The determination of carbon in aluminium

Aluminium generally contains less than a few/xg/g carbon. In rolled sheet the concentration may be higher, especially in zones near the surface (8). Cast products also have higher carbon concentrations, especially when the melt was treated with carbon separating mixtures for refining purposes. 

In earlier times, carbon was determined in aluminium using wet chemical methods. This was mainly due to the difficulties encountered in the combustion of the metal. The sample may be dissolved in chromic-sulphuric acid (2), or a combined wet-dry method may be used to dissolve it in non-oxidizing mediums (3), or it is volatilized in a hydrochloric acid or chlorine gas stream (4,5,6,7), the remaining insoluble parts or rests being burnt. The problems with these procedures are obvious  

Fundamental work was carried out by Fischer and Schmidt (8) using a tube furnace and combustion boats of pythagoras-mass. Carbon dioxide is absorbed in barium hydroxide solution (pH = 9.9) and determined by back titration 

The above procedure does however not guarantee complete combustion and aluminium pearls are always observed in the combustion slags. Satisfactory conditions are however reached when metallic bismuth is used as fluxing agent in a 1 1 ratio to aluminium, the further procedure being similar to the above one. 

Sulzberger (9) analyzed this recommendation thoroughly and applied it to aluminium alloys as well. The operational conditions are summarized in Table V-1. 

---

The procedure is described in detail under "the determination of carbon in aluminium" (8). 

Table V-8 Results of BCR round robins on the determination of carbon in primary ingot aluminium...

Applications Basic methods for the determination of halogens in polymers are fusion with sodium carbonate (followed by determination of the sodium halide), oxygen flask combustion and XRF. Crompton [21] has reported fusion with sodium bicarbonate for the determination of traces of chlorine in PE (down to 5 ppm), fusion with sodium bisulfate for the analysis of titanium, iron and aluminium in low-pressure polyolefins (at 1 ppm level), and fusion with sodium peroxide for the complexometric determination using EDTA of traces of bromine in PS (down to 100ppm). Determination of halogens in plastics by ICP-MS can be achieved using a carbonate fusion procedure, but this will result in poor recoveries for a number of elements [88]. A sodium peroxide fusion-titration procedure is capable of determining total sulfur in polymers in amounts down to 500 ppm with an accuracy of 5% [89]. 

A method has been reported for the determination of calcium, copper, iron, magnesium, potassium, sodium and zinc in cellulose [169]. The sample (10 g) was air-dried and then ashed at 575°C until all the carbon was removed. Hydrochloric acid (5 ml of 6M) was added to the residue and evaporated to dryness twice before taking up the sample in a third aliquot, diluting to 100 ml and aspiration into an air /acetylene flame. It is likely that volatile elements such as cadmium may be lost at such an elevated ashing temperature and temperatures below 500°C may be preferable. Alternatively wet ashing with nitric acid has been proposed for the determination of aluminium, cadmium, potassium and zinc in pressed boards [170] or sodium in gypsum glass board [171]. For the determination of lead in confection wrappers, the sample may be treated with concentrated nitric acid at 70—80°C and diluted for flame analysis [172]. In the full method, the wrapper was wiped clean with a damp tissue, cut up to 0.5 X 0.5 mm pieces and dried at 110°C (for paper, for plastic 80°C) for 1 h. The sample (0.5 g) was heated with concentrated nitric acid (1ml) at... 

Garcia Gutierrez [19] has described an azo coupling spectrophotometric method for the determination of nitrite and nitrate in soils. Nitrite is determined spectrophotometrically at 550 nm after treatment with sulfuric acid and N-1 -naphlhylclhylcnediamine to form an azo dye. In another portion of the sample, nitrate is reduced to nitrite by passing a pH 9.6 buffered solution through a cadmium reductor and proceeding as above. Soils were boiled with water and calcium carbonate, treated with freshly precipitated aluminium hydroxide and active carbon, and filtered prior to analysis by the above procedure. 

Solid samples may frequently require fusions. Examples of such procedures in the literature are (a) the determination of aluminium, silicon and sodium in molecular sieves using a nitrous oxide/acetylene flame following fusion (1 part to 10 of fusion mixture) with sodium carbonate, sodium... 

Between pH 4.5 and 10, 8-hydroxyquinoline (oxine) forms the chelate AKCgHeON), which is sparingly soluble in water but dissolves readily in CHCI3. The yellow extract of aluminium oxinate is the basis for the determination method [2,14]. Carbon tetrachloride and trichloroethylene are also used as extraction solvents. The absorption maximum of the chloroform extract is at 390 nm (e= 7.310, a = 0.27). The absorption of oxine in CHCI3 increases rapidly below 390 nm. 

General methods for the estimation of bismuth present in quantity include direct weighing as sulphide precipitation as sulphide, conversion to carbonate and ignition to oxide and precipitation as phosphate. Recently the use of EDTA as a titrant for bismuth has considerably simplified the determination of this metal in pharmaceutical mixtures. Bismuth forms a complex at pH 1 to 2 and at this degree of acidity few other metals likely to be encountered (with the exception of iron) interfere bismuth may therefore be selectively titrated in the presence of, say, aluminium, magnesium, or calcium. Various methods available have been discussed by Brookes and Johnson and the following general procedure is recommended ... 

Discussion. This method is based upon the precipitation of lead chlorofluoride, in which the chlorine is determined by Volhard s method, and from this result the fluorine content can be calculated. The advantages of the method are, the precipitate is granular, settles readily, and is easily filtered the factor for conversion to fluorine is low the procedure is carried out at pH 3.6-5.6, so that substances which might be co-predpitated, such as phosphates, sulphates, chromates, and carbonates, do not interfere. Aluminium must be entirely absent, since even very small quantities cause low results a similar effect is produced by boron ( >0.05 g), ammonium (>0.5 g), and sodium or potassium ( > 10g) in the presence of about 0.1 g of fluoride. Iron must be removed, but zinc is without effect. Silica does not vitiate the method, but causes difficulties in filtration. 

---