Pipettes, calibration

For MPN determination, sterile pipettes calibrated in 0.1-ml increments are used. Other equipment includes sterile screw-top dilution bottles containing 99 ml of water and a rack containing six sets of five lactose broth fermentation tubes. A sterile pipette is used to transfer 1.0-ml portions of the sample into each of five fermentation tubes. This is followed by dispensing 0.1 ml into a second set of five. For the next higher dilution (the third), only 0.01 ml of sample water is required. This small quantity is very difficult to pipette accurately, so 1.0 ml of sample is placed in a dilution bottle containing 99 ml of sterile water and mixed. The 1.0-ml portions containing 0.01 ml of the surface water sample are then pipetted into the third set of five tubes. The fourth set receives 0.1 ml from this same dilution bottle. The process is then carried one more step by transferring 1.0 ml from the first dilution bottle into 99 ml of water in the second for another hundredfold dilution. Portions from this dilution bottle are pipetted into the fifth and sixth tube sets. After incubation (48 h at 35 C), the tubes are examined for gas production and the number of positive reactions for each of the serial dilutions is recorded. 

Microlitre syringe pipettes are available with capacities ranging from 10 to 250 ph and with the body of the pipette calibrated. When fitted with a needle tip they are particularly useful for introducing liquids into gas chromatographs (Chapter 9). 

Modem Erlenmeyer flasks and beakers have approximate volume calibrations fused into the glass, but these are very approximate. Somewhat more accurate volumetric measurements are made in the 10-mL graduated cylinders. For volumes less than about 4 mL, use a graduated pipette. Never apply suction to a pipette by mouth. The pipette can be fitted with a small rubber bulb. A Pasteur pipette can be converted into a calibrated pipette with the addition of a plastic syringe body [see Fig. 11(d)] or you can calibrate it at 0.5, 1.0, and 1.5 ml and put three file scratches on the tube this eliminates the need to use a syringe with this Pasteur pipette in the future. Also see the Pasteur pipette calibration marks in the back of this book. You should find among your equipment a 1-mL pipette, calibrated in hundredths of a milliliter [Fig. 11(a)]. Determine whether it is designed to... 

In some cases, we apply an observation method that in a similar way leads to the introduction of a random error in the final outcome. For example, we always read 0.1 mL if we use a 10 mL measuring pipette, calibrated in tenths of a millilitre. Acmally we round off the observation to one decimal place without mentioning or realising that. 

Additional Pasteur pipettes can be calibrated easily by holding them next to the pipette calibrated in Laboratory Exercise 2 and scoring a new mark on each pipette at the same level as the mark placed on the calibrated pipette. We recommend that several Pasteur pipettes be calibrated at one time for use in future experiments. 

Deduce whether each of these operations results in a random ora systematic error, a Using a pipette calibrated at 20°C to measure volumes of hot solutions at 85°C. b Using a BOcm measuring cylinder to measure 50.00cm of solution. 

The collection of this volume of blood with a pipette induces a first uncertainty based on the precision of the pipette used. Also, collection of blood with a pipette calibrated by the manufacturer for distilled water induces a non-negligible error because the viscosities and densities of water and blood are significantly different. 

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. 

When the correct solvent for recrystallisation is not known a procedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than 10 mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(B)) and about o i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(B)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a calibrated pipette with shaking to determine whether crystallisation will now take place, indicated by a cloudiness or by the separation of solid.]... 

Amount of material required. It is convenient to employ an arbitrary ratio of 0 10 g. of solid or 0 20 ml. of liquid for 3 0 ml. of solvent. Weigh out 0 10 g. of the finely-powdered solid to the nearest 0 01 g. after some experience, subsequent tests with the same compound may be estimated by eye. Measure out 0-20 ml. of the liquid either with a calibrated dropper (Fig. 11,27, 1) or a small graduated pipette. Use either a calibrated dropper or a graduated pipette to deliver 3 0 ml. of solvent. Rinse the delivery pipette with alcohol, followed by ether each time that it is used. 

Much time will be saved if each of the solvents (Water, Ether, 5 per cent. Sodium Hydroxide, 5 per cent. Sodium Bicarbonate and 5 per cent. Hydrochloric Acid) be contained in a 30 or 60 ml. bottle fitted with a cork carrying a calibrated dropper. The concentrated sulphimic acid should be kept in a glass-stoppered bottle and withdrawn with a dropper or pipette as required. 

A liquid may be transferred from one vessel to another with a dropper pipette (Fig. XII, 1, 2, a or b). If the dropper pipette is calibrated, it may be employed for measuring out a definite volume of liquid. 

Aufnahme-fahigkeit, /. absorbability, absorptivity, absorbing power capacity. -ge-Bchwindigkeit, /. absorption rate, -kolbeu, m. absorption flask, -pipette,/, a pipet calibrated to take up a definite volume. Cf. Ausflusspipette. -vermogen, n. absorptive power. 

Calibration of apparatus and application of corrections. All instruments (weights, flasks, burettes, pipettes, etc.) should be calibrated, and the appropriate corrections applied to the original measurements. In some cases where an error cannot be eliminated, it is possible to apply a correction for the effect that it produces thus an impurity in a weighed precipitate may be determined and its weight deducted. 

The above procedure may be adapted to the determination of molybdenum in steel. Dissolve a 1.00 g sample of the steel (accurately weighed) in 5 mL of 1 1 hydrochloric acid and 15 mL of 70 per cent perchloric acid. Heat the solution until dense fumes are evolved and then for 6-7 minutes longer. Cool, add 20 mL of water, and warm to dissolve all salts. Dilute the resulting cooled solution to volume in a 1 L flask. Pipette 10.0 mL of the diluted solution into a 50 mL separatory funnel, add 3 mL of the tin(II) chloride solution, and continue as detailed above. Measure the absorbance of the extract at 465 rnn with a spectrophotometer, and compare this value with that obtained with known amounts of molybdenum. Use the calibration curve prepared with equal amounts of iron and varying quantities of molybdenum. If preferred, a mixture of 3-methylbutanol and carbon tetrachloride, which is heavier than water, can be used as extractant. 

Procedure. Allow the whole of the sample solution (1 L) to flow through the resin column at a rate not exceeding 5 mL min . Wash the column with 250 mL of de-ionised water and reject the washings. Elute the copper(II) ions with 30 mL of 2M nitric acid, place the eluate in a small conical flask (lOOmL, preferably silica) and evaporate carefully to dryness on a hotplate (use a low temperature setting). Dissolve the residue in 1 mL of 0.1 M nitric acid introduced by pipette and then add 9 mL of acetone. Determine copper in the resulting solution using an atomic absorption spectrophotometer which has been calibrated using the standard copper(II) solutions. 

Calcium, D. of - continued in limestone or dolomite, (fl) 813 in presence of barium, (ti) 333 with CDTA, (ti) 333 with lead by EDTA, (ti) 333 with magnesium by EDTA, 328 by EGTA, (ti) 331 by flame emission, (aa) 804 Calcium oxalate, thermal analysis 498 Calcon 318 Calculators 133 Calibration of apparatus, 87 of burettes, 88 of graduated flasks, 88 of pipettes, 88 of weights, 74... 

Sanq>ling With Capillaries. Earlier it was pointed out that with the aid of a constriction pipette a sample can be measured accurately. A similar effect can be obtained by allows ing the serum to enter a capillary open at both ends. The serum will reach the opposite end of the capillary if it is held at an angle and then stoppered, and if the capillary has been carefully calibrated an accurate volume can be measured out. Such capillaries are now available commercially, and have been in use in automated equipment in the authors laboratory for at least 10 years. It is now possible to take the capillary and empty its contents into a container for analysis, or into a stream for the purpose of determining any of the materials which can be determined with the autoanalyzer. Figure 33 shows an instrument which is used for this purpose (58). 

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