Calcium interference with barium

The bomb method for sulfur determination (ASTM D129) uses sample combustion in oxygen and conversion of the sulfur to barium sulfate, which is determined by mass. This method is suitable for samples containing 0.1 to 5.0% w/w sulfur and can be used for most low-volatility petroleum products. Elements that produce residues insoluble in hydrochloric acid interfere with this method this includes aluminum, calcium, iron, lead, and silicon, plus minerals such as asbestos, mica, and silica, and an alternative method (ASTM D1552) is preferred. This method describes three procedures the sample is first pyrolyzed in either an induction furnace or a resistance furnace the sulfur is then converted to sulfur dioxide, and the sulfur dioxide is either titrated with potassium iodate-starch reagent or is analyzed by infrared spectroscopy. This method is generally suitable for samples containing from 0.06 to 8.0% w/w sulfur that distill at temperatures above 177°C (351°F). 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

Hardness Calcium, magnesium, barium and strontium salts expressed as CaCOa Chief source of scale in heat exchange equipment, boilers, pipe lines, etc. forms curds with soap interferes with dyeing, etc. Softening, distillation, internal boiler water treatment, surface active agents, reverse osmosis, electrodialysis... 

The barium carbonate is sufficiently soluble to initiate the reaction, and the final products, barium sulphate and calcium carbonate, are much less soluble than calcium sulphate and so do not interfere with deflocculation. 

Koirtyohann and Pickett first reported on molecular spectral interferences caused by alkali halides. CaOH absorption occurs in the region of the barium line at 5535.6 A thus the presence of calcium in the analytical sample can interfere with the determination of barium. 

Evaporate a solution of the chlorides (sodium, potassium, barium and strontium do not interfere with the determination, but calcium must be absent) to dryness in a small beaker and volatilise ammonium salts by heat. Grind up the residue with a small glass rod and add 25 ml of dry acetone containing 1 drop of concentrated hydrochloric acid (to dissolve lithium hydroxide) stir the mixture well and allow to settle. Pour the... 

Calcium cyclamate may be present with saccharin salts in pharmaceutical preparations the latter may be determined, after preliminary extraction of interference with chloroform, by ultra-violet absorption at 266 m/t or by extraction with ether from an acidified solution and then using one of the methods given above, such as fusion and barium sulphate precipitation. 

Sichere et al. [25] determined bromine concentrations in the 0.06-120mg/1 range in brines, directly by X-ray fluorescence using selenium as an internal standard to eliminate interference effects. Lower concentrations of bromine must be concentrated on filter paper containing an ion exchange resin. The same concentrations of chlorine can be determined with the addition of barium to reduce the interferences from carbonates and sulfates. Relative standard deviation was better than 1%. The interference of some other ions (e.g., calcium, potassium, magnesium, sodium, and iron) was examined. 

Polarography has also been applied to the determination of potassium in seawater [535]. The sample (1 ml) is heated to 70 °C and treated with 0.1 M sodium tetraphenylborate (1 ml). The precipitated potassium tetraphenylborate is filtered off, washed with 1% acetic acid, and dissolved in 5 ml acetone. This solution is treated with 3 ml 0.1 M thallium nitrate and 1.25 ml 2M sodium hydroxide, and the precipitate of thallium tetraphenylborate is filtered off. The filtrate is made up to 25 ml, and after de-aeration with nitrogen, unconsumed thallium is determined polarographically. There is no interference from 60 mg sodium, 0.2 mg calcium or magnesium, 20 pg barium, or 2.5 pg strontium. Standard eviations at concentrations of 375, 750, and 1125 pg potassium per ml were 26.4, 26.9, and 30.5, respectively. Results agreed with those obtained by flame photometry. 

Spectral interferences are not common in atomic absorption but can occur. An element with an absorption line sufficiently close to the one of the test element that it overlaps would cause a positive interference. Fassel et a/.20) have discussed the problems of spectral interference. This type of interference, especially in biological samples, occurs only rarely, but the analyst should be aware of it. It is more serious if a continuous source is used. Molecular absorption is a more common spectral interference and occurs when a molecular absorption band overlaps with the atomic absorption line. For example, the CaOH species absorbs in the region of the barium 5535.5 A line. A 1 % calcium solution gives an absorption equivalent to what is expected from about 75 ppm barium21). 

Test for borate Make a paste of the original substance with calcium fluoride and concentrated sulphuric acid. Hold some of this in a platinum loop just outside the base of the Bunsen flame. A green flame, due to boron trifluoride, indicates borates. Barium and copper do not interfere when the test is carried out in the above manner. 

Analytical Group I-in metals interfere, and, in principle, they should be separated. Microgram amounts of some of them can be masked with EDTA [29]. Magnesium scarcely reacts with Chlorophosphonazo III in acidic media, so calcium can be determined in the presence of a 10-fold amount of magnesium [29]. At about pH 7, magnesium does react with Chlorophosphonazo III. Strontium and barium ions give colour reactions over a wide pH range. Preliminary separation of calcium from Mg, Sr and Ba is possible by extraction with Azo-azoxy BN. 

Molecules formed in the flame can have absorption bands at the analytical wavelengths of particular elements. ITiese molecular bands occur most often when low temperature flames or special burner systems of peculiar design are used. With Perkin-Elmer s air-acetylene burner, one such interference has been noted when barium is determined in the presence of a large excess of calcium, calcium molecules absorb radiation at the barium wavelength. Even this interference vanishes in the higher temperatures generated by a nitrous-oxide-acetylene flame. 

Spectral interferences in AAS due to direct overlapping of the emission lines from the primary radiation source and the adsorption line of another element in the atom cell are very rare. To give rise to a spectral interference, the lines must not merely be within the band pass of the monochromator, but must actually overlap with each other s spectral profile (i.e., be within 0.01 nm). Spectral interferences resulting from the overlap of molecular bands and lines are more of a problem in AAS. Examples of this type of interference are the nonspecific absorption at 217.0 nm which affects lead (e.g., sodium chloride gives a strong molecular absorption at this wavelength), and the calcium hydroxide absorption band on barium at 553.55 nm. Spectral interferences may usually be eliminated by the use of background correction. 

In gravimetric analysis, the solution-preparation step has its own special significance. The analyte must be separated from interfering species, or the interferents must be masked. For example, if iron(III) is to be estimated as its hydrated oxide in the presence of chromium(III), then the mixture is initially treated with perchloric acid so as to oxidize chromium(III) to chromate (chromium(VI), Cr04 ), followed by addition of ammonia to precipitate the hydrated iron oxide. Sometimes it is necessary to remove interferents, for example, when calcium is to be estimated as calcium sulfate in the presence of barium. The barium is removed as its chromate and the calcium is precipitated quantitatively as its sulfate. 

The usual way to avoid this interference is to prevent the formation of calcium phosphate during the desolvation of the sample solution in the burner. This can be done by adding another element in excess which binds the phosphate more strongly than calcium. Elements such as strontium, barium, and landianum have been used successfully for that purpose. The other possibility is to add a complexing agent such as EDTA, which forms a complex with calcium and hence prevents the analyte element from reacting with phosphate. 

Another example of CO sensor with increase selectivity is shown in Fig. 12.3. Kitsukawa et al. (2000) tested different materials, such as Nation, Si02-Al203, zeolite, Y-AI2O3, BaO, and CaO, and established that the ethanol interference for CO sensor is almost removed by Nation filter without a sensitivity change to CO gas. It was also found that this effect was remarkably improved by the treatment of the Nation to be formed into complete acidic type before measurement. In the case of barium oxide and calcium oxide, no effect of elimination was observed. 

Spectral interferences can be expected from relatively large concentrations of barium, strontium, calcium, and sodium, although the use of the cool flame will reduce the effect of the first three to a minimum. These interferences will of course be most severe with filter instruments and corrections must be applied in the usual ways. When background correction is being applied with a monochromator instrument by measuring the intensity to one side of the line, care must be exercised to avoid the weak lithium line at 6100 A, the ends of the band systems of CaO and the oxide or hydroxide systems of strontium and barium. 

---