Ignition of precipitates

Many precipitates are formed with a composition unsuitable for weighing, or containing varying amounts of water (or other solvent) that need removal most precipitates therefore need to be heated to convert them to compounds of known composition. Water may be present as superficial adherent water clinging to the moist 

The weight of an ignited precipitate is itself not necessarily represented accurately by the balance reading there may be equilibration of the precipitate and container with atmospheric moisture (the atmosphere in a desiccator probably is not dry), and absorption of carbon dioxide or ammonia. 

Potassium tetraphenylborate, normally dried at 105 to 120°C, is stable up to 265°C, according to the thermolysis curve of Wendlandt. At 715 to 825°C the metaborate KBO2 is formed. 

Calcium oxalate normally is precipitated as the monohydrate from hot solution. The thermolysis curve shows several plateaus, corresponding to the monohydrate from room temperature to 100°C, anhydrous calcium oxalate from 226 to 398°C, calcium carbonate from 420 to 660°C, and calcium oxide above 840 to 850°C. Sandell and Kolthoff concluded that the monohydrate is not a reliable weighing form because of its tendency to retain excess moisture. Coprecipitated ammonium oxalate also remains undecomposed, so the results are usually 0.5 to 1.0% too high when the precipitate is dried at 105 to 110°C. Anhydrous calcium oxalate also is unsuitable as a weighing form because of its hygroscopicity. 

Willard and Boldyreff concluded that calcium carbonate is an excellent weighing form if the oxalate is ignited at a temperature of 500 25 C. The necessity of this closeness of temperature control is evident from the following consideration. The minimum temperature is determined by the rate of the irreversible decomposition 

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In addition to the evolution of water, the ignition of precipitates often results in thermal decomposition reactions involving the dissociation of salts into acidic and basic components, e.g. the decomposition of carbonates and sulphates the decomposition temperatures will obviously be related to the thermal stabilities. 

Solution reaction between analytes and reagents to give sparingly soluble products filtration, drying or ignition of precipitates electrolytic deposition of metals weighing. 

Cd(OH) j. The hydroxide is precipitated from aqueous solution by OH", it does not dissolve in excess OH". Ignition of Cd(OH)2 or CdCO, gives CdO which varies in colour from red-brown to black because of lattice defects. 

The first report concerning barium compounds occurred in the early part of the seventeenth century when it was noted that the ignition of heavy spar gave a peculiar green light. A century later, Scheele reported that a precipitate formed when sulfuric acid was added to a solution of barium salts. The presence of natural barium carbonate, witherite [14941-39-0] BaCO, was noted in Scodand by Withering. 

Solid carbon, however produced, presents a hazard. It may be burnt off at low temperatures, with free access to air, without harm to the crucible, but it should never be ignited strongly. Precipitates in filter paper should be treated in a similar manner strong ignition is only permissible after all the carbon has been removed. Ashing in the presence of carbonaceous matter should not be conducted in a platinum crucible, since metallic elements which may be present will attack the platinum under reducing conditions. 

After a precipitate has been filtered and washed, it must be brought to a constant composition before it can be weighed. The further treatment will depend both upon the nature of the precipitate and upon that of the filtering medium this treatment consists in drying or igniting the precipitate. Which of the latter two terms is employed depends upon the temperature at which the precipitate is heated. There is, however, no definite temperature below or above which the... 

Determination of uranium with cupferron Discussion. Cupferron does not react with uranium(VI), but uranium(IV) is quantitatively precipitated. These facts are utilised in the separation of iron, vanadium, titanium, and zirconium from uranium(VI). After precipitation of these elements in acid solution with cupferron, the uranium in the filtrate is reduced to uranium(IV) by means of a Jones reductor and then precipitated with cupferron (thus separating it from aluminium, chromium, manganese, zinc, and phosphate). Ignition of the uranium(IV) cupferron complex affords U308. 

The reduction is avoided by first charring the paper without inflaming, and then burning off the carbon slowly at a low temperature with free access of air. If a reduced precipitate is obtained, it may be re-oxidised by treatment with sulphuric acid, followed by volatilisation of the acid and re-heating. The final ignition of the barium sulphate need not be made at a higher temperature than 600 800 °C (dull red heat). A Vitreosil or porcelain filtering crucible may be used, and the difficulty of reduction by carbon is entirely avoided. 

If there is any doubt concerning the purity of the potassium titanyl oxalate, standardise the solution by precipitating the titanium with ammonia solution or with cupferron solution, and igniting the precipitate to TiOz. 

Rapid passage of gas into a cone, nitrate solution caused an explosion, or ignition of a slower gas stream. The explosion may have been caused by rapid oxidation of the precipitated silver phosphide derivative by the co-produced nitric acid or dinitrogen tetraoxide. 

For analysis, a silver-containing solution was made alkaline with 25% sodium hydroxide solution and filtered, then the precipitate was washed with ammonium hydroxide to redissolve the silver. Hydrazine (as sulfate) was added to precipitate the silver, and when the mixture was heated, an explosion occurred. This could have been caused by precipitation of explosive silver nitride, rapid catalytic decomposition by silver compounds of the hydrazine salt, and/or ignition of the hydrogen evolved. 

Excer A process for making uranium tetrafluoride by electrolytic reduction of a uranyl fluoride solution, precipitation of a uranium tetrafluoride hydrate, and ignition of this. 

However, it is pertinent to mention here that quite a few techniques related to measurement of pharmaceutical substances and reagents involved is more or less common to both gravimetric and volumetric analysis. Besides, in gravimetric analysis, some more additional techniques play a vital role, namely precipitation, filtration, washing of the precipitate and ignition of the precipitate. 

Bismuth trioxide is commercially made from bismuth subnitrate. The latter is produced by dissolving bismuth in hot nitric acid. Addition of excess sodium hydroxide followed by continuous heating of the mixture precipitates bismuth trioxide as a heavy yellow powder. Also, the trioxide can be prepared by ignition of bismuth hydroxide. 

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