Weighing, estimation

When the test piece is removed from the liquid, the volume or mass will decrease with time as the liquid permeates back out. For volatile liquids this dictates that the time between removal and test is critical and it may be necessary- to extrapolate back to zero time. It is also possible to dry the test piece and by further weighing estimate the amount of matter extracted, but this is liable to be inaccurate. The physical properties after drying can be measured and may be particularly relevant for some applications. 

The sulphate ion is detected by addition of barium chloride in the presence of hydrochloric acid a white precipitate of barium sulphate is obtained. The same test can be used to estimate sulphate, the barium sulphate being filtered off, dried and weighed. 

Addition of calcium nitrate solution to a fluoride gives a white precipitate of calcium fluoride, CaFj. If the latter is precipitated slowly, it can be filtered off and weighed to estimate the fluoride. Fluoride can also be determined by the addition of sodium chloride and lead nitrate which precipitate lead chlorofluoride, PbClF. This is filtered off and weighed. 

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

Digestion. 20-25 mg. of the substance whose nitrogen content is to be estimated are weighed out in a stoppered weighing-tube and then transferred to the flask L that has been previously dried in an oven at izo C. With care, the substance may be transferred directly into the bulb of the flask without any adhering to the sides. If any material sticks on the way down, the flask should be tapped gently to cause the substance to fall to the bottom. 2 G. of the catalyst mixture (32 g. of potassium sulphate, 5 g. of mercury sulphate and ig. of selenium powder, well mixed) are added and 3 ml. of A.R. cone, sulphuric acid are measured out carefully and poured into the digestion... 

The estimation. Label two 250 ml. conical flasks A and B, and into each measure 5 ml. of urine solution (or about o i g. of solid urea, accurately weighed). Add to each about 20 ml. of water and bring the temperature to about 60°. To A add 3 drops of phenolphthalein solution and to B add i ml. of 0-5% mercuric chloride solution. Now to each solution, add 10 ml. of the urease solution and mix well. The mixture A soon turns red. 

Amount of material required. It is convenient to employ an arbitrary ratio of 0 10 g. of solid or 0 20 ml. of liquid for 3 0 ml. of solvent. Weigh out 0 10 g. of the finely-powdered solid to the nearest 0 01 g. after some experience, subsequent tests with the same compound may be estimated by eye. Measure out 0-20 ml. of the liquid either with a calibrated dropper (Fig. 11,27, 1) or a small graduated pipette. Use either a calibrated dropper or a graduated pipette to deliver 3 0 ml. of solvent. Rinse the delivery pipette with alcohol, followed by ether each time that it is used. 

A billet of PVC weighing ISO g is to be compression moulded into a long playing record of diameter 300 mm. If the maximum force which the press can apply is 100 kN estimate the time needed to fill the mould. The density and viscosity of the the PVC may be taken as 1200 kg/m and 10 Ns/m respectively. 

In conclusion, llic purpose of Ihe loxicily assessment is to weigh available evidence regarding the potential for particular contaminants to cause adverse effects in exposed individuals and to provide, where possible, an estimate of the relationship between the extent of exposure to a contaminant and the increased likelihood and/or severity of adverse effects. 

Hydrocyanic acid may be approximately estimated by dissolving 1 gram of oil in 5 c.c. of alcohol, and adding 50 c.c. of water. Then add ammonio-silver nitrate solution and shake well. Acidify slightly with nitric acid, and collect, wash, and dry the silver cyanide precipitated. Ignite and weigh the silver, 4 parts of which correspond to practically 1 of hydrocyanic acid. 

As regards the use of hydroxylamine for the estimation of ketones, it was recommended by Kremers in 1901 for the estimation of oarvone in spearmint oil, the ketoxime being formed by treating the oil with hydroxylamine, and the remainder of the oil removed by steam distillation, the crystalline ketoxime which is left being separated, dried, and weighed. 

Several processes for the estimation of aldehydes and ketones have been based on these reactions, some depending on the separation and weighing of the insoluble hydrazone, others on treatment of the substance with an excess of pbenylhydrazine, and estimation of the unused reagent. 

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