add a spin vane. Add 5 milliliters of DMSO.

After this, dissolve the sample in two milliliters of the prepared hydroxylamine hydrochloride solution. Add two milliliters of prepared triethanolamine solution. Next, tightly cap the vial and place it in a block heater preheated to 80 degrees Celsius.

Stir for two hours. After stirring is complete, remove the sample from the heat block and allow it to cool. Then transfer the sample to a titration vessel.

Wash the reaction vials several times separately with ethanol and water such that the final solution is 80%ethanol. Then use an automatic titrator to titrate with the acid solution to the endpoint. Repeat this validation twice to obtain three endpoint values.

First, weigh out approximately 100 milligrams of the bio-oil sample to a five milliliter vial. Record the actual sample weight. After this, add a spin vane.

Add 5 milliliters of DMSO. Next, dissolve the sample in two milliliters of the prepared hydroxylamine hydrochloride solution. Add two milliliters of prepared triethanolamine solution.

Then cap the vial tightly and stir at 80 degrees Celsius for two hours. After stirring is complete, transfer the sample to a titration vessel. Wash the reaction vial several times separately with ethanol and water such that the final solution is 80%ethanol.

Using an automatic titrator, titrate with the hydrochloric acid solution to the inflection point on the titration curve. Record this endpoint and repeat the bio-oil analysis two additional times to obtain a total of three endpoint values. After this, analyze the data as outlined in the text protocol.

In this study, a potentiometric titration technique is used to accurately quantify carbonyl compounds in pyrolysis bio-oils. Representative titrations for both a raw bio-oil sample and a blank titration are shown here. The first derivative is plotted to easily identify the inflection point in each titration curve.

These values are then used to calculate the total carbonyl content in the pyrolysis oil samples. The hydrochloric acid titrant concentration has been determined to be 07032 moles per liter. Average acid consumption of the three blanks has been determined to be 13.0085 milliliters while the corresponding endpoint for the first titration has been determined to be 4.8909 milliliters.

Subtracting the two volumes dividing by the weight of the sample and multiplying by the acid concentration gives a carbonyl content of 4.9724 millimoles per gram for the bio-oil sample. The traditional Nicolaides method of bio-oil analysis is performed at room temperature with a typical stirring time of 18 to 24 hours which is insufficient for complete reaction of all carbonyls present in this sample due to steric hindrance of some carbonyls. Heating the mixture drastically increases the reaction rate, but the volume of the reagents for the Nicolaides method required can be difficult to stir in heat reliably.

The presented Black/Faix method however provides more accurate results more quickly and utilizes fewer materials. Here, hydroxylamine hydrochloride is present in excess and unconsumed hydroxylamine hydrochloride has no impact on titration. This method also includes multiple independent samples per test to check against titration errors.

This is a quick and easy method to perform. Up to 11 unique samples can be analyzed in less than an eight-hour shift. Once the procedure starts, it's important to complete the titration as quickly as possible.

Triethanolamine can form if the samples are allowed to stand overnight which leads to inaccuracies. This video should provide you with the tools necessary to perform oxidation reactions on carbonyl constituents and biomass-derived materials. This method is rapid, accurate and uses relatively low-hazard reagents.

While performing this procedure, be aware that pyrolysis oils, hydroxylamine hydrochloride, triethanolamine and ethanol all have hazards associated with them. Personal protective equipment such as a lab coat, gloves and safety glasses should be worn while using these chemicals.