An Important Improvement in Ferron-Timed Spectrophotometry

Abstract

Ferron dosage ([Ferron]) is key to ferron-timed spectrophotometry (ferron assay). In order to clarify some important questions, the following studies were conducted: (1) The effect of [Ferron] on the sensitivity of total aluminum (Al_T) determination was experimentally investigated and ²⁷Al nuclear magnetic resonance (²⁷Al NMR) spectroscopy was used to verify our developed Ferron assay; (2) the chemical equilibrium calculation was employed to analyze the effect of [Ferron] on the determination of Al_T; and (3) the main reason for insufficient [Ferron] was further analyzed. This is the first study to standardize a ferron assay operating procedure.

Keywords

Ferron timed spectrophotometry Ferron dosage Sensitivity Chemical equilibrium calculation

INTRODUCTION

Spectrophotometric methods have often been applied to trace metal determination due to such advantages as accuracy and precision, low cost, and simple operation. The ferron assay is the most widely used method for Al^III determination in the field of water treatment. It can achieve the quantitative determination of Al^III and speciation of aqueous Al simultaneously;^1–7 however, the major problem of this method is that a uniform and standardized operating procedure has not been established.⁸

Ferron was first proposed for Fe^III determination. Then, three main stages were established. The present ferron assay for the speciation of hydroxyl–Al and efficiency evaluation of Al-based flocculants was finally formed.

Establishment of the Method Measuring Total Aluminum Concentration. Combining the results reported by Yoe et al.^9,10 and Swank et al.,¹¹ Davenport et al.¹² initially used the ferron assay to determine total aluminum concentration (Al_T) in solutions containing both Al^III and Fe^III. Later, more researchers optimized this method and explored the influence of a series of experimental basic parameters. Rainwater et al.¹³ recommended the addition of hydroxylamine and orthophenanthroline to the ferron colorimetric system to minimize the inference of Fe^III on Al^III determination. However, addition of hydroxylamine gradually alters ferron to a different chemical species, which influences the reaction between Al^III and ferron. Hsu et al.¹⁴ examined the influence mechanism of hydroxylamine and modified the procedure for Al_T determination by eliminating hydroxylamine from reagent preparation. The effect of acidity on the ferron and the Al^III–ferron reaction was also investigated.^13,14 Goto et al. tried to use a cationic surfactant to improve the ferron assay. It was because the addition of cationic surfactants could enhance the acid dissociation of ferron that the reaction characteristics of Al–ferron changed. Accordingly, the reproducibility and the linearity of the calibration curve of the ferron assay for Al^III were greatly improved. The useful pH range was also widened.^15,16 Langmyhr et al.¹⁷ measured the two dissociation constants of 2.50 and 7.11 for ferron by potentiometric titration, and the stepwise stability constants of the Al–ferron complexes were 10^7.6, 10^7.1, and 10^5.6.

Application of the Ferron Assay in Hydroxyl–Al Speciation. Smith¹⁸ first utilized the kinetic reaction of Al^III with ferron to partition mononuclear from polynuclear Al species. He assumed that mononuclear Al reacts instantaneously with ferron, intermediate polymers reacted by pseudo–first-order kinetics, and large polymers or initial solid phases reacted ferron extremely slowly or were completely inert. Since then, the ferron assay was transformed from a thermodynamic method to a dynamic analysis technique—ferron timed spectrophotometry. In agreement with the findings of Smith,¹⁸ several researchers further employed linear and nonlinear dynamic model (pseudo–first order, pseudo–second order) to analyze the curve of absorbance versus time determined in hydroxyl–Al solutions and hoped to eliminate arbitrary separation of mononuclear and polynuclear Al.^19–24 The ability of three spectrophotometric methods (ferron, aluminum, and 8-hydroxyquinoline) for differentiating mono- and polynuclear hydroxyl–Al complexes was compared with kinetic analyses. Ferron was the preferred method, based on its simplicity, level of precision, and moderate reaction rate with Al^III.²⁵

Application of the Ferron Assay in the Determination of Hydroxyl–Al Species in the Al-Based Flocculants. Developed from the initial ferron timed spectrophotometry, the ferron assay was further optimized in the two aspects: a reagent-added approach (separated or combined) and interference limits of Fe^III for Al^III determination.^19,20,26,27 At present, the ferron assay is the only one that can be used to determine the speciation of Al^III under water treatment conditions.²⁸ Using this technique, researchers have tried to establish the relationship between the Al^III species distribution and water quality characteristics, and explore the mechanism of flocculation.^4,6,28–34

Generally, when spectrophotometry is used to measure a solution's absorbance, the colorimetric reaction in this solution must reach equilibrium. The dosage of the chromogenic agent is one of the important factors and directly influences the sensitivity of colorimetric determination.^35–37 For the ferron assay, although many factors such as composition of ferron reagent, pH, the maximum absorption wavelength, and the way the reagent was added had been investigated, [Ferron] had never been explored. As a result, the ferron assay had not been normalized and unified. The objectives of this work were (1) to investigate the effect of [Ferron] on the sensitivity of Al_T determination and ascertain optimal [Ferron], (2) to prove the correctness of [Ferron] proposed in this experiment by the chemical equilibrium calculation, and (3) to analyze the main reason of the insufficient [Ferron] in Al^III–ferron system.

EXPERIMENTAL

All chemicals were at least of analytical grade unless otherwise stated. Quartz doubly distilled water and polyethylene containers were used for the preparation of all aqueous solutions. The colorimetric studies were performed at 25 °C.

Materials. To prepare the ferron colorimetric solution, 39 g of anhydrous NaAc (99%, Alfa Aesar) was dissolved in about 400 mL of water, 16 mL of HCl (1:1) was added, and the solution was finally diluted to 500 mL with water and mixed well (solution I). Under rapid stirring, about 1 g of ferron (MP Biomedicals, Inc., France) was added to solution I, which was pulverized thoroughly with a glass rod and continuously stirred for 10 h (solution II). Solution II was filtered with filter paper, having been dried to a constant weight.^{14,2 0,38} The accurate concentration of ferron in this solution was measured by the gravimetric method: [Ferron] = 5.2 × 10⁻³ M.

The stock standard solution of mononuclear aluminum (mononuclear Al; 0.223 M) was prepared by dissolving 0.601 g of Al power (99.99%, Shanghai, China) in 35 ml of HCl (1:1), and then diluted to 100 mL with water. The stock standard solution of NaAl(OH)₄ (0.100 M) was prepared by dissolving 0.677 g of Al power (99.99%, Shanghai, China) in 35.0 mL NaOH (4.0 M) and then diluting it to 250 mL with water. Similar preparations of these reagents can be found elsewhere.^12,39–41

Instrumentation. The UV-VIS 3600 Spectrophotometer (1 cm quartz cell; SHIMADZU, Japan), the DRX-500 superconducting nuclear magnetic resonance (NMR) spectrometer (Bruker, Germany), an inductively coupled plasma atomic emission spectrometer (ICP-AES; JA1100, Kempston Controls, USA), and the PHS-3B pH meter (Shanghai, China) were used to obtain the various measurements.

Effect of pH on the Nature of Ferron. Various amounts of diluted HCl or NaOH were mixed with 1.0 mL of ferron colorimetric solution and then diluted to 50 mL. The final pH of each mixture was measured, and the changes of absorbance for these solutions were monitored between 200 and 600 nm against a water blank.^14,20

Effect of pH on the Al^III–Ferron Reaction. Suitable aliquots of mononuclear Al solutions were pipetted into appropriate volumes of water. Then, various amounts of diluted HCl or NaOH solutions and 11.0 mL of ferron colorimetric solution were added and diluted to 25 mL. The pH and the absorbance at 370 nm of the resulting solutions were determined.

Investigation of the Effect of [Ferron] on the Sensitivity of [Al_T] Determination. Five concentrations of the mononuclear Al solutions (Al_T = 2.0 × 10⁻⁵, 4.0 × 10⁻⁵, 6.0 × 10⁻⁵, 8.0 × 10⁻⁵, and 1.0 × 10⁻⁴ M) were used to perform this study: 0.13, 0.25, 0.38, 0.50 and 0.63 mL of the mononuclear Al solution was transferred into appropriate volumes of water, followed by the addition of ferron colorimetric solution (for each Al_T level, various [Ferron] concentrations were used.). The final volume was 25 mL, and was then mixed well (pH = 5.2 ± 0.2). The absorbance for these solutions was recorded at 370 nm against the respective reagent blanks.

²⁷Al Nuclear Magnetic Resonance Spectroscopy. The instrumental settings and data acquisition conditions were set length = 0.4 s, spectra (n) = 200, delay for pulse length–delay–set length sequence = 500 μs, temperature = 300 K, pulse length = 12 μs, spectrum width = 40 KHz, delay = 0.5 s, set points = 32 768, line horizontal stretch = 10 Hz, and data mass = 16 384. For the coaxial NMR tubes, a 5 mm NMR tube was used with a 1 mm tube, held in a coaxial position by a pressure-equalizer plug. The sample (0.8 mL) was held in the 5 mm NMR tube. To the 1 mm tube in this case were added equal volumes of standard NaAl(OH)₄ solution and D₂O (100 μL), which served as both quantitative reference and lock, respectively. The standard curve of ²⁷Al NMR determination was prepared by plotting the integrated intensity (S) of the ²⁷Al NMR determination for [Al(H₂O)₆]³⁺ against Al:^24,39,40,42

[A 1] = (5.99 \times 10^{- 5}) \times S, R^{2} > 0.999

(1)

Analyzing the Effect of [Ferron] on the Sensitivity of [Al_T] Determination by Chemical Equilibrium Calculation. Ferron is a dibasic acid (H₂Q), and dissociates to HQ and Q²⁻ with increased pH.¹⁷ According to the law of conservation of mass, the equilibrium equations of Al_T and [Ferron] in the Al^III–ferron colorimetric system are as follows:

\begin{array}{l} [{Al}_{T}] & = & [{Al}^{3 +}] + [Al Q^{+}] + [Al Q_{2}^{-}] + [Al Q_{3}^{-}] + [{Al (OH)}^{2 +}] \\ + [{Al (OH)}_{2}^{+}] + [{Al (OH)}_{3}] + {[{Al (OH)}_{4}^{-}]}^{2} \\ = & [{Al}^{3 +}] (1 + K_{1} k_{a_{2}} \frac{[{HQ}^{-}]}{[H^{+}]} + K_{1} K_{2} K_{a_{2}}^{2} \frac{{[{HQ}^{-}]}^{2}}{{[H^{+}]}^{2}} \\ + K_{1} K_{2} K_{3} K_{a_{2}}^{3} \frac{{[{HQ}^{-}]}^{3}}{{[H^{+}]}^{3}} + \frac{K_{h_{1}}}{[H^{+}]} + \frac{K_{h 1} K_{h 2}}{{[H^{+}]}^{2}} \\ + \frac{K_{h 1} K_{h 2} K_{h 3}}{{[H^{+}]}^{3}} + + \frac{K_{h 1} K_{h 2} K_{h 3} K_{h 4}}{{[H^{+}]}^{4}}) \end{array}

(2)

[Ferron]

\begin{array}{l} [Ferron] \\ = [H_{2} O] + [{HQ}^{-}] + [Q^{2 -}] + [{AlQ}^{+}] + 2 [{AlQ}_{2}^{-}] + 3 [{AlQ}_{3}^{3 -}] \\ = \frac{[H^{+}] [{HQ}^{-}]}{K_{a 1}} + [{HQ}^{-}] + \frac{[{HQ}^{-}] K_{a 2}}{[H^{+}]} + K_{1} K_{2} \frac{[{Al}^{3 +}] [{HQ}^{-}]}{[H^{+}]} \\ + 2 K_{1} K_{2} K_{a 2}^{2} \frac{[{Al}^{3 +}] {[{HQ}^{-}]}^{2}}{{[H^{+}]}^{2}} + 2 K_{1} K_{2} K_{3} K_{a 2}^{3} \frac{[{Al}^{3 +}] {[{HQ}^{-}]}^{3}}{{[H^{+}]}^{3}} \end{array}

(3)

The distribution of Al species, ferron, and Al-ferron complex at various pH values was calculated with MATLAB 7.0 software, and the step (ΔpH) was 0.01; meanwhile, the distribution of Al–ferron complex at pH = 5.2 under different [Ferron] obtained by the Eqs. (2) and (3), and the step (Δ[Ferron]) was 3 × 10⁻⁵ M.

RESULTS AND DISCUSSION

Reconfirmation of the Basic Experimental Conditions. As illustrated in Figs. 1a and 1b, the nature of ferron was greatly affected by pH, and the maximum absorptions determined in ferron colorimetric solutions changed with pH variation. In the pH range from 7.8 to 10.3, the 366 nm absorption was at its maximum, whereas on the acidic side of this pH range (4.6–5.3), the maximum absorption moved to 435 nm. Accordingly, the pH could influence the reaction between ferron and Al^III (Figs. 1c, 1d). The results determined in this study and reported in the literature all verified that the maximum absorption of Al^III–ferron colorimetric system was at 368–370 nm (Fig. 1c) after the reagent blank was subtracted.^14,20 Thus, 370 nm was used in this study.

Fig. 1.

Effect of pH on the absorption spectra of ferron reagent and Al–ferron complexes. (a) Spectral scan plots of ferron colorimetric solution under different pH conditions: [Ferron] = 1.0 × 10⁻⁴ M, ( 1 ) pH = 2.4, ( 2 ) pH = 4.0, ( 3 ) pH = 4.6, ( 4 ) pH = 5.0, ( 5 ) pH = 5.2, ( 6 ) pH = 5.3, ( 7 ) pH = 6.4, ( 8 ) pH = 7.8, ( 9 ) pH = 8.1, ( 10 ) pH = 9.4, ( 11 ) pH = 10.3; λ = 200–600 nm, T = 25 °C. (b) The change of the absorbance (A) determined at λ_max = 370 nm in the ferron colorimetric solution under different pH conditions ([Ferron] = 1.0 × 10⁻⁴ M). (c) Spectral scan plots of mononuclear Al solution with various Al concentration determined at pH = 5.2: ( 1 ) Al_T = 2.0 × 10⁻⁵ M, ( 2 ) Al_T = 4.8 × 10⁻⁵ M, ( 3 ) Al_T = 6.4 × 10⁻⁵ M, ( 4 ) Al_T = 8.8 × 10⁻⁵ M; [Ferron] = 2.2 × 10⁻³ M. (d) The change of the absorbance determined in the mononuclear Al–ferron colorimetric system at various pH (3.5–6.5 ± 0.2) ([Ferron] = 2.2 × 10⁻³ M; λ_max = 370 nm, T = 25 °C).

In order to further confirm the optimal pH of the Al^III–ferron reaction, four Al concentrations of the mononuclear Al solution were employed (Fig. 1d). The results indicated that the absorbance determined at 370 nm and pH 5.2 ± 0.2 was at the highest; moreover, in this pH range, the absorbance measured at 370 nm for ferron was near its minimum (Fig. 1b). Thus, the background interference was the weakest at this pH. The pH of 5.2 ± 0.2 is favorable for the ferron assay. The above experimental results were consistent with those reported in the literature.^12,14,20

Effect of [Ferron] on the Sensitivity of Total Aluminum Concentration Determination. The change trend of absorbance determined at 370 nm with increasing [Ferron] was analyzed. As shown in Fig. 2a, the absorbance increased with increases in [Ferron], ranging from 2 × 10⁻⁴ to 2 × 10⁻³ M. Then, as the [Ferron] concentration rose, the absorbance basically kept constant and a platform presented, which implies that [Ferron] = 2 × 10⁻³ M was sufficient for Al determination, and all the Al^III in this color system could be converted to the stable complexes. Therefore, we can conclude that [Ferron] should be at least 2 × 10⁻³ M in order to ensure AlT measurement is the most sensitive when Al_T is within 2.0 × 10⁻⁵ to 8.0 × 10⁻⁵ M. Jardine et al.²¹ and Parker et al.²⁴ had been proposed the condition of [Ferron]/AlT ≥ 50. It was established according to the kinetic reaction characteristic and had nothing to do with the accurate selection of the colorimetric reaction condition. More importantly, in the case of the absorbance versus [Ferron]/Al_T curve being employed to ascertain the lower limit of [Ferron] (Fig. 2b), Al_T should be considered simultaneously. For those samples with unknown Al concentrations that are difficult to measure, the ferron assay will lose general meaning.^21,24 Furthermore, when the Al_T is less than 4.0 × 10⁻⁵ M, the color reaction of Al^III–ferron is conducted under insufficient [Ferron], and follow-up studies should be conducted on the incorrect experimental results. Thus, the [Ferron]/AlT ratio is not used as a criterion for [Ferron] selection, but only as a reference condition.

Fig. 2.

Effect of [Ferron] on the colorimetric reaction between the mononuclear Al solutions and ferron (pH = 5.2 ± 0.2, T = 25 °C) (average of three experiments). (a) The changing trend of absorbance with rising [Ferron]. (b) The changing trend of absorbance with rising [Ferron]/Al_T ratio. (c) The curves of absorbance vs. Al_T under different [Ferron] conditions. (d) The change trend of the ε value with rising [Ferron].

Figures 2c and 2d show the effect of [Ferron] on the sensitivity of the Al^III–ferron reaction. With the increase of [Ferron], the slope of absorbance–Al_T curves also increased. Therefore, [Ferron] has an obvious influence on the sensitivity of the Al^III–ferron reaction, i.e., when [Ferron] < 2 × 10⁻³ M, five absorbance–AlT curves approaching the x-axis in Fig. 2c could be clearly identified. The insufficient [Ferron] decreased the sensitivity of the Al determination, and the determined absorbance was low. When [Ferron] ≥ 2 × 10⁻³ M, most of the absorbance–AlT curves overlapped. It is because the sufficient [Ferron] made the Al^III completely complex, the colorimetric sensitivity reached the utmost, and the absorbance became stable. Accordingly, the e value of Al^III–ferron system is not a constant before [Ferron] reaches 2 × 10⁻³ M. The change of e with increasing [Ferron] had the following relationship:

ɛ (M^{- 1} {cm}^{- 1}) = C_{1} - C_{2} \times (- \frac{[Ferron]}{C_{3}}), R^{2} > 0.999

(4)

C₁, C₂, and C₃ were 9690, 5105, and 0.00054, respectively. Under sufficient amounts of [Ferron], the e value of Al^III–ferron system is 9737 ± 420 M⁻¹·cm⁻¹.

Because the question concerning the dosage of the chromogenic reagent of the ferron assay has never been unified and standardized, 7000 to ≥ 10 000 M⁻¹·cm⁻¹ for the e value can be found in the works referenced in Table I. The change trend of e with increasing [Ferron] summarized in the literature was in line with that proposed in our study (Fig. 3). The reason for the formation of this trend is that in the Al^III–ferron reaction system, with the increases in [Ferron], Al^III and ferron can form a series of complexes with different coordination numbers:

TABLE I.

The ε values of Al–ferron colorimetric system reported in literature.

Colorimetric conditions
Al_T (M)	[Ferron] (M)^a	ε (M⁻¹·cm⁻¹)	Reference
8.9 × 10⁻⁵	5.7 × 10⁻⁴	6742^b	15
8.0 × 10⁻⁵	5.7 × 10⁻⁴	7000^b	43
7.6 × 10⁻⁵	5.7 × 10⁻⁴	7123^b	20
1.5 × 10⁻⁵ to 8.9 × 10⁻⁵	4.6 × 10⁻⁴	7335	12
2.0 × 10⁻⁵ to 1.0 × 10⁻⁴	5.7 × 10⁻⁴	7491	44
8.0 × 10⁻⁵	to 6 × 10⁻⁴	7500^b	2
3.7 × 10⁻⁵	8.1 × 10⁻⁴	7568^b	45
7.4 × 10⁻⁵	5.7 × 10⁻⁴	7940	14
1.0 × 10⁻⁵ to 8.0 × 10⁻⁵	5.7 × 10⁻⁴	7995	46
7.4 × 10⁻⁵	1.0 × 10⁻³	9450^c	21
1.0 × 10⁻⁵ to 5.0 × 10⁻⁵	≥2.0 × 10^−3b	10 049	47
0.5 × 10⁻⁵ to 4.0 × 10⁻⁵	1.0 × 10⁻³	10 657	48
1.0 × 10⁻⁵ to 6.0 × 10^−5b	≥2.0 × 10^−3b	10 753	27
2.4 × 10⁻⁵	1.2 × 10⁻³	10 800	24
0 to 5.0 × 10⁻⁵	≥2.0 × 10^−3d	11 060	49

Apparent concentration.

Estimated according to the conditions given in the literature.

This value had been corrected according to the condition given in the literature.

Inferred according to the literature cited by original.

Fig. 3.

Comparison of the change trend of ε values with increasing [Ferron] proposed in this work and literature (this work: λ_max = 370 nm, pH = 5.2 ± 0.2, T = 25 °C).

{Al}^{3 +} \overset{+ Q^{2 -}}{⇄} {AlQ}^{+} \overset{+ Q^{2 -}}{⇄} {AlQ}_{2} \overset{+ Q^{2 -}}{⇄} - {Al}_{3}^{3 -}

whereas the absorbance at 370 nm only comes from AlQ₃³⁻.^15,21,35 Hence, when the [Ferron] concentration increases, the distribution ratio of the four species Al³⁺, AlQ⁺, AlQ₂⁻, and AlQ₃³⁻) continuously changes. The concentrations of Al³⁺, AlQ⁺, and AlQ₂⁻ falls, and the concentration of AlQ₃³⁻ rises. The direct result is the increasing of the absorbance. After all the Al^III in the colorimetric solution is transformed to AlQ₃³⁻, further increase in [Ferron] effects no increase in the absorbance. Thus, the trend of ε first increases, and then gradually ends on a plateau with the rising [Ferron]. Only at the plateau does the determined absorbance match well with the Al_T.

²⁷Al NMR was used to verify the Al_T as measured by this optimized ferron assay. For the same mononuclear Al solution (Al^III concentration was 0.401 M, determined by the ICP-AES spectrometer.), the ferron and ²⁷Al NMR assays were simultaneously employed to determine their mononuclear Al content, Al_T and Al_m, respectively. As shown in Fig. 4, Al_T and Al_m were almost the same. Thus, Al_T could be accurately determined when the experimental conditions were [Ferron] ≥ 2 × 10⁻³ M and Al_T = 2.0 × 10⁻⁵ to 8.0 × 10⁻⁵ M.

Fig. 4.

Comparison between Al_T determined by ferron assay and Al_m determined by ²⁷Al NMR (the red line indicates equality). The Al solutions used in the ferron assay were all diluted from that used in ²⁷Al NMR. [Ferron] = 2 × 10⁻³ M, λ_max = 370 nm, pH = 5.2 ± 0.2, T = 25 °C.)

Analysis of the Effect of [Ferron] on the Sensitivity of [Al_T] Determination by Chemical Equilibrium Calculation. Under the required pH condition (approximately 5.2), the distribution of various species in the Al^III–ferron reaction system calculated with chemical equilibrium calculation were that Al^III mainly existed as Al³⁺, Al(OH)²⁺, and Al(OH)₂⁺; HQ⁻ was the most prevalent species of ferron. The Al^III–ferron complexes mainly existed as AlQ₃³⁻ (Figs. 5a–5c). The results (shown in Fig. 5d) indicated that with the increasing [Ferron] in the Al^III–ferron reaction, the proportion of the complex of Al:ferron = 1:1 and 1:2 declined, whereas, the complex of Al:ferron = 1:3 increased. When [Ferron] ≥ 2 × 10⁻³ M, >90% of Al^III transformed to AlQ₃³⁻ and reached a constant. Al^III and ferron could react completely and form a stable complex at this point. This result is well in accordance with the rule proposed in the experiment. Thus, it can be further deduced that [Ferron] in the Al^III–ferron reaction system is at least 2 × 10⁻³ M.

Fig. 5.

The distribution of mononuclear Al species, ferron, and Al–ferron complexes under different pH conditions (a–c) and the content change of three kinds of Al–ferron complexes with [Ferron] (d). (a) [Ferron] = 2 × 10⁻³ M, (b) Al_T = 6.0 × 10⁻⁵ M. (c) Al_T = 6.0 × 10⁻⁵ M, [Ferron] = 2 × 10⁻³ M. (d) Al_T = 6.0 × 10⁻⁵ M, pH = 5.2. Hydrolysis constants of Al³⁺: pK_h1 = 5:0; pK_h2 = 5:1; pK_h3 = 6:7; pK_h4 = 5:9⁵⁰; dissociation constants of ferron: K_a₁ = 10^−2.5; K_a₂ = 10^−7.1; formation constants for Al–ferron complexes: log K₁ = 7.6, log K₂ = 7.1, log K₃ = 5.6.¹⁷

Analysis of the Main Reason for Insufficient [Ferron]. When [Ferron] is insufficient, the sensitivity of Al^III–ferron colorimetric detection declines, the linear range reduces, and the error of Al_T detection increases. Insufficient [Ferron] is mainly introduced by the uncertainty of the ferron concentration in the prepared ferron colorimetric solution (Table SI, Supplemental Material). Although ferron solubility obtained from reagent manual is 0.2 g/100 mL, the actual solubility of ferron cannot reach this limit, and our experimental results indicate that the dissolved ferron was only about 92% (Table II). Meanwhile, (1) whether ferron reagent added in water is completely pulverized and (2) the length of its stirring time obviously influences the solubility of ferron. Usually reported in the literature is that ferron is dissolved in water slowly and contains insoluble impurities; when the ferron colorimetric solution is prepared, it should be stirred overnight and then filtered. Nevertheless, the accurate solubility of ferron and its effect on Al determination was neglected by some researchers.^14,21,24,38 Our experiment simulated various situations that can exist in ferron colorimetric solution preparation, and compared the dissolved quantity of ferron.

TABLE II.

Comparison of ferron solubility under different dissolution means.

The amount of ferron added (g/500 mL)	Dissolved amount (g)	Solubility (g/mL)	%	Actual concentration of ferron (M)
a. 1.00	0.92	1.8 × 10⁻³	92	5.2 × 10⁻³
b. 1.01	0.90	1.8 × 10⁻³	89	5.1 × 10⁻³
c. 1.00	0.86	1.7 × 10⁻³	86	4.9 × 10⁻³
d. 1.01	0.69	1.4 × 10⁻³	68	3.9 × 10⁻³
e. 1.01	0.40	8.0 × 10⁻⁴	40	2.3 × 10⁻³
f. 1.01	0.38	7.6 × 10⁻⁴	38	2.2 × 10⁻³
g. 1.00	0.28	5.6 × 10⁻⁴	28	1.6 × 10⁻³
h. 1.00	0.18	3.6 × 10⁻⁴	18	1.0 × 10⁻³

a.–d. After ferron as added to Solution I (NaAc–HCl), it was pulverized completely and continuously stirred 10 h, 4 h, 1 h, and 30 min, respectively, and then filtered. e.–h. After ferron was added to solution I (NaAc–HCl), it was not pulverized, only stirred 2 h, 1 h, 20 min, and 5 min, respectively, and then filtered.

As shown in Table II, because the operation of preparation could not be unified, the dissolved quantity of ferron decreased by 3–74%. In other words, if the above situation was neglected when the ferron colorimetric solution was prepared, the actual concentration of ferron could be far less than the apparent concentration of ferron obtained by theoretical calculations. Thus, the following operation was suggested to prepare ferron colorimetric solution: After ferron is dissolved in NaAc–HCl buffer solution, the bigger particles clumped together should be pulverized with a glass rod completely. Then, the NaAc–HCl buffer solution dissolving the ferron should be continuously stirred for 10 h.

CONCLUSION

The uniformity and standardization of the ferron assay provides a correct experimental method for the further determination of hydroxyl–Al species. The reconfirmed colorimetric conditions of the Al^III–ferron system are pH = 5.2 ± 0.2 and λ_max = 370 nm. [Ferron] is the key for Al_T determination. The main conclusions of this study can be drawn as follows:

(1)

The ε of Al^III–ferron system is not a constant before [Ferron] reaches 2 × 10⁻³ M. The change of s with increasing [Ferron] is ε(M⁻¹ ·cm⁻¹) = C₁ – C₂ × exp(—[Ferron]/C₃). Under insufficient [Ferron] conditions, the sensitivity of Al^III–ferron reaction is low. When the sensitivity reaches the utmost, the s value is 9737 ± 420 M⁻¹·cm⁻¹. Thus, the optimized reaction conditions for the Al^III–ferron system are Al_T = 2.0 × 10⁻⁵ to 8.0 × 10⁻⁵ M and [Ferron] ≥ 2 × 10⁻³ M. The ²⁷Al NMR method verified the developed the ferron assay.

(2)

The influence of [Ferron] on the sensitivity of Al^III–ferron reaction analyzed by the chemical equilibrium calculation is in line with that obtained from our experiment. The optimal [Ferron] should be at least 2 × 10⁻³ M, and the experimental result is then reasonable.

(3)

The main reason for the insufficient [Ferron] in the Al^III–ferron system is the uncertainty of the ferron concentration in the prepared ferron colorimetric solution.

Footnotes

ACKNOWLEDGMENTS

We thank State Key Laboratory of Analytical Chemistry for Life Science of Nanjing University for help with all experiments and NMR determination. The present study was financially supported by Natural Science Foundation of Ningbo City (No. 2010A610020).

Notes

References

Shen

Y.H.

Dempsey

B.A.

. “Synthesis and Speciation of Polyaluminum Chloride for Water Treatment”. Environ. Int. 1998. 24(8): 899–910.

Chen

Z.Y.

Liu

C.J.

Luan

Z.K.

Zhang

Z.G.

Y.Z.

Jia

Z.P.

. “Effect of Total Aluminum Concentration on the Formation and Transformation of Nanosized Al₁₃ and Al₃₀ in Hydrolytic Polymeric Aluminum Aqueous Solutions”. Chin. Sci. Bulletin. 2005. 50(18): 2010–2015.

Kuan

W.H.

Wang

M.K.

Huang

P.M.

C.W.

Chang

C.M.

Wang

S.L.

. “Effect of Citric Acid on Aluminum Hydrolytic Speciation”. Water Res. 2005. 39(15): 3457–3466.

Yan

M.Q.

Wang

D.S.

J.H.

W.J.

Chow

C.W.K.

. “Relative Importance of Hydrolyzed Al^III species (Al-a, Al-b, and Al-c) During Coagulation with Polyaluminum Chloride: A Case Study with the Typical Micro-polluted Source Waters”. J. Colloid Interf. Sci. 2007. 316(2): 482–489.

Wei

J.C.

Gao

B.Y.

Yue

Q.Y.

Wang

W.W.

Zhu

X.B.

. “Comparison of Coagulation Behavior and Floc Structure Characteristic of Different Polyferric-Cationic Polymer Dual-Coagulants in Humic Acid Solution”. Water Res. 2009. 43(3): 724–732.

Anderson

M.A.

Berkowitz

. “Aluminum Polymers Formed Following Alum Treatment of Lake Water”. Chemosphere. 2010. 81(7): 832–836.

Zhao

Liu

H.J.

J.H.

. “Aluminum Speciation of Coagulants with Low Concentration: Analysis by Electrospray Ionization Mass Spectrometry”. Colloids Surf. A. 2011. 379(1–3): 43–50.

Tang

H.X.

. “Chapter 2. Preparation and Characterization of Inorganic Polymer Flocculant”. Theory of Inorganic Polymer Flocculation and Flocculants. Beijing: Architecture Press, 2006. Pp. 11–25.

Yoe

J.H.

. “7-Iodo-8-hydroxyquinoline-5-sulfonic Acid as a Reagent for the Colorimetric Determination of Ferric Iron”. J. Am. Chem. Soc. 1932. 54(11): 4139–4143.

10.

Yoe

J.H.

Hall

R.T.

. “A Study of 7-Iodo-8-hydroxyquinoline-5-sulfonic Acid as a Reagent for the Colorimetric Determination of Ferric Iron”. J. Am. Chem. Soc. 1937. 59(5): 872–879.

11.

Swank

H.W.

Mellon

M.G.

. “Determination of Iron with 7-Iodo-8-hydroxyquinoline-5-sulfonic acid”. Ind. Eng. Chem. Anal. Ed. 1937. 9(9): 406–409.

12.

Davenport

W.H.

. “Determination of Aluminum in Presence of Iron Spectrophotometric Method Using Ferron”. Anal. Chem. 1949. 21(6): 710–711.

13.

Rainwater

F.H.

Thatcher

L.L.

. “Section D. Analytical Procedures”. In: Pecora

W.T.

, editor. Methods for Collection and Analysis of Water Samples. Washington, DC: United States Government Printing Office, 1960. Pp. 97–100.

14.

Hsu

P.H.

Cao

. “Effects of Acidity and Hydroxylamine on the Determination of Aluminum with Ferron”. Soil Sci. 1991. 152(3): 210–219.

15.

Goto

Tamura

Onodera

Nagayama

. “Spectrophotometric Determination of Aluminum with Ferron and a Quaternary Ammonium Salt”. Talanta. 1974. 21(3): 183–190.

16.

Goto

Taguchi

Miyabe

Haruyama

. “Effect of Cationic Surfactant on the Formation of Ferron Complexes”. Talanta. 1982. 29(7): 569–575.

17.

Langmyhr

F.J.

Storm

A.R.

. “Complex Formation of Aluminium with 7-Iodo-8-Hydroxy-quinoline-5-sulphonic Acid (Ferron)”. Acta Chem. Scand. 1961. 15(7): 1461–1466.

18.

Smith

R.W.

. “Relations Among Equilibrium and Nonequilibrium Aqueous Species of Aluminum Hydroxy Complexes”. Adv. Chem. Ser. 1971. 106: 250–279.

19.

Batchelor

McEwen

J.B.

Perry

. “Kinetics of Aluminum Hydrolysis—Measurement and Characterization of Reaction Products”. Environ. Sci. Technol. 1986. 20(9): 891–894.

20.

Feng

C.H.

Shi

B.Y.

Wang

D.S.

G.H.

Tang

H.X.

. “Characteristics of Simplified Ferron Colorimetric Solution and Its Application in Hydroxy–Aluminum Speciation”. Colloids Surf. A. 2006. 287(1–3): 203–211.

21.

Jardine

P.M.

Zelazny

L.W.

. “Mononuclear and Polynuclear Aluminum Speciation Through Differential Kinetic Reactions with Ferron”. Soil Sci. Soc. Am. J. 1986. 50(4): 895–900.

22.

Jardine

P.M.

Zelazny

L.W.

. “Influence of Inorganic Anions on the Speciation of Mononuclear and Polynuclear Aluminum by Ferron”. Soil Sci. Soc. Am. J. 1987. 51(4): 889–892.

23.

Jardine

P.M.

Zelazny

L.W.

. “Influence of Organic Anions on the Speciation of Mononuclear and Polynuclear Aluminum by Ferron”. Soil Sci. Soc. Am. J. 1987. 51(4): 885–889.

24.

Parker

D.R.

Bertsch

P.M.

. “Identification and Quantification of the Al₁₃ Tridecameric Polycation Using Ferron”. Environ. Sci. Technol. 1992. 26(5): 908–914.

25.

Parker

D.R.

Zelazny

L.W.

Kinraide

T.B.

. “Comparison of 3 Spectrophotometric Methods for Differentiating Mononuclear and Polynuclear Hydroxy–Aluminum Complexes”. Soil Sci. Soc. Am. J. 1988. 52(1): 67–75.

26.

Parthasarathy

Buffle

. “Study of Polymeric Aluminum^III Hydroxide Solutions for Application in Waste-Water Treatment—Properties of the Polymer and Optimal Conditions of Preparation”. Water Res. 1985. 19(1): 25–36.

27.

Feng

Luan

Z.K.

Tang

H.X.

. “[Study on Speciation Analysis Methods for Hydrolysis Polymerization of Aluminum]”. Environ. Chem. 1993. 12(5): 373–379.

28.

Yan

M.Q.

Wang

D.S.

J.R.

J.H.

Chow

C.W.K.

Liu

H.L.

. “Mechanism of Natural Organic Matter Removal by Polyaluminum Chloride. Effect of Coagulant Particle Size and Hydrolysis Kinetics”. Water Res. 2008. 42(13): 3361–3370.

29.

Luan

Z.K.

Tang

H.X.

. “[Coagulation–Flocculation Characteristics and Mechanism of Polyaluminum Salts]”. Acta Sci. Circum. 1992. 12(2): 129–137.

30.

C.Z.

Liu

H.J.

J.H.

Wang

D.S.

. “Coagulation Behavior of Aluminum Salts in Eutrophic Water: Significance of Al₁₃ Species and pH Control”. Environ. Sci. Technol. 2006. 40(1): 325–331.

31.

Shi

B.Y.

G.H.

Wang

D.S.

Feng

C.H.

Tang

H.X.

. “Removal of Direct Dyes by Coagulation: The Performance of Preformed Polymeric Aluminum Species”. J. Hazard. Mater. 2007. 143(1–2): 567–574.

32.

Lin

J.L.

Huang

C.P.

Chin

C.J.M.

Pan

J.R.

. “Coagulation Dynamics of Fractal Flocs Induced by Enmeshment and Electrostatic Patch Mechanisms”. Water Res. 2008. 42(17): 4457–4466.

33.

Zhao

H.Z.

Zhang

X.J.

Wang

H.Y.

. “Stability and Coagulation Behavior of Solid Al₁₃ Purified with an Ethanol–Acetone Fractional Precipitation Method”. Chem. Eng. J. 2012. 179(1): 203–208.

34.

Zhao

H.Z.

Wang

H.Y.

Dockko

Zhang

. “The Formation Mechanism of Al₁₃ and Its Purification with an Ethanol–Acetone Fractional Precipitation Method”. Sep. Pur Tech. 2011. 81(3): 466–471.

35.

Balogh

I.S.

Andruch

Kovacs

. “Spectrophotometric Determination of Manganese with Derivatives of 1,3,3-Trimethyl-2- 3-(1,3,3-trimethyl-1,3-H-indol-2-ylidene)propenyl-3H-indolium”. Anal. Bioanal. Chem. 2003. 377(4): 709–714.

36.

K.A.

. “Chapter 10. UV–Visible Spectrophotometry”. In: Liu

Chang

W.B.

Jiang

Z.W.

Liao

Y.P.

, editors. Analytical Chemistry Tutorial. Beijing: Peking University Press, 2006. Pp. 339–344.

37.

Sharma

A.K.

Singh

. “A Rapid Spectrophotometric Method for Trace Determination of Zinc”. Food Anal. Meth. 2009. 2(1): 311–316.

38.

Wang

D.S.

Sun

Tang

H.X.

Gregory

. “Speciation Stability of Inorganic Polymer Flocculant–PACl”. Colloids Surf. A. 2004. 243(1–3): 1–10.

39.

Akitt

J.W.

. “Multinuclear Studies of Aluminum Compounds”. Prog. Nucl. Magn. Reson. Spectrosc. 1988. 21(1): 1–149.

40.

Chen

Z.Y.

Luan

Z.K.

Fan

Zhang

Z.G.

Y.Z.

Jia

Z.P.

. “[Quantitative Studies on Keggin Polycation Al₁₃ and Al₃₀ in Hydrolytic Polymeric Aluminum Aqueous Solution by ²⁷Al Nuclear Magnetic Resonance]”. Anal. Chem. 2006. 34(1): 38–42.

41.

Jin

L.Y.

. “Aluminum”. In: Chen

Y.J.

Zhang

Chen

C.J.

Chen

S.J.

Xing

D.R.

Wang

Z.H.

Wei

J.R.

Yang

, editors. X.L.E. Standard Examination Methods for Drinking Water–Metal Parameters (GB/T5750.6-2006 of People's Republic of China). Beijing: China National Standardization Management Committee, 2006. Pp. 1–2.

42.

Bertsch

P.M.

Barnhisel

R.I.

Thomas

G.W.

Layton

W.J.

Smith

S.L.

. “Quantitative Determination of Aluminum₂₇ by High-Resolution Nuclear Magnetic Resonance Spectrometry”. Anal. Chem. 1986. 58(12): 2583–2585.

43.

Feng

C.H.

Tang

H.X.

Wang

D.S.

. “Differentiation of Hydroxyl–Aluminum Species at Lower OH/Al Ratios by Combination of ²⁷Al NMR and Ferron Assay Improved with Kinetic Resolution”. Colloids Surf. A. 2007. 305(1–3): 76–82.

44.

Wang

D.S.

Tang

H.X.

Luan

Z.K.

. “[Characterization of Coagulation–Flocculation Species in Particle Suspension System (I): Application of Al–Ferron Method]”. Environ. Sci. 1999. 20(2): 1–5.

45.

Bersillon

J.L.

Hsu

P.H.

Fiessinger

. “Characterization of Hydroxyaluminum Solutions”. Soil Sci. Soc. Am. J. 1980. 44(3): 630–634.

46.

Liu

Fang

Wang

Guo

. “[Speciation Method for the Characterization of Industrial PACl]”. Tech. Equip. Environ. Pollut. Control. 2004. 5(5): 52–55.

47.

Gao

B.Y.

Yue

Q.Y.

Wang

. “[Study on the Aluminum Species of PACS by Using Al–Ferron Timed Complex Colorimetric Method]”. Environ. Chem. 1996. 15(3): 234–239.

48.

Chen

W.N.

Zhong

H.P.

Chen

Y.J.

. “[Study of Charge Characteristics and Hydrolysis State of Polysilicic Alumina Chloride Prepared by Various Preparation Methods]”. Fine Chem. 2001. 18(5): 262–264.

49.

Liu

W.X.

Long

Y.Q.

Wang

Q.C.

Xie

L.S.

. “[Study on the Aluminum Species of PASS by Using Al–Ferron Complex Timed Colorimetric Method]”. Trans. China Pulp Paper. 2001. 16(2): 57–64.

50.

Nordstrom

D.K.

May

H.M.

. “Chapter 1. Aluminum—Environmental Aspects”. In: Sposito

, editor. The Environmental Chemistry of Aluminium. Boca Raton, FL: CRC Press, 1989. Pp. 35–36.