Magnesium determination

Magnesium, determination by x-ray emission spectrography, 32 in cements, 260, 261 Magnetic lenses, use, 263, 293 Major constituents, determination by comparison with a standard, 179-185... 

There are a number of interferences in magnesium determination in the air-acetylene flame, the best known being silicate, phosphate, and aluminium, so a releasing agent, e.g. lanthanum at a final dilution of 5 mg ml" must always be used. 

Haemolysis of the sample will cause a significant increase in plasma magnesium level. Lanthanum is added to samples and standard for convenience so that calcium and magnesium determinations may be made on each sample. 

Several papers deal with magnesium determination in blood and urine. Willis (WIO) analyzed serum in the air-acetylene flame and found no effect from the presence of sodium, potassium, calcium, or phosphate, but states that an enhancement was seen in serum diluted with water only, probably due to serum proteins. This interference was controlled by addition of strontium or EDTA. Sensitivities were the same in the eoal gas-air and air-acetylene flame, indicating complete atomization of magnesium. In urine (W13) no interference was encountered and determinations were performed on samples directly diluted with water. 

Comparisons of instruments for free magnesium determinations have been reported." Electrodes are warranted by the manufacturer for as little as a couple days to several months. Differences in measured free magnesium were apparent between analyzers, mainly because of interference from free calcium. Further improvements in ISEs and instruments for free magnesium would improve performance and increase the availability of these determinations. 

Discordance between total and free magnesium measurements has been reported in selected patient populations, including those with cardiovascular disorders, diabetes meUitus, alcoholism, migraine headaches, asthma, renal transplants, head trauma, and in pregnant women. Interferences, such as that from thiocyanate, in measurement of fr ee magnesium may explain some of these discrepencies. Free magnesium determinations may be helpfiil in others of these disorders and in critically ill patients and during cardiopulmonary bypass, preeclampsia, neonatal distress, and therapy with a number of... 

Stone M, Chowdrey PE, Maill P, Price CP. Validation of an enzymatic total magnesium determination based on activation of modified isocitrate dehydrogenase. Clin Chem 1996 42 1474-7. 

Equation (12.2) indicates two ways to determine the formula of RMg X (I) Mg/RIl must equal n anu to Hi/RFi must equal a-I. the nuclcar-itics of magnesium determined for the products of cryosynlhesis from various aryl halides arc presented in Table 12.1 along with comparison data for products obtained from some alkyl chlorides. The data in Table 12.1 indicate that the nuclearities obtained by elemental analyses and in two ways from the compositions of hydrolysis products are rather similar. 

The selectivity of hydroxyquinoline in itself is very poor. One reference, which deals with the reagent in general and also its use for magnesium determinations in some detail, has a table (Table 6.3) of cations that can be precipitated by the reagent at different pH values. The lack of selectivity is also underlined in another reference, which states that the reagent can be used for identification of magnesium, but that all other metals except sodium and potassium must be absent. ... 

The co-occurrence of cations may also contribute to the formation of refractory compounds. The presence of aluminum has such an effect on the determination of magnesium. It is assumed that a refractory aluminium—magnesium oxide is formed which reduces the sensitivity of the magnesium determination. 

Fig. 30 Effect of alloying and contaminant metals on the corrosion rate of magnesium, determined by alternate immersion in 3% NaCi solution [35].

Two chelating agents, ethylenediaminetetraacetic acid (EDTA) and 8-hydroxyquinoline, also have been used for calcium and magnesium determinations. They are effective in controlling a number of interfering ions, including phosphate, sulfate, aluminum, silicon, boron, and selenium. 

There are two methods commonly used for magnesium determination. The first one is based on reaction, in sodium hydroxide media, with Titan Yellow (sodium salt of dehydrothio-p-toluidine sulfonic acid) to produce a colored red colloidal suspension. The other method is based on reaction with Solo-chrome Black to give a red soluble complex. 

In general, any strong acid can be used for the extraction. The choice of acid depends on the sample matrix. Acceptable results have been obtained with hydrochloric and perchloric acids. Nitric acid may oxidize organic matrices making certain constituents in the matrix soluble. Sulfuric acid is another acceptable acid for pretreatment extraction. However, sulfuric acid will cause disturbances if electrothermal AAS is used as the method of magnesium determination. The addition of a surfactant such as Triton X-100 can assist in the digestion of hydrophobic material. 

Spectral absorbance and electrochemical detectors, in conjunction with single or dual 1C columns, can also be used for magnesium determination. The sample pretreatments can be ultrafiltration, incineration, or acid extraction. These have proven to be reliable methods for the quantification of magnesium in plant material [106] and in serum and plasma [107]. 

Graphite furnace atomic absorption spectrometry (GFAAS) is an excellent method to provide sub-ng/mL minimum detection limits [110]. Continuing advancements such as Zeeman correction, and stabilized temperature platform furnaces, have made GFAAS an effective analytical method for magnesium determination. Depending on the sample matrix, pretreatment can vary from direct analysis of fluids, to wet mineralization, dry ash, acid extraction, and by using PPRs (e.g., Triton X-100). 

If the residues for magnesium determination have been obtained during the course of separation of the alkaline earths, it is advisable to eliminate an excess of ammonium oxalate by evaporating the solution to dryness in the presence of nitric acid and heating to volatilise most of the ammonium salts, finally dissolving the residue in a minimum of hydrochloric acid and filtering before precipitating as above. 

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