A, primary standard

H0CH3)3CNHH3 121.137 Tris(hydroxymethyl)aminomethane is available commercially as a primary standard. Dry at 100-103°C (<110°C). In titrations with a strong acid the equivalence point is at about pH 4.5-5. Equivalent weight is the formula weight. [J. H. Eossum, P. C. Markunas, and J. A. Riddick, Anal. Chem., 23 491 (1951).]... 

Solutions of Mn04 are prepared from KMn04, which is not available as a primary standard. Aqueous solutions of permanganate are thermodynamically unstable due to its ability to oxidize water. 

FeSO (NH 2S04 6H20, is used as a primary standard for iron. It is soluble in water but insoluble in alcohols. Both the soHd and its solution are more stable to oxidation than iron(II) sulfate. 

For this purpose the potassium salt K2Cr20v is preferred since it lacks the hygroscopic character of the sodium salt and may therefore be used as a primary standard. 

In titrimetry certain chemicals are used frequently in defined concentrations as reference solutions. Such substances are referred to as primary standards or secondary standards. A primary standard is a compound of sufficient purity from which a standard solution can be prepared by direct weighing of a quantity of it, followed by dilution to give a defined volume of solution. The solution produced is then a primary standard solution. A primary standard should satisfy the following requirements. 

A secondary standard is a substance which may be used for standardisations, and whose content of the active substance has been found by comparison against a primary standard. It follows that a secondary standard solution is a solution in which the concentration of dissolved solute has not been determined from the weight of the compound dissolved but by reaction (titration) of a volume of the solution against a measured volume of a primary standard solution. 

Solutions of EDTA of the following concentrations are suitable for most experimental work 0.1M, 0.05M, and 0.01 M. These contain respectively 37.224 g, 18.612g, and 3.7224 g of the dihydrate per litre of solution. As already indicated, the dry analytical grade salt cannot be regarded as a primary standard and the solution must be standardised this can be done by titration of nearly neutralised zinc chloride or zinc sulphate solution prepared from a known weight of zinc pellets, or by titration with a solution made from specially dried lead nitrate. 

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). 

Discussion. Sodium thiosulphate (Na2S203,5H20) is readily obtainable in a state of high purity, but there is always some uncertainty as to the exact water content because of the efflorescent nature of the salt and for other reasons. The substance is therefore unsuitable as a primary standard. It is a reducing agent... 

Potassium bromate is readily available in a high state of purity the product has an assay value of at least 99.9 per cent. The substance can be dried at 120-150 °C, is anhydrous, and the aqueous solution keeps indefinitely. It can therefore be employed as a primary standard. Its only disadvantage is that one-sixth of the relative molecular mass is a comparatively small quantity. 

A pharmacopoeial reference substance is intended for the determination of the main component of a substance or for the active ingredient of a pharmaceutical formulation which is usually present at a high proportion of the total. The reference substance is to be used as a primary standard in a specific method validated as prescribed in the ICH Guideline Validation of Analytical Procedure Methodology" (Technical Guide for the Elaboration of Monographs 1996 ICH Guideline 1997). the reproducibility of which is known. This is taken into account when the limits of acceptance (tolerance) for the substance or product are fixed (Daas and Miller 1997,1998). 

Step 2 Weigh out an appropriate amount of an acidic material, suitable for use as a primary standard, such as potassium hydrogen phthalate (KHP). 

The prepared NaOH solution is a secondary standard because its concentration is determined by titration against a primary standard. 

This last paragraph inevitably leads to the questions, So how do we know what the exact temperature is and How do I know if my thermometer follows profile (a) or profile (b) in Figure 1.4 Usually, we do not know the answer. If we had a single thermometer whose temperature was always accurate then we could use it as a primary standard, and would simply prepare a calibrated thermometer against which all others are calibrated. 

Experiment 6 Standardization of a Solution Using a Primary Standard... 

Because of the cost of primary standards, it is normal to use them very sparingly (certainly not as calibration standards), but to include them as unknowns in the analytical run as a measure of the quality of the analysis (see QA, below). Even so, this becomes prohibitively expensive, and it is common to use materials which can be included in each analysis, which may not be fully certified, but whose values can be related to a primary standard. These are often referred to as in-house standards, although they may be more generally available. They can be home-made, providing sufficient attention has been paid to homogenizing the material thoroughly. A simple example is... 

The alternative to the above is using an accurately weighed solid material as the known standard. Such a material is called a primary standard. Thus, a primary standard is a material that can be weighed accurately either for the purpose of preparing a standard solution (which then does not have to be standardized) or for comparison to a solution with which it reacts for the purpose of standardizing that solution. For standardization with a primary standard, Equation (4.21) becomes... 

The molarity of the EDTA solution, MEDTA in Equation (5.52), can be known directly through its preparation with the use of an analytical balance and a volumetric flask. That is, one can purchase pure disodium dihydrogen EDTA and use it as a primary standard. In that case, the solution is prepared and the concentration calculated according to the discussion in Chapter 4 (see Section 4.3, especially Example 4.2, and Section 4.4.1). 

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