Primary standard sodium oxalate

In this connection it must be pointed out that standard samples which have been analysed by a number of skilled analysts are commercially available. These include certain primary standards (sodium oxalate, potassium hydrogenphthalate, arsenic(III) oxide, and benzoic acid) and ores, ceramic materials, irons, steels, steel-making alloys, and non-ferrous alloys. 

Standardization of potassium permanganate with a primary standard, sodium oxalate... 

Method A With arsenic(III) oxide. This procedure, which utilises arsenic(III) oxide as a primary standard and potassium iodide or potassium iodate as a catalyst for the reaction, is convenient in practice and is a trustworthy method for the standardisation of permanganate solutions. Analytical grade arsenic(III) oxide has a purity of at least 99.8 per cent, and the results by this method agree to within 1 part in 3000 with the sodium oxalate procedure (Method B, below). 

Note A good number of Standard Samples, including primary standards, such as arsenic trioxide, benzoic acid, potassium hydrogen phthalate, sodium oxalate, are available as ... 

Examples of primary standards include sodium carbonate (NajCOj), oxalic acid (H2C20. 2H20) and potassium hydrogenphthalate. 

A solution of potassium permanganate was standardized against sodium oxalate (Na C ) primary standard. A 50.0 mL 0.01 MNa2C204 standard solution required 37.8 mL KMn04 solution. Determine the strength (molarity) of KMn04 solution. 

Unfortunately, potassium permanganate is not obtainable in high enough purity and can undergo decomposition by exposure to sunlight. Therefore it cannot be used as a primary standard (p. 143). However, it can be used in redox titrations provided it is standardized with sodium oxalate (which is available in high purity). The redox reaction involving oxalate is as follows ... 

Although sodium oxalate is commonly used as a primary standard, its use is recommended only when oxalate is to be determined. For best absolute accuracy As(III) oxide is recommended. The direct titration of As(III) in acid solution does not proceed readily without a catalyst, probably because of the stabilization of Mn(III) by complex formation with As(V). With potassium iodate (1 drop of0.0025 M solution) as catalyst, the potentiometric end point was found to coincide with the visual end point, using ferroin, to within 0.01%, and the accuracy, tested against pure potassium iodide, was within 0.02%. 

Sodium oxalate has been used as a primary standard substance for Ce(IV) in sulfuric acid. In the absence of a catalyst a temperature of 70 to 75°C is necessary. Smith and Getz found that in 1 to 2 M perchloric acid solution, sodium oxalate can be titrated at room temperature with Ce(IV) perchlorate or nitrate but not with sulfate. Rao, Rao, and Rao carried out the titration at room temperature in the presence of barium chloride to remove sulfate, which retards the reaction between oxalate and Ce(IV) and between oxalate and oxidized ferroin. Alternatively, some Fe(III) was added, and the trace of Fe(II) produced photochemically then reacted with the indicator. Rao, Rao, and Murty carried out the titration in 0.5 M HNOj with ammonium hexanitratocerate(IV) instead of the sulfate. With a small amount of KI and KIO3, a satisfactory end point was obtained at room temperature with ferroin as indicator. 

Sodium oxalate is a widely used primary standard. In acidic solutions, the oxalate ion is converted to the undissociated acid. Thus, its reaction with permanganate can be described by... 

Sodium oxalate (primary standard). Dissolve 30 g of the commercial salt in 1 L of water, make slighdy alkaline with sodium hydroxide, and let stand until perfectly clear. Filter and evaporate the filtrate to 100 mL. Cool and filter. Pulverize the residue and wash it several times with small volumes of water. The procedure is repeated until the mother liquor is free from sulfate and is neutral to phenolphthalein. 

In this experiment, you will standardize (determine precisely the concentration) a solution of sodium hydroxide, NaOH, using oxalic acid dihydrate, H2C204 2H20, as a primary standard acid. A primary standard acid is a solid acid whose mass is an accurate measure of the number of moles of protons the acid will furnish. Oxalic acid, H2C2O4, is a diprotic acid and provides two reactive protons per molecule according to the following net ionic equation for the neutralization reaction. 

In this experiment as in Experiment 25, you will standardize a solution of sodium hydroxide using oxalic acid dihydrate, H2C2O4 2H20, as the primary standard. This solution of base will then be used to titrate the unknown mixture. Phenolphthalein will be used as the indicator of the endpoint in all titrations. Your goal is to determine the percent composition by mass of the mixture. 

Nitrite was measured by a spectrophotometric procedure described in Section 18-4. There is no convenient primary standard for nitrite, so a titration is used to standardize a NaN02 solution that serves as the standard for the spectrophotometric procedure. As always, the validity of any analytical procedure ultimately depends on knowing the composition of a primary standard, which is sodium oxalate (Na2C204) in this case. 

Sodium tetraborate, Na2B407, dissociates to form an equimolar concentration of metaboric acid, HBO2, and metaborate ion, B02, estabhshing such a reproducible hydrogen ion concentration in the solution that this material is used as a primary standard in pH measurement. (This interpretation is oversimplified account should also be taken of the tendency of boric acid and borate ion to form concentration-dependent polymers.) Another example is potassium tetroxalate which is an equimolar mixture of potassium hydrogen oxalate and oxalic acid. 

Other primary standards for the standardization of cerium(IV) solutions are pure iron or sodium oxalate. For the standardization of cerium(IV) against sodium oxalate, a hot solution... 

Both sodium oxalate and sodium arsenite (prepared from A.R. AS2O3) are more convenient because they are stable in air and are primary standards. Ferrous sulphate is not a primary standard and is oxidised by atmospheric oxygen. So the operation has to be carried out in the absence of air. Sodium arsenite though poisonous, costlier and preparation of its solution being cumbersome, gives more reliable results than sodium oxalate because oxalic acid is decomposed at higher temperatures into CO and CO2. The extent of decomposition at a sulphuric acid concentration below 20% is however very small. 

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