Piston burettes

In piston burettes, the delivery of the liquid is controlled by movement of a tightly fitting plunger within a graduated tube of uniform bore. They are particularly useful when the piston is coupled to a motor drive, and in this form serve as the basis of automatic titrators. These instruments can provide automatic plotting of titration curves, and provision is made for a variable rate of delivery as the end point is approached so that there is no danger of overshooting the end point. 

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Grasshoff and Wenck [29] have described a version of the Mohr-Knudsen silver nitrate procedure for the determination of the chlorinity of sea water. In this method, which overcomes the disadvantages of conventional burettes, use is made of a motor-driven piston burette of 20 ml capacity, which is sufficient for a chlorinity range of 0 - 45 per thousand. The accuracy is the same as for conventional titration. The apparatus is compact and portable. 

For addition of liquid bromine, a 5-mL piston burette was used delivering 0.4629 p.L s . In the calorimeter was placed 400 mL of a 0.4 M alkylmagnesium bromide in diethyl ether. The measuring procedure followed the same principles as used for HBr addition. 

If the total salt content of a solution is high or if only small amounts of sample are available the injection technique may be used. If a piston burette is used for the injection, individual errors cannot be avoided, and reproducibility becomes somewhat poorer than with continuous aspiration. Also, the uncertainty of the volume measurement with small injection volumes becomes noticeable in the reproducibility. In Table 4 the data and statistical values of a simultaneous copper and nickel determination are shown. 

The solvent 1,2-dichloroethane was washed twice with deionized water. Tetraalkylammonium bromide Q Br" was converted to picrate salt Q pic" by solvent extraction method. A portion of 1,2-dichloroethane saturated with water (40 ml, = 0.13 mol dm ) was titrated with 10" mol dm tetraalkylammonium picrate by a piston burette in a thermostated vessel (25.0 °C). Conductivity was measured by Toa-denpa Model CG-511C. The cell constant of the electrode was calibrated by KCl aqueous solution and obtained as 0.0920 cm. A portion of titrant was dried and dissolved in an alkaline aqueous solution. Then the concentration of picrate ion was determined by spectrophotometry. 

The flow resistance will increase, thus preventing the use of a low-pressure drive (peristaltic pump, air drive, piston burette). 

A calibrated 1 mL (10 mL) precision piston burette with at least +0.1 % precision and reading ability (display) to 0.1 % of the piston volume should be used for the titration. The piston drive should permit addition of at least 5 000 increments per burette volume. 

Then 10.00 mL of the iodate standard solution are added with a caUbrated pipette or a precision piston burette. The bottle is filled to about the sample volume (to the neck) with pure water. 

The same instrumentation as used for the determination of dissolved oxygen is required for the determination of thiosulphate in seawater (see Chapter 4). The same type of sample bottles is used, but with a content of about 100 mL instead of SOmL. The piston burette should be calibrated carefully and be readable to 0.01 mL. 

Accurate burette For example Metrohm piston burette, calibrated, with subdivisions of 0.005 mL. 

Accurate burette For example, a Metrohm piston burette, calibrated in subdivisions of 0.005 mL, needed only if the titration curve is performed by conventional procedure (e.g., as described under 5a of this section). 

Accurate burette For example, Metrohm Dosimat, 20 mL, used for the Mohr-Knudsen titration. The connections and the piston burette should be either amber glass or painted black to avoid any photochemical reaction of the silver ions (as for apparatus under 6.). 

Weigh another 10 mL of SSW in a titration beaker and add approximately 0.2 mL less than the amount of concentrated AgN03 solution (with a piston burette) as determined above (no chromate indicator is added at this time). Then weigh again and titrate with the dilute AgNOs solution (potentiometrically) until the endpoint as described for the calibra-ation of dilute AgNOs solution . Take care that no air bubbles are trapped in the burette while titrating. 

Accurate burette (For example a piston burette), calibrated with subdivisions of 0.01 mL. 

Standardize the 0.002 mol/L thiosulphate solution with a potassium bromate solution prepared by dissolving 2.7834 g of KBrOa (recrystallized 3-4 times from hot water and dried at 180°C) in distilled water and diluted to lOOOmL, giving a 1.667 1(F mol/L solution. Prepare the working standard (3.334 10 mol/L) by accurate dilution with distilled water 1 50. This solution has an oxidation concentration of 0.0020 mol/L of electrons. Transfer 40.00 mL aliquots (with an accurate burette, e.g., piston burette) of the working standard into an Erlenmeyer flask, add about 0.25 g of potassium iodide, 4 drops of 3 % ammonium molybdate, 10 mL of 3 mol/L sulphuric acid and titrate in the same manner as described for the procedure in Section 11.2.6.4. 

Refluxing storage vessel, /2a,2b/ Nitrogen inlets, /3a,3b/ Water cooled condensers, /4/ PTFE three-way stopcock, /5/ 10 ml piston burette, /6/ Dissolution vessel (60 ml), / / Sample inlet, /8/ Inlet for washing, /9/ Adaptor to absorption/titration vessel,... 

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