Neumann's Method

NEUMANN'S METHOD (Neumann) of incineration is based on the use of a strong oxidizing mixture consisting of equal parts of concentrated nitric and sulfuric acids. Incineration is performed in a round-bottom flask made of Jena glass with a capacity of about ³/₄ liter, into which the mixture is added little by little from a dropping funnel. The amount of liquid added is regulated by a stopcock, and the entire funnel is secured on a stand. With this method of incineration, which is simpler and more convenient than ashing (see), all volatile acids, such as HCl, CO₂, etc., are lost, and only non-volatile acids (for example, phosphoric) and metals can be determined. During incineration, it is necessary to ensure that the contents of the flask do not evaporate to dryness. If a liquid needs to be incinerated, then depending on its concentration, it is either incinerated directly (e.g., blood) in the flask or pre-concentrated in the incineration flask (e.g., urine). If the substance contains a large amount of fat or carbohydrate (milk), it is first treated with 1% KOH (in the case of milk, 15 cm³ of KOH per 25 cm³ of milk), evaporating it to the consistency of syrup; the solution is then neutralized with nitric acid and incinerated. In case it is necessary to concentrate large quantities (500-600 cm³) of liquid (e.g., urine), the measured portion is mixed with strong nitric acid (¹/₁₀ of the volume), gradually added through a dropping funnel into the round-bottom incineration flask, where 30 cm³ of nitric acid is located, and evaporated over a small flame. The addition is regulated so that the amount of liquid in the flask does not exceed 100 cm³. When all the liquid has been added, the flame is increased and the entire liquid is concentrated to a small volume (50 cm³), after which incineration begins. A small amount of the oxidizing mixture (5-10 cm³) is added to the material processed by the indicated methods or directly to the substance under investigation, and it is heated over a low flame. After the main mass of dark brown vapors has departed, the same amount of the mixture is added dropwise and heated again. To determine the end of incineration, the addition of the mixture is suspended and it is boiled until the brown vapors are removed. If the liquid in the flask remains almost black or dark brown, then the addition of the mixture is continued and after some time the test is repeated. Upon reaching the end of the reaction, the liquid should become colorless or only slightly yellow and transparent. Upon cooling, the contents of the flask are diluted with 3-4 volumes of water and boiled for about 10 minutes to remove residues of nitrogen oxides. This method of incineration serves as a preparatory step for a whole series of qualitative and quantitative determinations of inorganic substances.