Journal of the American Oil Chemists' Society

, Volume 94, Issue 2, pp 285–299

Bias and Imprecision in the Determination of Free Glycerin in Biodiesel: The Unexpected Role of Limitations in Solubility

  • Richard W. Heiden
  • Sigurd Schober
  • Martin Mittelbach
Original Paper

DOI: 10.1007/s11746-016-2930-6

Cite this article as:
Heiden, R.W., Schober, S. & Mittelbach, M. J Am Oil Chem Soc (2017) 94: 285. doi:10.1007/s11746-016-2930-6
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Abstract

Residual free glycerin (FG) is a critical marker of fatty acid methyl ester (biodiesel) fuel quality. Yet, routine determinations by standard methods display excessive imprecision, and the uncertainties undermine confidence in the data. Various degrees of agitation are used here to evaluate the possibility of heterophase formation as a contributor to imprecision and potential bias. Statistical markers from the analysis of 13 biodiesel samples reveal that seven contain unexpected heterophases, which, due to settling, cause underestimates of 15–68%. Agitation alone re-suspends heterophases for brief periods, easing potential biases, but also increases imprecision. A promising new sample pretreatment is presented, which reduces the deleterious effects of heterophases. Solubility limitations are explored as possible contributing factors, and measurements made at 23 °C in purified soy FAME reveal an equilibrium solubility which is more than 80% below previously published estimates. Strong interactions of liquid FG with small amounts of moisture reduce solubility to below international quality limits, while interactions of initially dissolved forms of moisture and FG produce bias-inducing heterophases at 0.02% FG. The unexpected low solubility of FG, exacerbated by interactions with impurities, is seen as an important contributor to underestimates and imprecision, and a strong influencing factor on the fate of residual FG in commercial biodiesel fuels.

Keywords

Biodiesel Glycerin Bias Solubility 

Abbreviations

0%D

FAME saturated with moisture at 40 °C

20% D

0%D FAME diluted to 20% v/v DDMF (100%D)

40%D

0%D FAME diluted to 40% v/v DDMF (100%D)

60%D

0%D FAME diluted to 60% v/v to DDMF (100%D)

80%D

0%D FAME diluted with 80% v/v to DDMF (100%D)

100%D

100% DDMF, moisture <50 ppm

95CI

95% Confidence interval

95CI1

95% Confidence interval, n = 1

95CI3

95% Confidence interval, n = 3

AG

Concentration of FG after agitation

AGQM

Arbeitsgemeinschaft Qualitätsmanagement Biodiesel, e.V.

ASTM

American Society for Testing and Materials

ASUB

An approximately 100.0 mg subsample of B100

ASUBINT

Time interval in seconds between ~100.0 mg subsamples

B100

Commercial Grade of FAME biodiesel

BSTFA

Bis-(trimethylsilyl)trifluoroacetamide

CDC

US Center for Disease Control

CEN

Comité Européen de Normalisation-European Committee for Standardization

CV

Coefficient of variation

DDMF

Distilled, dry, methanol free

EI

Electron ionization

EN

European normalization

EU

European Union

FAME

Fatty acid methyl esters: here refers to a research grade of biodiesel

FG

Free glycerin

GC

Gas chromatography

GC/MS

Gas chromatography/mass spectrometry

GC-FID

Gas chromatography-flame ionization detection

ILS

Interlaboratory study

IRMM

Institute for Reference Materials and Measurements

ISUB

An approximately 6.0000 g subsample for pretreatment by pyridine augmentation (PA)

ISUBINT

Time interval between 6 g intermediate subsamples

MG

Monoglyceride

MS1

Magnetic stirring with 2.5 cm stir bar, surface turbulence, no vortex

MS2

Magnetic stirring with 3.7 cm stir bar and 2 cm vortex

MSTFA

N-Methyl-N-trimethylsilyltrifluoroacetamide

NIST

National Institute of Science and Technology

P

Treatment procedure

PA

Pyridine augmentation at a level of 30.0000% of B100 ISUB weight

R

Reproducibility-between lab variations

r

Repeatability-within lab variations

REF

Results of six consecutive injections of a single derivatized subsample

SMP

Portion of a B100 process stream or fuel lot submitted to a lab in a container for analysis

STD

Standard deviation

STD2

Variance

SUB

A portion of an SMP for analysis by a test method

TMS

Trimethylsilyl

U

Unagitated-concentration of FG before agitation

US DOE

US Department of Energy

VA

Vigorous wrist agitation for 10 s

VA INT

10 s interval between agitation and 100 mg subsample

X

EU rejection limit

Copyright information

© AOCS 2016

Authors and Affiliations

  • Richard W. Heiden
    • 1
  • Sigurd Schober
    • 2
  • Martin Mittelbach
    • 2
  1. 1.R. W. Heiden Associates LLC, Laboratory/Burle Business ParkLancasterUSA
  2. 2.Institute of ChemistryUniversity of Graz-NAWI GrazGrazAustria

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