Burettes

In gas analysis the burettes are generally vertical graduated tubes provided with a tap at the upper end. The lower end is connected by means of tubing to a reservoir containing mercury or water, by means of which the pressure on the gas enclosed between the tap and the liquid surface may be adjusted and ascertained. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

The transference of a liquid from one vessel to another is best carried out by means of a dropping pipette A (Fig. 30). For measuring out a definite volume of liquid it is obviously an advantage to have a calibrated pipette B (Fig. 30) of i or 5 ml. total capacity. Alternatively, semi-micro burettes reading to 0 02 ml. are particularly convenient for class work. 

Prepare a solution of 12 5 g. of hydroxylamine hydrochloride in 20 ml. of water contained in a too ml. conical flask. Dissolve 7 g. of powdered sodium hydroxide in 20 ml. of water, cool the solution in ice-water, and then add it to that of the hydroxylamine hydrochloride. Place a thermometer in the mixed solution, and chill the flask in ice-water until the temperature of the solution is between 5 and 10 . Now add 12 ml. (9 5 gO of dry acetone (preferably from a burette to ensure... 

Place 2 1 ml. (measured from a micro-burette) of nitro-benzene and 5 g. of granulated tin in a 150 ml. round-bottomed flask fitted with a small reflux water-condenser. (A large flask is employed because the mixture when subsequently boiled may bump violently.) Pour 10 ml. of cone. HCl down the condenser on this scale the reaction is not sufficiently vigorous to get out of control. Heat over a gauze for 15 minutes. Cool the flask and add a solution of 7 5 8- of NaOH in 10 ml. of water to redissolve the initial precipitate. Add about... 

Dilute the solution (if necessary) to about 150 ml., add i ml. of concentrated hydrochloric acid, and then heat in a covered beaker almost to boiling. Meanwhile dissolve a small excess (o 4-o 5 g.) of barium chloride in about 50 ml. of water, bring to the boil, and then transfer to a clean 50 ml. burette. Now run this solution slowly drop hy drop into the sulphuric acid solution, keeping the latter steadily stirred throughout the addition. Then boil the solution gently in the... 

The air may be collected directly into a Hempel gas-burette (Fig. 76) and there measured. This burette consists of a glass tube H calibrated in ml. from the tap E downwards, and connected by a piece of rubber tubing to the reservoir R, the height of which can be adjusted. The tap E is a 3-way tap. by which the tube H can be connected directly through to the capillary tube above, or either tube can be connected through the left- hand end of the tap to the atmosphere. 

Prepare the acetylating mixture by adding i volume of acetic anhydride to 4 volumes of pure anhydrous pyridine, and shaking thoroughly. Immediately before use, transfer the mixture to a clean dry burette having a welUfitting glass tap, and then close the top of the burette by means of a soda-lime tube. 

Place about 50 ml. of 40% formalin solution in a conical flask and add at least 10 drops of phenolphthalein solution. No add ery carefully from a burette dilute sodium hydroxide solution (il/ 10 will serve the purpose) until the solution is just faintly pink. 

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. 

The method is based on the conversion of urea to amnionium carbonate and the estimation of the latter by titration with standard acid. For this purpose, two equal quantities of urea (or urine) are measured out into two flasks A and B. A is treated with 10 ml. of a strong urease preparation and some phenol-phthalein, warm water is added and the mixture is adjusted by the addition of V/io HCl from a burette A until the red colour is just discharged. This brings the mixture to about pH 8 (the optimum for urease) and also prevents loss of ammonia. 

The contents of B, which act as a control, are treated with mercuric chloride in order to inhibit the action of the enzyme, and then 10 ml. of urease solution are added. The solution is diluted with water and ammonium chloride added (in order to balance the ammonium chloride subsequently formed in A). Meth) l-red is then added and the solution is titrated with Mj 10 HCl from a second burette B until a bright red colour is obtained. 

The solution in A is now treated with mercuric chloride and methyl-red, and then titrated with Ml 10 HCl until its colour matches that of the solution in B. The difference in the volume of HCl run in from the burettes A and Bi is a measure of the amount of urea present. 

Fill two burettes A and B w ith A//10 HCl. Run in from A, drop by drop, sufficient A/,To hydrochloric acid just to discharge the red colour in A. Maintain the temperature at about 60° and keep the colour just discharged by cautiously adding the HCl from time to time. Care must be taken not to add an excess of acid, otherwise the proteins will be precipitated and the enzyme rendered inactive. The reaction is o>m plete in about 5 minutes, but allow the mixture to stand for a further 5 minutes after the final discharge of the colour. 

The success of this preparation depends upon the use of the apparatus (1) depicted in Fig.///, 57, 1, which permits of the automatic separation of the water produced in the reaction this will be termed a water-separator tube. Convenient dimensions for students preparations are indicated in the diagram. Determine the volume v of the tube up to the neck, i.e., between A and B, by adding water from a burette. The quantity of water which should be eliminated, assuming a quantitative conversion of the alcohol into the ether, may be computed from the equation ... 

Dissolve 10 g. of sym.-tribromoaniline (Section IV,47) in 60 ml. of rectified spirit and 15 ml. of benzene in a 200 ml. bolt-head flask by heat ing on a water bath. Add, from a burette or small graduated pipette, 5-3 g. (3-5 ml.) of concentrated sulphuric acid to the hot solution and gently swirl the liquid. Attach a reflux condenser to the flask and heat on a water bath until the clear solution boils. Detach the condenser, remove the flask from the water bath, and add 3 5 g. of powdered sodium... 

The apparatus to use is seen in figure 13 which consists of a burette, thermometer, Erlenmeyer flask and a two-holed rubber stopper that has a small V-shaped wedge cut out of one side of the rubber stopper to allow the inside contents to vent. 31.5g of orangy-red fuming nitric acid (see chemicals section) is poured into the Erlenmeyer flask and the rubber stopper with its burette and thermometer is placed on to the... 

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