Photometric determination Subject

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

Blood is obtained from the test subjects by venipuncture and mixed with K-EDTA (1 mg/ml) or heparin (5 IU/ml heparin sodium) to prevent clotting. Erythrocyte aggregation is determined in whole blood of 40 % haematocrit. A sample of 40 xl blood is transferred to the measuring device. The red cells are dispersed at a shear rate of 600/s. After 20 s, flow is switched to stasis and the extent of erythrocyte aggregation is determined photometrically. 

However, there are instances when no visible indicator is available to identify the equivalence point of a titration. Also, some color changes can be very subtle, which can introduce subjectivity (and hence errors) in determining the equivalence point. In these situations, a more reliable detector than the human eye is required. In situations where the analyte, reagent, or titration product absorbs radiation, photometric measurements can be employed to determine the equivalence point of the titration. 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

When subjected to alkaline hydrolysis streptomycin yields maltol, 2-methyl-3-hydroxy-y-pyrone, quantitatively and this fact may be used in the assay of streptomycin, the maltol being determined either spectro-photometrically (maximum absorption at 322 m//) or colorimetrically (after reaction with ferric iron to give a purple-red colour). Dihydrostreptomycin will not yield maltol under the conditions given below so that streptomycin can be determined in its presence as, for example, in Streptoduocin for Injection, U,S.P, which contains equal amounts of the two antibiotics. 

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