Graduated flasks

Weigh out accurately about 2-5 g. of pure powdered succinic acid, transfer to a 100 ml. graduated flask, dissolve in distilled water, make the solution up to the graduation mark and mix well. Now, by means of a pipette, transfer 25 ml. of the solution to a 150 ml. conical flask, add a drop of phenolphthalein solution and titrate with A/ 2 NaOH or KOH solution to obtain consistent results. 

Note, (i) In view of the large volume of. "l/.NaOH solution required in the above titrations, the contents of the flask A after hydrolysis niay alternatively be washed carefully into a 100 ml. graduated flask, and the solution made up to the mark and well mixed. 25 ml. of the-solution are then withdrawn with a pipette, and titrated with the A/.NaOH solution. The 100 ml. flask is then washed out repeatedly with distilled water, and used similarly for the contents of the flask B. 

Solution A. Dissolve 17 320 g. of powdered crystalline copper sulphate, CuS04,5H20, in water and make the solution up to 250 ml. in a graduated flask. 

Titration. The solution of ammonia absorbed in saturated boric acid may now be titrated as an alkali directly with 0 025 A.HCl (best obtained by dilution of commercially available standard A.HCl in a graduated flask). Three drops of indicator (mixed methyl-red/methyl-ene-blue being most satisfactory) are added to the liquid in the receiver and the 0 025 A.HCl run in from an accurate burette. 

Five c.c. of the oil to be examined are weighed and brought into a glass-stoppered burette graduated to c.c., and is diluted with about, an equal volume of petroleum ether a 5 per cent, potassium hydroxide solution is added, and the mixture shaken for a short time, then the liquid is left standing until separation is complete. Then the alkaline solution is allowed to run into a 100 c.c. graduated flask. This operation is repeated until no further decrease in the volume of the oil takes place. 

The most commonly used pieces of apparatus in titrimetric (volumetric) analysis are graduated flasks, burettes, and pipettes. Graduated cylinders and weight pipettes are less widely employed. Each of these will be described in turn. 

A graduated flask (known alternatively as a volumetric flask or a measuring flask), is a flat-bottomed, pear-shaped vessel with a long narrow neck. A thin line etched around the neck indicates the volume that it holds at a certain definite temperature, usually 20 °C (both the capacity and temperature are clearly marked on the flask) the flask is then said to be graduated to contain. Flasks with one mark are always taken to contain the volume specified. A flask may also be marked to deliver a specified volume of liquid under certain definite conditions these are, however, not suitable for exact work and are not widely used. Vessels intended to contain definite volumes of liquid are marked C or TC or In, while those intended to deliver definite volumes are marked D or TD or Ex. 

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. 

Procedure. Dissolve 0.0393 g of pure copper(II) sulphate pentahydrate in 1 L of water in a graduated flask. Pipette 10.0 mL of this solution (containing about 100 jug Cu) into a beaker, add 5.0 mL of 25 per cent aqueous citric acid solution, render slightly alkaline with dilute ammonia solution and boil off the excess of ammonia alternatively, adjust to pH 8.5 using a pH meter. Add 15.0mL of 4 per cent EDTA solution and cool to room temperature. Transfer to a separatory funnel, add lOmL of 0.2 per cent aqueous sodium diethyldithiocarbamate solution, and shake for 45 seconds. A yellow-brown colour develops in the solution. Pipette 20 mL of butyl acetate (ethanoate) into the funnel and shake for 30 seconds. The organic layer acquires a yellow colour. Cool, shake for 15 seconds and allow the phases to separate. Remove the lower aqueous... 

Procedure. Weigh out 0.0226 g of hydrated ammonium iron(III) sulphate and dissolve it in 1 L of water in a graduated flask 50 mL of this solution contain 100 g of iron. Place 50.0 mL of the solution in a 100 mL separatory funnel, add 10 mL of a 1 per cent oxine (analytical grade) solution in chloroform and shake for 1 minute. Separate the chloroform layer. Transfer a portion of the latter to a 1.0 cm absorption cell. Determine the absorbance at 470 nm in a spectrophotometer, using the solvent as a blank or reference. Repeat the extraction with a further 10 mL of 1 per cent oxine solution in chloroform, and measure the absorbance to confirm that all the iron was extracted. 

Procedure. Dissolve 0.0079 g of pure lead nitrate in 1 L of water in a graduated flask. To 10.0 mL of this solution (containing about 50 p.g of lead) contained in a 250 mL separatory funnel, add 75 mL of ammonia-cyanide-sulphite mixture (Note 1), adjust the pH of the solution to 9.5 (pH meter) by the cautious addition of hydrochloric acid (CARE ), then add 7.5 mL of a 0.005 per cent solution of dithizone in chloroform (Note 2), followed by 17.5 mL of chloroform. Shake for 1 minute, and allow the phases to separate. Determine the absorbance at 510 nm against a blank solution in a 1.0 cm absorption cell. A further extraction of the same solution gives zero absorption indicative of the complete extraction of the lead. Almost the same absorbance is obtained in the presence of 100 pg of copper ion and 100 pg of zinc ion. 

Magnesium may conveniently be determined by atomic absorption spectroscopy (Section 21.21) if a smaller amount (ca 4 mg) is used for the separation. Collect the magnesium effluent in a 1 L graduated flask, dilute to the mark with de-ionised water and aspirate the solution into the flame of an atomic absorption spectrometer. Calibrate the instrument using standard magnesium solutions covering the range 2 to 8 ppm. 

Analyses, (a) Original zinc-ion solution. Dilute 2.00 mL (pipette) to 100 mL in a graduated flask. Pipette 10.0 mL of the diluted solution into a 250 mL conical flask, add ca 90 mL of water, 2 mL of the buffer solution, and sufficient of the solochrome black indicator mixture to impart a pronounced red colour to the solution. Titrate with standard 0.01 M EDTA to a pure blue colour (see Section 10.59). 

Reagents. Standard copper (II) solutions. Dissolve 100 mg of spectroscopically pure copper metal in a slight excess of nitric acid and dilute to 1 L in a graduated flask with de-ionised water. Pipette a 10 mL aliquot into a 100 mL graduated flask and make up to the mark with acetone (analytical grade) the resultant solution contains 10 g of copper per mL. Use this stock solution to... 

Sample solution. Prepare a sample solution containing 100 pg of copper(II) in 1 L of 0.5M sodium chloride solution in a graduated flask. 

An alternative elution technique is to transfer the powder (e.g. for bromophenol blue) to a glass column fitted with a glass-wool plug or glass sinter, and elute the dye with ethanol containing a little ammonia. The eluted solution, made up to a fixed volume in a small graduated flask, may be used for colorimetric/ spectrophotometric analysis of the recovered dye (see Chapter 17). A calibration curve must, of course, be constructed for each of the individual compounds. 

Note. If 1M hydrochloric acid is required, use 90 mL of the concentrated acid. If 0.01 M acid is required, dilute two 50 mL portions of the approximately 0.1 M acid, removed with a 50 mL pipette, to 1 litre in a graduated flask. 

Notes. (1) For elementary students, an approximately 0.05 M solution of sodium carbonate may be prepared by weighing out accurately about 1.3 g of pure sodium carbonate in a weighing bottle or in a small beaker, transferring it to a 250 mL graduated flask, dissolving it in water (Section 3.28), and making up to the mark. The flask is well shaken, then 25.00 mL portions are withdrawn with a pipette and titrated with the acid as described above. Individual titrations should not differ by more than 0.1 mL. Record the results as in Section 10.30. 

Procedure. Weigh an empty stoppered weighing bottle, add about 2 g of syrupy phosphoric(V) acid and re-weigh. Transfer the acid quantitatively to a 250 mL graduated flask, and then proceed as detailed for sulphuric acid, but using the phenolphthalein-1-naphtholphthalein mixed indicator. 

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