Measurement of Trace Elements in Biodiesel

These methods are adapted from methods for the determination of sodium and potassium in fatty-acid methyl ester samples. The sample is diluted with an organic solvent (xylene, cyclohexane or petroleum ether) plus a stabiliser.

The calibration standards which have a similar matrix to the prepared sample solutions, are prepared from sodium and potassium organometallic salts which are dissolved in a solution of xylene with a stabiliser.

These European Standards specify a method for the determination of potassium of equal to or greater than 0.5 mg/kg and sodium level equal to or greater than 1 mg/kg.  This method is applicable to fatty acid methyl esters intended to be added to mineral oil.

Principle

The vegetable oil methyl ester sample is diluted with a xylene solution and stabiliser.  (Note: xylene can be replaced by cyclohexane or light petroleum in laboratories which are not authorised to use aromatic solvents.)

Solutions

The calibration solutions used are prepared from organometallic salts dissolved in a mixture of xylene and stabiliser (potassium) or stock oil (sodium). The addition of a stabiliser or stock oil to the calibration solutions is necessary in order to improve storage life and linearity of calibration.

If possible all solutions should be kept in plastic, this is due to sodium plating out on the surface of glassware which will reduce the concentration and give an incorrect reading of sodium concentration.

Procedure

Potassium

Preparation of Stabiliser Solution

The stabiliser is supplied by CONOSTAN (CONOSTAN standard, CONOSTAN Division, Continental Oil Company, Ponca City, OK 74601, USA).

The stabiliser solution is prepared by diluting 20 g of stabiliser with xylene to a total volume of 200 ml (100 g/l).

Preparation of Sample

Weigh to within 0.001g approximately 5.0g of the ester sample into a 50 ml volumetric flask using a polypropylene pipette, add to the flask 5.0 ml of the prepared stabiliser solution and dilute to volume with xylene.

Preparation of Calibration Solutions

Prepare calibration solutions having the following potassium concentrations: 0.1mg/l, 0.2mg/l, 0.3mg/l and 0.5mg/l.  Using a variable volume pipette transfer 1.0ml, 2.0ml, 3.0ml and 5.0ml of the 5mg/l potassium solution to 50ml volumetric flasks.

With a polypropylene pipette add to each flask 5 ml of the 100g/l solution of stabiliser in xylene and dilute to mark with xylene.  Prepare the blank in the same manner without adding any potassium solution.

Prepare the calibration solutions just prior to the measurement as the prepared solutions are not stable when stored.

Sodium

Preparation of Stock Oil Solution

The Stock Oil 75 is supplied by CONOSTAN (CONOSTAN standard, CONOSTAN Division, Continental Oil Company, Ponca City, OK 74601, USA).

The stock solution is prepared by diluting 40g of the oil with xylene to a total volume of 200ml (200 g/l).

Weigh to within 0.001g approximately 5.0g of the ester sample into a 50ml volumetric flask using a polypropylene pipette and dilute to volume with xylene.

Sample should be diluted with xylene by a factor of at least 25 to ensure comparison of diluted sample and standard is valid.

Prepare calibration solutions having the following sodium concentrations: 0.3mg/l, 0.5mg/l and 1.0mg/l.  Using a variable volume pipettes transfer 3.0ml, 5.0ml and 10.0ml of the 5mg/l sodium solution to 50 ml volumetric flasks.

With a polypropylene pipette add to each flask 10 ml of the 200g/l solution of stock oil in xylene and dilute to mark with xylene.  Prepare the blank in the same manner without adding any of the sodium solution.

Preparation of Standard Graph

Set the flame photometer in accordance to MultiPoint/Single Ion Calibration found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium emission. Nebulise the working standard solutions and adjust the controls until steady zero and maximum readings are obtained. Nebulise the intermediate working standard solutions and construct a graph relating raw emission data (known as RAW in  BWB the flame photometer) to concentration of all the standard solutions.