Gooch and Pulman — Estimation of Molybdic Acid. 449 



Art. XLYIII. — The Estimation of Molybdic Acid reduced by 

 Hydriodic Acid ; by F. A. Gooch and O. S. Pulman, Jr. 



' [Contributions from the Kent Chemical Laboratory of Yale University — CIY.] 



While the determination of molybdic acid by estimating the 

 iodine set free upon boiling a solution of that acid in hydro- 

 chloric acid to which potassium iodide has been added, is a 

 possibility, the operation is beset with difficulties. If due 

 attention be paid to the proportion of acid, the excess of potas- 

 sium iodide over the amount theoretically necessary to produce 

 the ideal reduction, and the final degree of concentration of the 

 liquid, the change of the molybdic acid to the condition of oxi- 

 dation of the pentoxide, Mo 2 5 , may, as has been shown in 

 former articles from this laboratory,* be closely realized. 

 Even when the conditions essential to the ideal reduction 

 obtain, however, the possibility of the interaction of atmospheric 

 oxygen with the hydriodic acid produced from the potassium 

 iodide and hydrochloric acid must be guarded against by con- 

 ducting the operation in an atmosphere of carbon dioxide. 

 Furthermore, the hydrochloric acid employed must be freed, 

 so far as may be, from dissolved oxygen by previous boiling. 

 The operation is capable of yielding accurate indications (unless 

 by a chance combination of opposite errors) only when the 

 precautions mentioned are scrupulously observed. 



On the other hand, the essential principles of the reaction 

 admit of very simple application when the residue of the dis- 

 tillation, instead of the iodine distilled, is made the object of 

 the analytical determination. This was the plan of a method 

 formerly proposed. f In this process the mixture of molybdic 

 acid, potassium iodide and hydrochloric acid was boiled 

 between defined limits of volume in an Erlenmeyer flask 

 simply trapped to prevent mechanical loss during the boiling, 

 and the residue, after the removal of all free iodine, was 

 treated with tartaric acid to prevent subsequent precipitation, 

 neutralized with an alkaline bicarbonate, and reoxidized by a 

 measured excess of standard iodine, this excess of iodine being 

 finally determined by titration with a standard arsenite. The 

 difference between the iodine value of the arsenite used and 

 the amount of standard iodine employed measures the molybdic 

 acid. The action of atmospheric or dissolved oxygen obviously 

 plays no important part in the operation providing all uncom- 

 bined iodine, however produced, is finally boiled out. At the 

 outset iodine is liberated by the air present as well by the 

 intended action of the molybdic acid, but as the boiling con- 

 tinues the hydriodic acid diminishes in strength, the iodine is 

 driven from the flask kept filled with steam, and if the condi- 

 tions of the operation have been properly adjusted the molyb- 



* Gooch and Fairbanks, this Journal, ii, 156; Gooch and Norton, this Journal, 

 vi, 168. f Gooch and Fairbanks, this Journal, ii, 160. 



Am. Jour. Sci.— Fourth Series, Yol. XII, No. 72 — December, 1901. 

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