The method described in Part 11 for determining "reactive" silicon has been shown to be suitable for analysing boiler waters containing phosphate and alkali. This paper describes the tests performed to confirm the suitability of the method in the presence of up to 5.0 mg of phosphate. Although the magnitude of interference from phosphate is small for silicon concentrations exceeding 0.1 p.p.m. of silica, the nature of the effect was found to be complex, and has been studied in detail. The effects of many other substances have also been determined. Coefficients of variation of about 0.4 per cent. have been obtained in the analysis of boiler waters.PART I11 describes a method for determining "reactive" silicon in relatively pure waters such as de-ionised water, feed-water, make-up water and steam. The method was expected to be suitable for analysing boiler waters that may contain appreciable concentrations of phosphate and alkali. However, the effect of these two substances had not been tested in sufficient detail for a definite recommendation to be made. Such tests have now been performed, and the results presented in this paper confirm the suitability of the method for analysing boiler waters.Two variants of the method are described in Part 1I.l In the first, portions of samples are analysed in 100-ml calibrated flasks; in the second (the modification for higher sensitivity), 100-ml portions are analysed in polythene bottles. In view of the alkalinity of many boiler waters, the latter technique was adopted as described below under "Method." The method given in this paper is suitable for all types of power-station waters but the tolerances given for times and reagent concentrations are smaller than those in Part I1.l This is to ensure that the effects caused by phosphate are small. METHOD APPARATUS-Polythene bottles, 4-OY 8-oz capacity-The analyses are carried out in polythene bottles, and the magnitude of the interference caused by phosphate may depend on the particular bottles that are used (see under (iii), p. 637). Therefore, before a set of bottles is used for analysis, it is prudent first to check that the effect of phosphate is adequately small. When the bottles have been shown to be satisfactory they should be reserved for this analysis only. REAGENTS-The water, tartaric acid solution, reducing-agent solution and standard solutions of silicon should be prepared as described in Part 1I.l AcidiJied molybdate solution-Dissolve 89 g of ammonium molybdate, (NH,) ,Mo70,,.4H2O, in about 800ml of water at room temperature. Add 63ml of 98 per cent. sulphuric acid cautiously to 100 ml of water, with stirring, and allow the mixture to cool. Add the acid to the molybdate solution, cool it to room temperature, and dilute it to 1 litre with water.Both the acidified molybdate solution and the tartaric acid solution have been found to be adequately stable for at least 6 months.* For details of earlier parts of this series, see reference list, p. 641.f "Reactive" silicon is defined in this paper as those forms of silicon, ...
An investigation has been made into the accuracy of ammonium-selective (ammonium-responsive) glass electrodes for determining ammonia (10 to 1000 pgl-l) in boiler feed-water and similar high-purity water samples from power stations. The electrode potential follows the Nernst equation in samples containing up to 10 000 mg 1-1 of ammonia, the pH of which is controlled between 8.0 and 8.4 by the addition of triethanolaminehydrochloric acid buffer solution. However, interfering species in the buffer solution cause a detectable deviation from Nernstian response at low ammonia concentrations (less than 1000 pg 1-1). By use of a calomel -0.1 N hydrochloric acid reference electrode, reproducible results have been obtained in static buffered solutions containing 10 to 1000 pgl-1 of ammonia. Of the other impurities likely to be present in power-station waters only sodium caused a serious effect (100 pgl-1 of sodium is equivalent to 25 pg1-1 of ammonia). The within-batch standard deviations of analytical results were 2, 7, 17 and 33 t concentrations of 10, 100, 500 and 1000 pg 1-1 of ammonia, respect-&: &. aDetails of a recommended analytical procedure for discrete samples are given, and the application of the ammonium-selective electrode to continuous on-stream analysis is briefly discussed.* The term "static" refers throughout this paper t o the system in which the electrodes are immersed in a magnetically stirred, buffered solution of the sample contained in a polythene beaker.
A simple, fast method for determining low concentrations of oxygen in power-station waters has been developed, based on the reaction of dissolved oxygen with the leuco-base of methylene blue to give a soluble blue oxidation product the absorbance of which is a function of the oxygen concentration.A special glass cell has been devised, which acts sequentially as a samplecollection vessel, a reaction vessel and a spectrophotometric cuvette. The cell design permits the easy addition of the leuco-base and also the airsaturated water used for calibration. A novel technique of "zero-time extrapolation" for the determination of the reagent/cuvette blank circumvents the difficulty of making this measurement with oxygen-free water.The calibration graph is linear up to 50 pgl-l, but satisfactory measuretnents may be made up to 100 pg 1-l. The criterion of detection is approximately 1.0 pg 1-l with standard deviations ranging between 0.4 and 1 . 7 pg l-l, depending on the concentration. The analysis time is 5-10 min for a single determination.Iron(I1) and copper(I1) ions are the only ions likely to be present in boiler waters that cause serious interference and these must be removed before analysis by passing the water sample through a cation-e:.;changc column.
A detailed investigation has been made of the accuracy of sodiumresponsive glass electrodes for determining sodium (1 to 50 pg per litre) in high purity waters (e.g., condensed steam and boiler feed-water) from power stations. The electrode potential can be made to follow the Nernst equation down to a sodium concentration of about 1 p g per litre by controlling the pH of the sample and by using a continuous flow of the sample past the electrode. Octadecylamine seriously affected the response of the electrodes, but other impurities likely to be present in power-station waters caused no significant effects. The standard deviation of analytical results varied from 0-4 to 1.2pg per litre a t concentrations of 2 and 26pg of sodium per litre. Details of a recommended analytical procedure for discrete samples are given.
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