Last updated: 10. July 2002

By: L. Evje

Author: ******

Procedure for aluminium fractionation

1Aluminium fractionation

1.1Aluminium fractions:

Fractionation of monomeric aluminium (Ala) from polymeric forms is accomplished by complexation with 8-hydroxyquinoline at pH 8.3 with subsequent extraction into MIBK organic phase. The procedure is based on the method to Barnes and Driscoll (Driscoll, 1984; Barnes, 1975)

Organic bound monomeric aluminium (Alo) is separated from inorganic aluminium (mainly labile) (Ali) by trapping the Ali fraction at an Amberlight IR-120 ion exchange column.

The Al concentrations in the organic extracts are determined photometrically.

The Ali fraction consist mainly of Al3+, Alx(OH)y, AlxFy, Alx(SO4)y and AlxSiy as well as mixed complexes. The Ali may be speciated into these various compounds by the use of the computer program ALCHEMI.

1.2Reagents:

Use PA grade chemicals only.

Phenol red: Dissolve 0.1 g phenolsulfonephatein in 28 mL of 0.01 M NaOH and dilute to 250 mL with Milli-Q water.

~100% acetic acid glacial (CH3COOH): This reagent is commercially available.

8-hydroxyquinoline: Dissolve 5 g 8-hydroxyquinoline in 10 mL acetic acid (CH3COOH) and dilute to 100 mL with Milli-Q water.

25% Aqueous NH3: This reagent is commercially available.

Extraction buffer: Mix 223 mL 25% aqueous NH3 carefully with 120 mL acetic acid (WARNING EXPLOSIVE). Dilute to 1000 mL with Milli-Q water. Adjust pH in temperate sample to 8.3±0.1.

MetylIsoButylKeton (MIBK): This reagent is commercially available.

Al-standards:

Stock solution - Dissolve 1.758 g KAl(SO4)2*12H2O in 50 mL Milli-Q water, add 10 mL 4 M H2SO4 and dilute to 1000 mL.

4 M H2SO4 is prepared by diluting 222 mL conc. (18 M) sulphuric acid to 1000 mL.

Working standard - 2 mg Al/L. Pipet out 5.0 mL of stock solution into a 250 mL volumetric flask half filled with ion exchange water. Dilute with ion exchange water until the mark.

Al-standards - Pipet out the following volumes into 100 mL volumetric flasks and fill up to the mark with ion exchanged water.

2 mL → 40 μg/L20 mL → 400 μg/L

5 mL → 100 μg/L30 mL → 600 μg/L

10 mL → 200 μg/L40 mL → 800 μg/L

1.3Extraction:

All chemicals should have same temperature (± 5°C) as sample.

At no time should the complexbinder (8-hydroxyquinoline) come in contact with glass equipment. This reagent must also be stored dark and cold.

1.3.1Procedure

Measure out 30 mL sample into the extraction funnel. Add fast and in a repepative fashion:

- 0.25 mL phenol red

- 1.0 mL 8-hydroxyquinoline

- 25% aqueous NH3) until red reaction (~0.25mL)

- 2.5 mL extraction buffer

- 10 mL MIBK

Shake the mix for exactly 20 sec. and set the extraction funnel to rest so that the phases may separate. Let the lower water layer go to waste and collect the yellow/green organic layer. The MIBK extracts should be stored cold and dark until photometric measurement is convenient.

The amount of ammonium hydroxide that is needed to shift phenol red to red colour (pH.8.3) you may determine once for all the samples.

In order to get a steady result it is important that the chemicals are added in a similar fashion and speed. This needs some practice and is only possible to manage by use of dispensers.

1.3.2Dilution

If the absorbance is grater than 2.0, the sample has to be diluted. The dilution is done by adding twice as much MIBK as described in 1.3.1 (Evje, 2002). However, this method needs some more research.

1.4Ion exchange:

Mobile phase (Eluent): Dissolve 0.2338 g NaCl in 10 L of ion exchanged water to 4*10-4 M NaCl. The eluent should have the same ionic strength as the samples.

Preparation of column: Make a glass column as shown in figure 1.

Take approx. 20 g Amberlight IR 120 and rinse:

3 times with 2 M NaCl

2 times with ion exchanged water

2 times with 4*10-4 M NaCl

Fill column with 9.5 mL resin. Adjust pH of eluent to sample pH ± 0.5 by pumping 0.12 M HCl or 2 M NaCl through the column. Example: To get the column pH to 5.8 you must pump through 75 mL 2 M NaCl and rinse through with at least 250 mL eluent before stable pH measurement is accomplished. When pH is less than 0.5 from sample pH dismantle column and mix the resin in the column (turn the column upside-down). Check pH before running sample.

Figure 1. The ion exchange column.

Sample exchanging: Peristaltic pump speed 38 mL/min. First pump through 60 mL eluent for conditioning, then shift over to sample. Pump through 60 mL sample[1] for waste and collect the next 35 mL for Alo analysis without stopping the pump. Shift back to eluent. Pump at least 60 mL of eluent through the system for rinsing. The resin must be replaced after approx. 40 samples.

Run Al complexation and extraction analysis on sample as described in 1.3 immediately after it has been pumped through the ion exchange column.

1.5Photometric measurement:

Measure the MIBK extracts photometrically at 395 and 600 nm. At 395 the blind samples will give a background signal (typically Abs. = 0.010 - 0.030) that must be zeroed out. The measurement at 600 nm is for Fe correction (Rudberg, 1995).

1.6Calculations:

Ali = Ala - Alo

Iron correction: Abscorr. = Abs395 - (Abs600*1.12)

Alcons. = A * Abscorr. + B

A & B standard regression analysis (least squared method).

1.7Detection limit

The detection limit of the aluminium fractionation method is 0.5 μM (Vogt et al., 1994) and a 1% standard deviation at 20 μM Al (Sullivan et al., 1986).

Referanser:

Barnes, R.B.; 1975. The determination of specific forms of aluminium in natural water. Chem. Geol., 15:177-191.

Driscoll, C.T.; 1984. A procedure for the fractonation of aqueous aluminium in dilute acidic waters. International Journal of Environmental Analytical Chemistry, 16:267-283.

Evje, L.G.; 2002. Fysiokjemiske egenskaper til løst naturlig organisk materiale i Norden. Hovedoppgave ved Kjemisk institutt, Universitetet i Oslo.

Rudberg, B.; 1995. Aluminium, jern og silisium i jordvæske fra HUMEX-feltet. Fraksjonering av aluminium og jern. Hovedoppgave ved Kjemisk institutt, Universitetet i Oslo.

Sullivan, T.J.; Seip, H.M.; Muniz, I.P.; 1986. A comparison of frequently used methods for the determination of aqueous aluminum. International Journal of Environmental Analytical Chemistry, 26:61-75.

Vogt, R.D.; Ranneklev, S.B.; Mykkelbost, T.C.; 1994. The impact of acid treatment on soilwater chemistry at the HUMEX site. Environment International, 20:277-286.

[1] If the set up has much dead volume (that should be avoided) then 60 ml of sample to waste may not be enough to flush the system.