Determination of calcium

Lochmuler, C. Atomic Spectroscopy—Determination of Calcium and Magnesium in Sand with a Statistical Treatment of Measurements published on the web at http //www.chem.duke.edu/ clochmul/exp4/exp4.html. 

Quigley, M. N. Determination of Calcium in Analgesic Tablets Using Atomic Absorption Spectrophotometry, ... 

Traces of many metals interfere in the determination of calcium and magnesium using solochrome black indicator, e.g. Co, Ni, Cu, Zn, Hg, and Mn. Their interference can be overcome by the addition of a little hydroxylammonium chloride (which reduces some of the metals to their lower oxidation states), or also of sodium cyanide or potassium cyanide which form very stable cyanide complexes ( masking ). Iron may be rendered harmless by the addition of a little sodium sulphide. 

Determination of calcium. Pipette two 25.0 mL portions of the mixed calcium and magnesium ion solution (not more than 0.01M with respect to either ion) into two separate 250 mL conical flasks and dilute each with about 25 mL of de-ionised water. To the first flask add 4 mL 8 M potassium hydroxide solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 30 mg each of potassium cyanide (Caution poison) and hydroxylammonium chloride and swirl the contents of the flask until the solids dissolve. Add about 50 mg of the HHSNNA indicator mixture and titrate with 0.01 M EDTA until the colour changes from red to blue. Run into the second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1 mL. Now add 4 mL of the potassium hydroxide solution, mix well and complete the titration as with the first sample record the exact volume of EDTA solution used. Perform a blank titration, replacing the sample with de-ionised water. 

G1 DETERMINATION OF CALCIUM IN THE PRESENCE OF MAGNESIUM USING EGTA AS TITRANT... 

Determination of calcium as oxalate. Discussion. The calcium is precipitated as calcium oxalate CaC204,H20 by treating a hot hydrochloric acid solution with ammonium oxalate, and slowly neutralising with aqueous ammonia solution ... 

Discussion. This method is based upon the formation of a fluorescent chelate between calcium ions and calcein [fluorescein bis(methyliminodiacetic acid)] in alkaline solution.29 The procedure described below30 has been employed for the determination of calcium in biological materials.31 ... 

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

The reliability of the results may be judged from the following. In sections of biological materials, about 10 pt thick, the absolute errors in the determination of calcium ranged between 0.15 and 0.2 g per square micron those for phosphorus and sulfur between 0.06 and 0.1 /igg per square micron. These errors often corresponded to less than 5% of the amount of element present in the sample. 

Calcium activities as low as 5 x 10 7 M can be measured, with selectivity coefficients ACaMg and ACaK of 0.02 and 0.001, respectively. Such potential response is independent of the pH over the pH range from 5.5 to 11.0. Above pH 11, Ca(OH)+ is formed, while below pH 5.5, protons interfere. Because of its attractive response characteristics, the calcium ISE has proved to be a valuable tool for the determination of calcium ion activity in various biological fluids. 

We have found that the use of serum standards for standardizing the instrument in the laboratory is useful. However, the serum standards cannot be used for urines. In urines, one runs into other problems and needs to use aqueous standards. Therefore, at present, while atomic absorption is the instrument of choice, there is much to be desired for the determination of calcium and magnesium in the routine laboratory of clinical chemistry. 

Cali JP, Mandel J, Moore L, and Young DS (1972) A Referee Method for the Determination of Calcium in Serum Standard Reference Materials. NBS Spec Publ 260-36. National Bureau of Standards, Washington, DC. 

Siong, T. E., Choo, K. W., and Shahid, S. M. (1989a). Determination of calcium in foods by the atomic absorption spectrophotometric and titrimetric method. Pertanika 12, 303-311. 

This dye fluoresces after binding Pb+2 and Ca+2 lead is considered an interferant to the determination of calcium by this approach. However, by complexing the divalent lead ion with the heavy metal chelator TPEN (N,N,N ,N -tetrakis(2-pyridylmethyl)ethylene-diamine) prior to the addition of the fluo-3, the fluorescent... 

Jagner [150] used computerised photometric titeration in a high-precision determination of calcium in seawater. 

Table 5.2. Reported studies on determination of calcium using EDTA ...

The presence of normal concentrations of sodium, magnesium, and strontium have no net effect on the determination of calcium above the approximate level of accuracy of about 0.1% so that no correction factor seems necessary. A sufficient amount of titrant must be added to complex at least 98% of dissolved calcium before the buffer is added this apparently reduces the loss of calcium by coprecipitation with magnesium hydroxide. 

Atomic absorption spectrophotometry [165,166] has been used in the determination of calcium and magnesium in seawater. 

Brenner et al. [ 169] applied inductively coupled plasma atomic emission spectrometry to the determination of calcium (and sulfate) in brines. The principal advantage of the technique was that it avoided tedious matrix matching of calibration standards when sulfate was determined indirectly by flame techniques. It also avoided time-consuming sample handling when the samples were processed by the gravimetric method. The detection limit was 70 ig/l and a linear dynamic range of 1 g/1 was obtained for sulfate. 

---