Buret

When reading the liquid level in a buret, your eye should be at the same height as the top of the liquid. If your eye is too high, the liquid seems to be higher than it really is. If your eye is too low, the liquid appears too low. The error that occurs when your eye is not at the same height as the liquid is called parallax. 

Philadelphia. PA ] (b) Digital titrator with plastic cartridge containing reagent solution is used for analyses in the field. [Courtesy Hach Co., Loveland, CO] (c) Battery-operated electronic buret with digital readout delivers 0.01-mL increments from a reagent bottle. This device can be used for accurate titrations in the field. [Courtesy Cote-Parmer Co.. Niles. ILJ 

Figuro 2-8 An air bubble trapped beneath the stopcock should be expelled before you use the buret 

Accuracy refers to the difference between the stated volume and the actual volume delivered. 

The thickness of the markings on a 50-mL buret corresponds to about 0.02 mL. For best accuracy, select one portion of the marking to be called zero. For example, you can say that the liquid level is at the mark when the bottom of the meniscus just touches the top of the mark. When the meniscus is at the bottom of the same mark, the reading is 0.02 mL greater. 

A drop of liquid from a 50-mL buret is about 0.05 mL. Near the end point of a titration, try to deliver less than one drop at a time so that you can locate the end point more precisely than 0.05 mL. To deliver a fraction of a drop, carefully open the stopcock until part of a drop is hanging from the buret tip. Then touch the inside wall of the receiving flask to the buret tip to transfer the droplet to the flask. Carefully tip the flask so that the main body of liquid washes over the newly added droplet. Then swirl the flask to mix the contents. Near the end of a titration, tip and rotate the flask often to ensure that droplets on the wall containing unreacted analyte contact the bulk solution. 

Before you fill a buret with fresh solution, it is a wonderful idea to rinse the buret several times with small portions of the new solution, discarding each wash. It is not necessary to fill the entire buret with wash solution. Simply tilt the buret so that its whole surface contacts the wash liquid. This same technique should be used with any vessel (such as a spectrophotometer cuvet or a pipet) that is reused without drying. 

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Close-up of buret, showing difficulty in estimating volume. With scale divisions every 0.1 ml it is difficult to read the actual volume to better than +0.01 - 0.03 ml. 

Gordus, A. A. Statistical Evaluation of Glass Data for Two Buret Readings, /. Chem. Educ. 1987, 64, 516-511. 

The volumes of water in two burets are read, and the difference between the volumes are calculated. Students analyze the data by drawing histograms for each of the three volumes, comparing results with those predicted for a normal distribution. 

Volumetric buret showing a portion of its graduated scale. 

Table 9.1), with the choice of buret determined by the demands of the analysis. The accuracy obtainable with a buret can be improved by calibrating it over several intermediate ranges of volumes using the same method described in Chapter 5 for calibrating pipets. In this manner, the volume of titrant delivered can be corrected for any variations in the buret s internal diameter. 

In practice, however, any improvement in the sensitivity of an acid-base titration due to an increase in k is offset by a decrease in the precision of the equivalence point volume when the buret needs to be refilled. Consequently, standard analytical procedures for acid-base titrimetry are usually written to ensure that titrations require 60-100% of the buret s volume. 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

Good measurement practices (GMPs) describe operations specific to a technique. In general, GMPs provide instructions for maintaining, calibrating, and using the equipment and instrumentation that form the basis for a specific technique. For example, a GMP for a titration describes how to calibrate a buret (if nec-... 

Write directives outlining good measurement practices for (a) a buret, (b) a pH meter, and (c) a spectrophotometer. 

Tertiary butyl alcohol (900 ml., 702 g., 9.47 moles) is dissolved in a solution prepared by mixing 28 ml. (0.50 mole) of concentrated sulfuric acid with 1.5 1. of water in a 5-1. round-bottomed flask (Note 1) equipped with a thermometer, stirrer, gas inlet tube, and two addition burets. One buret is charged with 86 ml. (1 mole) of 11.6iH hydrogen peroxide (Note 2), and the other with a solution of 278 g. (1 mole) of ferrous sulfate pentahydrate and 55.5 ml. (1 mole) of concentrated sulfuric acid in 570 ml. of water (Note 3). The reaction flask is swept out with nitrogen and cooled to 10° by means of an ice bath. Stirring is commenced and the two solutions are added simultaneously and equivalently over a period of 20 minutes. The temperature is held below 20°. 

It is convenient to calibrate the burets so that the liquid is divided into 20 equal portions. Then, in the addition of the reagents, these calibrations aid in synchronizing the rates. 

Calibrate the detector tube pump for proper volume measurement at least quarterly. Simply connect the pump directly to the bubble meter with a detector mbe in-line. Use a detector mbe and pump from the same manufacturer. Wet the inside of the 100 cc bubble meter with soap solution. For volume calibration, experiment to get the soap bubble even with the zero ml mark of the buret. For piston-type pumps, pull the pump handle all the way out (full pump stroke) and note where the soap bubble stops for bellows-type pumps, compress the bellows fully for automatic pumps, program the pump to take a full pump stroke. 

An alcohol-free solution of diazomethane in ether is prepared as in Chapter 17, Section III. This solution is approximately 0.5 M in diazomethane and may be standardized by titration as follows benzoic acid (0.6 g, approx. 0.005 mole) is weighed accurately into an Erlenmeyer flask and suspended in 5 ml of ether. The diazomethane solution (approx. 5 ml) is added from a buret with swirling, care being taken that an excess of unreacted benzoic acid remains (the yellow color of the diazomethane should be completely discharged). The excess benzoic acid is now titrated with standard 0.2 N sodium hydroxide solution, and the concentration of diazomethane is calculated. 

Btiretten-btirste, /. buret brush, -deckel, tn. buret cap. -flasche,/. volumetric flask, -ge-stell, n. buret stand, -hshn, tn. buret cock. 

Mass-ilnderung,/. change in dimension or size, -bestandigkeit, /. permanency of dimension or size, -burette, /. measuring buret(te). 

Nitrtir, n. (-ous) nitride. Cf. Chloriir. Nitrylsaure,/. nitrylic acid (nitrous acid). Niveau, n. level, -flache, /. level surface, -flasche,/. leveling bottle, leveling vessel (as for a gas buret), -rohr, n., -rohre, /. level tube, leveling tube, -stufe, /. (of electrons) energy level. ... 

Sprachschatz, m. vocabulary thesaurus, sprachwidrig, a. ungrammatical, sprang, pret. (of springen) buret, broke, etc. spratzen, spratzeln, v.t. spit, spurt, sputter, splutter, spatter, splash. 

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