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The processes of volumetric analysis are, in general, simpler than those of gravimetric analysis and accordingly serve best as an introduction to the practice of quantitative analysis.
From the weight of this substance and its known composition, the amount of the constituent in question is determined. In !volumetric! analysis, instead of the final weighing of a definite body, a well-defined reaction is caused to take place, wherein the reagent is added from an apparatus so designed that the volume of the solution employed to complete the reaction can be accurately measured.
Similarly, a burette used for the titration of the 50 cc. of solution removed should be calibrated under the same conditions as the measuring flasks or pipettes employed with it. The student should also keep constantly in mind the fact that all volumetric operations, to be exact, should be carried out as nearly at a constant temperature as is practicable.
As I have had to make numerous estimations of manganese in various compounds, as a public analyst, I have been induced to investigate the volumetric methods at present in use to find their comparative values, and if possible to work out a new one, setting aside one or more of the difficulties met with in the use of the older ones. This paper is a part summary of the results.
For a description of a procedure which is most convenient when many flasks are to be calibrated, the student is referred to the !Am. It cannot be too strongly emphasized that for the success of analyses uniformity of practice must prevail throughout all volumetric work with respect to those factors which can influence the accuracy of the measurement of liquids.
The methods of quantitative analysis are subdivided, according to their nature, into those of !gravimetric analysis, volumetric analysis!, and !colorimetric analysis!. In !gravimetric! processes the constituent to be determined is sometimes isolated in elementary form, but more commonly in the form of some compound possessing a well-established and definite composition, which can be readily and completely separated, and weighed either directly or after ignition.
It is, however, true that some of the advantage which pertains to the use of normal solutions as regards simplicity of calculations is lost when using these approximate solutions. The application of these general statements will be made clear in connection with the use of normal solutions in the various types of volumetric processes which follow.
The student is again reminded that variations in temperature of volumetric solutions must be carefully noted, and care should always be taken that no source of heat is sufficiently near the solutions to raise the temperature during use. It is then possible to run in rapidly approximately the required amount, after which it is only necessary to determine the end-point slowly and with accuracy.
Jackson virtually puts aside the volumetric theory with his statement, that "the balance of intra-ocular pressure is not maintained by the slight distensibility of the sclero-corneal coat." Further discussion on the inadequacy of the volumetric theory need not detain us. It is well to recall a few anatomical features because of their bearing on the theories herein considered.
Whatever other variations may occur in its composition, the ratio of 12 MoO :1 P seems to hold, and this fact is utilized in volumetric processes for the determination of phosphorus, in which the molybdenum is reduced to a lower oxide and reoxidized by a standard solution of potassium permanganate. The total volume of nitrate and washings should not much exceed 100 cc.
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