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Wort–15
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MAGNESIUM BY ATOMIC ABSORPTION
SPECTROPHOTOMETRY
Application of atomic absorption spectrophotometry
(AAS) in the analysis of beer for iron, copper, calcium,
sodium, potassium, and magnesium is described in
Beer-18B, Beer-19C, Beer-20C, Beer-36, Beer-37,
and
Beer-38A,
respectively. Its use has been extended to the
analysis of wort for magnesium and zinc (Wort-16).
Reagents
(a)
Hydrochloric acid,
concentrated; 1:1 (v/v); 1%
(v/v).
(b)
Magnesium stock solution,
1,000 mg/L. Dissolve
1.000 g magnesium ribbon in minimum amount of
hydrochloric acid (1:1). Make to 1 L with 1% (v/v)
hydrochloric acid.
(c)
Lanthanum solution,
5%. Wet 58.64 g La
2
O
3
with
distilled water. Add 250 mL concentrated HCl very
slowly until material is dissolved, cool to room tem-
perature, and make to 1 L with distilled water.
(d)
Reagent water
(resistivity 18 MΩ or greater).
Apparatus
(a)
Atomic absorption spectrophotometer.
(b)
Magnesium hollow cathode lamp.
(c)
Volumetric flasks.
(d)
Pipets.
Instrument Parameters
Set atomic absorption spectrophotometer according to
manufacturer’s specifications. Use an oxidizing (lean,
blue) air/acetylene flame. Set the wavelength (near 285.2
nm) at the maximum photomultiplier response.
Calibration
Prepare a 10 mg/L magnesium standard solution by
pipetting 10 mL of the 1,000 mg/L magnesium stock
solution (reagent b) into a 1-L volumetric flask and dil-
uting to 1 L with distilled water.
Prepare standards containing 0, 0.1, 0.2, 0.5, 0.7, 0.8,
and 1.0 mg/L magnesium by pipetting 0, 1, 2, 5, 7, 8,
and 10 mL of the 10 mg/L magnesium standard solution
into 100-mL volumetric flasks. Add 20 mL lanthanum
solution (reagent c) to each flask, and make to 100 mL
with distilled water. Set the atomic absorption spectro-
photometer to zero absorbance with the standard
containing only lanthanum. Measure absorbance of re-
maining standards and prepare a standard curve relating
absorbance to concentration of magnesium ion in mg/L
of the zero to 1.0 mg/L standards (see Note).
Method
Prepare samples as described under
Wort-1.
Pipet 1.0
mL of wort into a 100-mL volumetric flask. Add 20 mL
lanthanum solution (reagent c) and make to 100 mL with
distilled water.
Set the atomic absorption spectrophotometer to zero
absorbance with the standard containing only lanthanum.
Aspirate each diluted sample and record absorbance.
Read concentration of Mg
2+
from standard curve and
multiply by 100 to obtain the concentration in the origi-
nal wort sample.
Example
Diluted wort (1–100) gives absorbance indicating,
from the standard curve, a concentration of 0.89 mg/L
Mg
2+
, which for the undiluted wort becomes 89.0 mg/L.
Precision
Based on collaborative study (1), analyses of worts
having grand means of 99.0 and 112.4 mg/L magnesium
ion yielded coefficients of variation (CV), based on a
combined-laboratory error (S
c
), of 5.82 and 8.61%,
respectively.
Note
Dilution of the wort being tested is adequate to permit
use of a water solution of magnesium chloride to prepare
the standard curve, precluding the need for standard
addition curves as in
Wort-16, Beer-18B, Beer-19C,
and
Beer-20C.
Reference
1. American Society of Brewing Chemists. Report of Subcommittee
on Atomic Absorption Spectrophotometry.
Journal
38:116, 1980.
1980, rev. 2010
doi: 10.1094/ASBCMOA-Wort-15

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