DOI:
10.1039/A906307B
(Paper)
Analyst, 2000,
125, 221-225
Direct spectrophotometric determination of aluminium
oxide in Portland cement and cement clinker. An insight into the solution
equilibria and analytical aspects of the aluminium–quinizarin
system
Received 3rd August 1999, Accepted 8th November 1999
First published on UnassignedUnassigned7th January 2000
Abstract
A spectrophotometric study of the complexation reaction
between Al3+ and quinizarin (QUIN) was carried out to ascertain
the suitability of the complex formed for direct spectrophotometric
determination of aluminium. The absorbance at 550 nm, due to the
aluminium–QUIN complex, formed at pH 3.8, is recommended for the
determination of the alumina content of Portland cement and cement clinker.
The proposed method is simple and rapid and possesses reasonable
selectivity. Interference of iron(III), generally present in
Portland cement, is eliminated by addition of ascorbic acid. The results
obtained for several SRM Portland cement samples and for a variety of
cement materials demonstrate that the proposed method allows the precise
and accurate determination of Al2O3 content over the
concentration range 1.4–5.75 μg mL−1 of
aluminium. The determination of Al can be carried out successfully in the
absence of a masking agent by using first-derivative spectrophotometry.
Verification and the use of control charts in the spectrophotometric
determination of Al is achieved. The record of the verifier response during
routine operation establishes that the method is being maintained in
statistical control.
The determination of Al2O3 according to
the current ASTM method1 for chemical
analysis of hydraulic cements is lengthy and the final results obtained for
aluminium oxide are subject to the summation of possible errors involved in
the determination of the total oxides and the oxides of iron, phosphorus
and titanium.Over many years, many analysts and ASTM members have expressed the
opinion that ‘ASTM Methods for Chemical Analysis of Hydraulic
Cement’ (C-114) needs a direct method for determining alumina
(Al2O3) rather than determining it by difference as
in the present scheme of chemical analysis. This ‘by
difference’ approach has generated considerable controversy over the
years as to just what was to be subtracted from the ‘ammonium
hydroxide group’ (R2O3) to determine
‘Al2O3’ and what was to be used in
calculating the ‘Bogue potential compounds’2 both for research and for specification
purposes.
The ASTM specification for Portland cement (C-150)3 contains maximum limits for the aluminium oxide
and tricalcium aluminate contents for some types of cements. When an
accurate Al2O3 value is needed to meet specification
requirements or for other purposes, the interference caused by
P2O5 and TiO2 must be eliminated. This may
be done by either (1) determining these compounds separately and
calculating the Al2O3 content by difference or (2)
determining the Al2O3 content directly by a procedure
that eliminates the interfering components. Possible confusion exists
between ‘Specification C-150’ and ‘Test Methods
C-114’ of the ASTM Standards for Portland cement with regard to
calculation of Bogue potential compounds required in ‘Specification
C-150’ for the determination of conformance to specifications.
‘Test Methods C-114’ allow any method of demonstrated precision
and bias to be used, and consequently any method may be used which
legitimately obtains the Al2O3 value for the purpose
of calculating the potential mineralogical composition of the cement
clinker.
Numerous spectrophotometric methods for aluminium determination have
been published, many of which are not simple and usually have low
selectivity. The reagents most frequently used are 8-hydroxyquinoline,
aluminon, Eriochrome Cyanine R, Chrome Azurol S and stilbazo.4,5 Also, Hydroxy Naphthol Blue6 and morin7 have
been used for the determination of aluminium.
Although various techniques are utilized for the composition analysis of
cement, spectrophotometry continues to enjoy wide popularity. The common
availability and low cost of instrumentation, the simplicity of procedures
and the accuracy of the technique make spectrophotometry advantageous for
several investigations.1,8–12 Based on preliminary results on the reactivity
of quinizarin toward metal ions13 and
previous studies in this laboratory,9 this
reagent was chosen for a detailed spectrophotometric equilibrium study in
aqueous ethanol, the aim being to develop a rapid method for the direct
determination of the Al2O3 content of Portland
cement. An insight into the complex-forming equilibria in solution and the
analytical characteristics of the Al–quinizarin complex is given. The
proposed method shows considerable promise with respect to accuracy, and
possesses reasonable selectivity. It saves a lot of time in determining
aluminium oxide in Portland cement and could replace the present test
methods for alumina determination.
Experimental
Chemicals and solutions
A 10−3 mol L−1 stock standard solution
of quinizarin (1,4-dihydroxyanthraquinone) (QUIN) was prepared by
dissolving an accurately weighed amount of Aldrich (Gillingham, Dorset, UK)
pure grade reagent in absolute ethanol. A 10−2 mol
L−1 stock standard solution of aluminium nitrate was
prepared using the AnalaR grade product (E. Merck, Darmstadt, Germany). The
metal content of the solution was determined by conventional
methods.14 Solutions of perchloric acid,
sodium perchlorate, sodium acetate and, L-ascorbic acid and
standard sodium hydroxide solution were all prepared from
analytical-reagent grade reagents. Solutions of diverse ions used for
interference studies were prepared from AnalaR nitrate and chloride salts
of the metal ions and potassium or sodium salts of the anions to be
tested.Cement samples
National Institute of Standards and Technology (NIST; Gaithersburg, MD,
USA) Standard Reference Materials (SRMs) 1880, 1881, 1885, 1886 and 1889
were used as the Portland cement matrix in this study. Precautions for
handling and use were taken in accordance with the instructions15 on the NIST data sheet and the certificate of
analysis of percentage constituents. Other samples of ordinary Portland
cement, clinker, clay and raw meal were supplied by Assiut Cement (Assiut,
Egypt) and analysed using XRF spectrometry in the Assiut Cement XRF
laboratories or F. L. Smidth & Co. A/S (Copenhagen, Denmark).Dissolution of cement sample.. Weigh accurately 0.3–0.4 g of the sample (dried at 110 °C)
into a beaker and dissolve it in the minimum volume of hydrochloric acid.
Heat to dryness, add 10 mL of HCl (6 mol L−1) to the
residue, digest and filter the insoluble residue into a 100 mL calibrated
flask and then dilute to volume with doubly distilled water.
Procedures
(a) Ordinary spectrophotometry (procedure A).. Into a 25 mL calibrated flask transfer 6.5 mL of 10−3
mol L−1 QUIN solution and 6 mL of pure ethanol to ensure a
final ethanol content of 50% v/v. Adjust the pH to 3.8 using perchloric
acid (10−2 mol L−1). Add a suitable
volume of aluminium(III) solution containing <0.15 mg of
aluminium. Dilute to volume with doubly distilled water and measure the
absorbance of the solution at 550 nm against a reagent blank as the
reference.
(b) First-derivative spectrophotometry (procedure
B).. Operate as described above and record the first derivative spectrum from
750 to 500 nm against a reagent blank at a scan speed of 240 nm
min−1 and a slit width of 2 nm. A calibration curve
covering the range 0.35–6.75 μg mL−1 of aluminium
was established.
(c) Determination of aluminium(III) oxide
content in Portland cement.. Weigh accurately 0.3–0.4 g of the sample (dried at 110 °C)
into a beaker and prepare the sample solution as indicated earlier.
Transfer a 2.5–5 mL aliquot of the sample solution into a 25 mL
calibrated flask, add 6.5 mL of QUIN (10−3 mol
L−1) and 0.25 mL of ascorbic acid (10−1
mol L−1), then add 6 mL of pure ethanol. Adjust the pH to
3.8 by the addition of sodium acetate (1 mol L−1) and
dilute to volume while keeping final ethanol content at 50% v/v. Measure
the absorbance of the solution at 550 nm against a reagent blank as the
reference. If the first-derivative spectrophotometric method is used,
prepare the test solution as above but without addition of ascorbic
acid.
Apparatus
A Perkin-Elmer (Norwalk, CT, USA) Lambda 40 double beam
spectrophotometer was used for ordinary and first-derivative spectral
measurements using 1 cm matched quartz cells. pH values were measured using
a Radiometer (Copenhagen, Denmark) M 201 pH meter equipped with a
Radiometer combined glass electrode. The pH meter was calibrated regularly
before use with standard buffer solutions and the pH values in
water–ethanol medium were corrected as described elsewhere.16Results and discussion
Acid–base properties of the reagent
The QUIN reagent yields three coloured acid–base forms in
solutions of pH ≡2–11.2: LH2, LH−
and L2−. There is a pronounced transformation from the
yellow form (LH2) to the orange–red species
(LH−) at pH 8.2–9.0. The red form is converted into
the violet from (L2−) at pH > 10.8. Distinct isosbestic
points are observed for the particular acid–base equilibrium. The
protonation scheme of this reagent indicates that gradual association of
protons with the oxygen atoms of the bis(hydroxy) substituents occurs at pH
⩽ 10.8 and pH ⩽ 8.2. The absorbance versus pH graphs were
interpreted assuming that a particular equilibrium is established under
selected conditions. Under our experimental conditions,
pKa1 (LH2/LH−) =
8.5 ± 0.04 and pKa2
(LH−/L2−) = 10.65 ± 0.02
(I = 0.1, 25 °C).Complexation equilibria of Al3+ with
QUIN
The absorption spectra of the Al–QUIN system were recorded as a
function of pH in the presence of an excess of metal ion, in equimolar
solutions and in solutions with an excess of reagent. In acidic medium with
pH 3.8, the solution spectra have the same shape and exhibit the
characteristic double absorption maximum in the wavelength region
510–550 nm corresponding to the formation of the Al3+
complex with QUIN.The absorbance versus pH graphs of the Al–QUIN solution
with a concentration excess of aluminium were plotted at different
CM/CL ratios. All the graphs,
including those obtained for solution with excess of reagent, have the same
shape with a single formation branch in the pH range 2.5–4.0. The
already tested graphical logarithmic analysis of the absorbance curves for
the treatment of spectrophotometric data was employed.17–20
Absorbance versus pH graphs for the solutions investigated were
interpreted using the transformations given in Table 1.
Table 1 Summary of transformations useda
Conditions | Equilibrium/transformation |
---|
Symbols used: Z = 1 +
[H]/Kai, ΔA =
A−AL, differences in the overall
absorbances and absorbance of the reagent blank under the same conditions.
CL and CM = total concentration of
the ligand and metal ion, respectively. |
---|
M + LH2(εL2)
⇌ MLH(ε1) + H+ | (A) |
CM >
CL | CL/ΔA =
1/ε1 + ΔA[H]/A*K
ε1CM | (1) |
Log
[ΔA/(ε1CL−A
)] = pH + log CM + log *K | (2) |
CL
≈
CM | CM/ΔA =
1/(ε1−AL/CL)
+[H]
Z/(CL−A/ε1)
ε1 *K | (3) |
Log {ΔA
(ε1−AL/CL)
Z/[CM(ε1−A
L/CL)− |
ΔA/(CL−A/
ε1)]} = pH + log *K | (4) |
CL >CM | CM/ΔA =
1/ε1 + [H] Z/*K
ε1CL | (5) |
Log [ΔAZ/(ε1CM−ΔA
)] = pH + log CL + log *K | (6) |
MLH (ε1) ⇌ ML
(ε2) + H+ | (B) |
CM >
CL | CL/ΔA =
1/ε2 +
(ΔA−ε1CL)[H]/
ΔAKKa1ε2 | (7) |
Log
[(ΔA−ε1CL)/(
ε2CL−ΔA)] = pH +
log KKa1 | (8) |
CL >
CM | CM/ΔA =
1/ε2 +
(ΔA−ε1CM)[H]/
ΔAKKa1ε2
| (9) |
CM/ΔA =
1/ε1−(ε2CM
−ΔA)
KKa1/ΔA[H]
ε1 | (10) |
Log
[(ΔA−ε1CM)/(
ε2CM−ΔA)] = pH +
log KKa1 | (11) |
MLbHc(ε1
) + sLH2
⇌
MLnHz(ε2) +
qH+ | (C) |
CL >
CM | CM/ΔA =
1/ε1 +
(ΔA−ε2CM)
CLsK/ΔA
[H]qZsε1
| (12) |
CM/ΔA =
1/ε2 +
(ΔA−ε1CM)[H]
qZs/ΔAK
ε2CLs | (13) |
Log
[(ΔA−ε1CM)Z
s/(ε2CM
−ΔA)] = q pH + s log
CL + log
*K | (14) |
For the complexation equilibria in solutions with excess of
Al3+, the best agreement with the experimental conditions was
found for equilibrium (A) involving the molecular form of the reagent
LH2 and the formation of an Al–LH2+ complex.
This complex was also established in solutions with excess reagent or in
equimolar solutions. Equilibrium (B) was tested in solution with an excess
of one component using eqns. (7)–(11). Also, the equilibrium
representing complex transition (C) in solutions with excess of ligand was
tested using transformations (12)–(14). The results obtained indicate
no evidence for the existence of the deprotonation equilibrium (B) or the
formation of a biligand complex species in accordance with equilibrium (C).
The stoichiometry of the Al–QUIN complex was further verified by the
method of continuous variations. In solutions having Co
= CM + CL = 3.0 ×
10−4 mol L−1 and at pH 3.8, a component
ratio of 1:1 (metal to ligand) was obtained at 550 nm.
As far as the complexation equilibria are concerned, the complex
formation of QUIN with Al3+ does not compete with the
deprotonation equilibrium of the free ligand under the present experimental
conditions. According to our results, the only reacting species of the
reagent is the molecular form (LH2), which is the prevalent one
at the pH employed, and hence any contribution from the anionic form of
QUIN in the complexation reaction can be precluded.
The calculated values of the equilibrium constant (log
*K11) and stability constant (log
*β11) of the complex-forming reaction (A) were
found to be –0.54 (+ 0.02) and 8.1 (+ 0.03), respectively.
Analytical characteristics of the method
Under the optimum conditions, a linear calibration graph for the
Al–QUIN system was obtained up to a concentration of 6.75 μg
mL−1 of Al with a molar absorptivity of 3.15 ×
103 L mol−1 cm−1 at 550 nm. A
Ringbom plot showed that the optimum concentration range for the
determination of aluminium was 1.5–6.6 μg mL−1.
Sandell’s sensitivity of the reaction was found to be 11.7 ×
10−3
μg cm−2. The reproducibility of
the method was checked by analysing a series of five solutions with an Al
concentration of 2.7 μg mL−1. The relative standard
deviation (RSD) was found to be 0.96%.Effect of diverse ions
To assess the usefulness of the proposed method, the effects of diverse
ions that are often associated with Al3+ were studied. Aluminium
was then determined as Al–QUIN under the optimum conditions as
described in the given procedure. The tolerance of the method to foreign
ions was investigated with solutions containing 0.1 mg of Al per 25 mL and
various amounts of foreign ions. The tolerance criterion for a given ion
was taken to be the deviation of the absorbance values by more than
±2% from the expected value. The determination of aluminium as the
Al–QUIN complex was possible in the presence of Ti4+,
V5+, Cr3+, Zn2+, Zr4+,
Mo6+, Pd2+, Cd2+, Pb2+,
Hg2+, W6+ (about 2.5 mg) and of Cr3+,
Co2+, Ni2+, Cu2+ (about 4 mg). The
presence of 12 mg of any of the alkali or alkaline- earth metal ions
(Mg2+ , Ca2+, Si2+, Ba2+),
Cl−, Br−,
SO42−,
B4O72− and
PO43− had no effect on the procedure. The
existence of Fe3+ caused a serious interfering effect on the
determination of Al. In such a case, the solution spectrum of the
Al–QUIN system reveals an absorption band (at 590 nm) overlapped with
the characteristic double-maximum absorption of the Al–QUIN
complex.The reagent QUIN reacts with Fe3+ in 50% v/v ethanol at pH
2–3.5 to form a blue–violet complex with a characteristic
absorption maximum at 590 nm. According to our results, a monoligand
Fe–QUIN complex is formed in solution under the present experimental
conditions. The solution spectra of the iron(III)–QUIN
system in the presence of excess metal ion, in equimolar solutions and in
the presence of excess reagent were recorded. The analysis of the
absorbance versus pH graphs in the pH range studied indicated the
best fit for equilibrium (D):
| Fe3+ + LH2 FeLH2+ +
H+(D) |
(1)
|
The results obtained for several sample solutions indicate that
Al
3+ can be determined successfully in the presence of
Fe
3+ if the latter is masked with
L-ascorbic acid
(0.01 mol L
−1), whereby the absorption band due to
iron(
III)–QUIN complex at 590 nm is completely eliminated
(
cf.,
Fig. 1)
 |
| Fig. 1 Curves a and b, solution spectra of a cement sample (10 mg per 25 mL)
with QUIN (2.5 × 10−4 mol
L−1), pH 3.8, 50% v/v ethanol, in the absence of ascorbic
acid (a) and in the presence of 10−3 mol
L−1 ascorbic acid (b); curve c, absorption spectrum of
Al3+–QUIN complex (CL =
CM = 2.5 × 10−4 mol
L−1 , pH 3.8, 50% v/v ethanol); curve d, absorption
spectrum of 10−4 mol L−1 QUIN at pH 3.8,
50% v/v ethanol. | |
First derivative spectrophotometric determination of
aluminium
In the wavelength range 540–600 nm, the derivative spectrum of the
Fe3+ complex reveals an insignificant amplitude (approaching
zero), whereas that of the aluminium complex has a trough at 545 nm and a
peak at 560 nm. The presence of Fe3+ has no effect on the
first-derivative spectrum of the Al–QUIN complex in the wavelength
range 540–600 nm and, accordingly, this range seems to be suitable
for the determination of aluminium in the presence of iron(III),
using peak-to-trough measurement. The first-derivative spectra of a series
of solutions containing the reagent QUIN (2.5 × 10−4
mol L−1), increasing amounts of Al3+
(1.35–6.75 μg mL−1) and a fixed concentration of
Fe3+ (7 μg mL−1) are shown in Fig. 2. The aluminium(III) concentration
is found to be proportional to the sum of the amplitudes of the trough (at
545 nm) and the peak (at 560 nm). A linear calibration graph passing
through the origin is obtained on plotting the peak-to-trough vertical
distance versus aluminium concentration. The calculated regression
equation [95% confidence interval (CI), n = 5] is | D1 = 0.977 CAl
(±2.4 × 10−3) + 0.008 (±1.1 ×
10−3)(15) |
(2)
|
where D1 is the value of the
first-derivative signal (vertical distance from peak to trough) and
CAl is the aluminium concentration (μg
mL−1). The RSD for five determinations is 1.23% for a
sample solution containing 3.25 μg mL−1 aluminium.![First-derivative spectra of Al–QUIN system at pH 3.8,
CL = 2.5 × 10−4 mol
L−1, 50% v/v ethanol, [Fe3+] = 1.25 ×
10−4 mol L−1 , [Al3+] = (1) 5
× 10−5 (2) 1.0 × 10−4 (3) 1.5
× 10−4, (4) 2.0 × 10−4 and
(5) 2.5 × 10−4 mol L−1.](/image/article/2000/AN/a906307b/a906307b-f2.gif) |
| Fig. 2 First-derivative spectra of Al–QUIN system at pH 3.8,
CL = 2.5 × 10−4 mol
L−1, 50% v/v ethanol, [Fe3+] = 1.25 ×
10−4 mol L−1 , [Al3+] = (1) 5
× 10−5 (2) 1.0 × 10−4 (3) 1.5
× 10−4, (4) 2.0 × 10−4 and
(5) 2.5 × 10−4 mol L−1. | |
Determination of Al2O3 content of
Portland cement and cement materials
The potential of QUIN as a reagent for the direct spectrophotometric
determination of aluminium prompted us to explore the applicability of the
method for the determination of alumina in Portland cement and cement
clinker. The validity of both the normal and first-derivative methods was
thoroughly examined. It was observed that the determination of
Al2O3 content of cement can be achieved precisely
under the optimum conditions (representative spectra are shown in Fig. 3 and 4).
Replicate Al2O3 content analysis of NIST cement
samples SRM 1880, 1881, 1885, 1886, and 1889 were performed. In the
precision study, five determinations were carried out for each sample. A
good precision of the proposed method was obtained, which allows the
application of the method to the routine analysis of cement. The analysis
of cement materials containing various amounts of
Al2O3 is feasible over the concentration range
1.4–5.75 μg mL−1 of Al. |
| Fig. 3 First-derivative spectrophotometric determination of
Al2O3 content of cement sample (10 mg per 25 mL) with
QUIN, CL = 2.5 × 10−4 mol
L−1, pH 3.8, 50% v/v ethanol. Al2O3
found = 4.45%. | |
 |
| Fig. 4 Spectrophotometric determination of Al2O3 content
of cement materials with QUIN (CL = 2.5 ×
10−4 mol L−1, pH 3.8, 50% v/v ethanol,
10−3 mol L−1 ascorbic acid). Curves
1–3, 10 mg; and curve 4, 5 mg of sample per 25 mL. 1 = clinker; 2 =
cement; 3 = kiln feed; 4 = clay. Al2O3 found: = (1)
5.21; (2) 4.72; (3) 3.22; (4) 14.59%. | |
The detection limits (at the 95% confidence level) of the proposed
methods for the mean of five analyses (N1) were
calculated. The minimum detectable amount,
Δxmin , is given by21,22
|  |
(3)
|
where the subscript b refers to the blank determination. The
statistical parameter
t = 2.18 for 12 degrees of freedom and 95%
confidence. The calculated detection limits are 0.12 μg
mL
−1 and 56 ng mL
−1 for Al using the
normal and first-derivative spectrophotometric procedures,
respectively.
In real sample analyses, cement, clinker and clay were analysed for
Al2O3 content by XRF spectrometry. There was no
significant difference between the results obtained by the proposed method
and XRF. The method provides the rapid determination of
Al2O3 in Portland cement and could replace the
present test method for cement analysis.
 |
| Fig. 5 Control chart plot for monitoring aluminium in Portland cement. CL =
control limit (μg mL−1); U = upper; L = lower. | |
Table 2 Spectrophotometric determination of Al2O3 in some
Portland cement materials
Aluminium
determinationb | XRF: |
---|
| AL2O3 |
---|
Material | Samplec | xc | sc | 95% CI | (%) |
---|
Number of determinations for each sample: n = 5. |
---|
x = Mean recovery (% Al2O3); s
= standard deviation (%). |
---|
Test solutions of the samples investigated contained 4–15 mg of
cement material per 25 mL. |
---|
OPC = Ordinary Portland cement. |
---|
Certified amounts (% Al2O3): SRM 1889, 5.61;
1886, 3.99; 1885, 3.68; 1881, 4.16 and 1880, 5.03. |
---|
Cement (OPCd) | 1 | 4.90 | 9.79 × 10−2 | x
± 0.09 | 4.86 |
2 | 5.48 | 7.74 × 10−2 | x
± 0.07 | 5.46 |
3 | 4.86 | 10.10 × 10−2 | x
± 0.10 | 4.81 |
Clinker | 1 | 5.18 | 9.97 × 10−2 | x
± 0.10 | 5.05 |
2 | 5.24 | 9.40 × 10−2 | x
± 0.09 | 5.18 |
3 | 5.10 | 8.26 × 10−2 | x
± 0.08 | 5.14 |
Clay | 1 | 14.80 | 12.40 × 10−2 | x
± 0.12 | 14.72 |
2 | 14.05 | 10.68 × 10−2 | x
± 0.10 | 13.92 |
3 | 14.63 | 10.86 × 10−2 | x
± 0.11 | 14.50 |
NIST SRM |
Cementd | 1889 | 5.55 | 13.84 × 10−2 | x
± 0.13 | 5.64 |
1886 | 4.03 | 11.40 × 10−2 | x
± 0.11 | 3.96 |
1885 | 3.70 | 7.42 × 10−2 | x
± 0.07 | 3.64 |
1881 | 4.20 | 6.66 × 10−2 | x
± 0.06 | 4.16 |
1880 | 5.11 | 9.10 × 10−2 | x
± 0.09 | 5.05 |
Validation of the method
Based on running duplicates, a control chart was prepared for monitoring
aluminium in Portland cement analysis. The distribution of measurements and
range for the determination under investigation indicated that it is in
statistical control.References
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