Standard primary

Should there be more than one known material, a weighted average of the individual differences (x) should be taken. The value of 5- should be based on the combined estimate from the two or more materials (perhaps different primary standards for bases). Should the materials differ markedly in composition, a plot of the individual constant errors against composition should be made. If the constant error appear to depend upon the composition, they should not be pooled in a weighted average. 

Table 11.27 Primary Standards for Aqueous Acid-Base Titrations Table 11.28 Titrimetric (Volumetric) Factors...

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).]... 

Alkaline arsenite, O.IA As(lll) to As(V). Dissolve 4.9460 g of primary standard grade AsjOj in 40 mL of 30% NaOH solution. Dilute with 200 mL of water. Acidify the solution with 6N HCl to the acid color of methyl red indicator. Add to this solution 40 g of NaHC03 and dilute to 1 L. 

The list given below includes the substances that are most used and most useful for the standardization of solutions for precipitation titrations. Primary standard solutions are denoted by the letter (P) in Column 1. 

Selecting and Standardizing a Titrant Most common acid-base titrants are not readily available as primary standards and must be standardized before they can be used in a quantitative analysis. Standardization is accomplished by titrating a known amount of an appropriate acidic or basic primary standard. 

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

When CO2 is present, the volume of NaOH used in the titration is greater than that needed to neutralize the primary standard because some OH reacts with the CO2. 

Selected Primary Standards for the Standardization of Strong Acid and Strong Base Titrants... 

Primary Standard Standardization of Acidic Titrants Titration Reaction Comment... 

Under these conditions some OH is consumed in neutralizing CO2. The result is a determinate error in the titrant s concentration. If the titrant is used to analyze an analyte that has the same end point pH as the primary standard used during standardization, the determinate errors in the standardization and the analysis cancel, and accurate results may still be obtained. 

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. 

Solutions of Na2S203 are prepared from the pentahydrate and must be standardized before use. Standardization is accomplished by dissolving a carefully weighed portion of the primary standard KIO3 in an acidic solution containing an excess of KI. When acidified, the reaction between 103 and K... 

Thompson, R. Q. Identification of Weak Acids and Bases by Titration with Primary Standards, /. Chem. Educ. 1988, 65, 179-180. 

Calibrating the electrode presents a third complication since a standard with an accurately known activity for H+ needs to be used. Unfortunately, it is not possible to calculate rigorously the activity of a single ion. For this reason pH electrodes are calibrated using a standard buffer whose composition is chosen such that the defined pH is as close as possible to that given by equation 11.18. Table 11.6 gives pH values for several primary standard buffer solutions accepted by the National Institute of Standards and Technology. 

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