Making Standard Solutions

The following procedure may be useful in making standard solutions ... 

Volumetric flasks are used to measure very precise volumes of liquids for making standard solutions or weighing for density calibrations. Each flask has one measurement line for the specific volume of the flask (see Fig. 2.18). Volumetric flasks are of either Class A or Class B quality the tolerances for Class B are twice those for Class A. There are no general purpose volumetric flasks. The tolerances... 

The following questions are concerned with making standard solutions. It is essential to become familiar with and competent at calculations such as these. Mistakes in calculating and in making standard solutions are often the source of errors in analysis. Try to plan the steps you would use to make the solutions required. Avoid using pipettes smaller than 2 cm3 or larger than 25 cm3 because in inexperienced hands these can introduce large errors. You will find proposed answers at the end of the chapter. 

Making standard solutions (avoiding the use of pipettes smaller than 2 cm3). 

The oxidation efficiency for D-glucose was reported to be 100% (Hine and Bursill (1985)), in good agreement with the above results. Results rely on the correctness of information about carbon content of FA and HA received from IHSS, and a comparably small error in making standard solutions for FA... 

From the stock solution make standard solutions in C-PBS (at least 6 different concentrations). 

Volumetric flasks Convenient sizes for making standard solutions. 

If the sensitivity of the standard determination is seriously reduced and none of the above tests indicates any trouble, use pure water from a different source or run standard additions to a natural water. This may show if the water used for making standard solutions or reagents is contaminated with interferents. 

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

Weigh out accurately about 1 25 g. of the sample, dissolve it in water and make the solution up to 250 ml. Titrate this solution against 25 ml. of the standard Fehling s solution, precisely as before. 

Bismuth standard solution (quantitative color test for Bi) dissolve 1 g of bismuth in a mixture of 3 mL of concentrated HNO3 and 2.8 mL of H2O and make up to 100 mL with glycerol. Also dissolve 5 g of KI in 5 mL of water and make up to 100 mL with glycerol. The two solutions are used together in the colorimetric estimation of Bi. 

Cyanide is determined at concentrations greater than 1 ppm by making the sample alkaline with NaOH and titrating with a standard solution of AgN03, forming the soluble Ag(CN)2 complex. The end point is determined using p-dimethylaminobenzalrhodamine as a visual indicator, with the solution turning from yellow to a salmon color in the presence of excess Ag+. 

The concentration of Ca + in a water sample was determined by the method of external standards. The ionic strength of the samples and standards was maintained at a nearly constant level by making each solution 0.5 M in KNO3. The measured cell potentials for the external standards are shown in the following table. 

The analysis of N03 in aquarium water was carried out by CZE using 104 as an internal standard. A standard solution of 15.0-ppm N03 and 10.0-ppm 104 gives peak heights (arbitrary units) of 95.0 and 100.1, respectively. A sample of water from an aquarium is diluted 1 100, and sufficient internal standard added to make its concentration 10.0 ppm. Analysis gives signals of 29.2 and 105.8 for N03 and 104 , respectively. Report the parts per million of N03 in the sample of aquarium water. 

Graduated flasks are available in the following capacities 1, 2, 5, 10, 20, 50, 100, 200, 250, 500, 1000, 2000 and 5000 mL. They are employed in making up standard solutions to a given volume they can also be used for obtaining, with the aid of pipettes, aliquot portions of a solution of the substance to be analysed. 

In some circumstances it may be considered preferable to prepare the standard solution by making use of one of the concentrated volumetric solutions supplied in sealed ampoules which only require dilution in a graduated flask to produce a standard solution. 

Discussion. Very pure silver can be obtained commercially, and a standard solution can be prepared by dissolving a known weight (say, 10.787 g) in nitric acid in a conical flask having a funnel in the neck to prevent mechanical loss, and making up to a known volume (say, 1 L for a 0.1 M solution). The presence of acid must, however, be avoided in determinations with potassium chromate as indicator or in determinations employing adsorption indicators. It is therefore preferable to employ a neutral solution prepared by dissolving silver nitrate (relative molecular mass, 169.87) in water. 

Procedure. Prepare an approx. 0.02M standard solution of potassium bromate by weighing accurately about 1.65 g of the analytical grade reagent, dissolving it in water and making it up to 500 mL in a graduated flask (Note 2). 

Make sure that the readings are zero when the plungers just touch the bottoms of the cups. Place the standard solution in one cup, and an equal volume of the unknown solution in the other do not fill the cups above the shoulder. Set the unknown solution at a scale reading of 10.0mm and adjust the standard until the fields are matched. Carry out at least six adjustments with the cup containing the standard solution, and calculate the mean value. 

Procedure. Prepare standard solutions of pseudoephedrine hydrochloride by weighing out accurately about 60 mg into a 500 mL graduated flask. Add about 50 mL 0.1M hydrochloric acid to dissolve the solid and then make up to the mark with 0.1 M hydrochloric acid. In a series of 50 mL graduated flasks place... 

Using the calibration curve it is a simple matter to interpolate from the measured absorbance of the test solution the concentration of the relevant element in the solution. The working graph should be checked occasionally by making measurements with the standard solutions, and if necessary a new calibration curve must be drawn. 

Preparation of the standard solutions. For procedure (i) it is necessary to incorporate a releasing agent in the standard solutions. Three different releasing agents may be used for calcium, (a) lanthanum chloride, (b) strontium chloride and (c) EDTA of these (a) is the preferred reagent, but (b) or (c) make satisfactory alternatives. 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

Preparation of the standard solutions. The standard solutions are prepared from a solution of vanadium naphthenate in white spirit which contains about 3 per cent of vanadium. Weigh out accurately about 0.6 g of the vanadium naphthenate into a 100 mL graduated flask and make up to the mark with white spirit this stock solution contains about 180/igmL-1 of vanadium. Dilute portions of this stock solution measured with the aid of a Grade A 50 mL burette to obtain a series of working standards containing from 10-40/igmL-1 of vanadium. 

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