Calibration volumetric

For greatest accuracy, we calibrate volumetric glassware to measure the volume actually contained in or delivered by a particular piece of equipment. We do this by measuring the mass of water contained or delivered by the vessel and using the density of water to convert mass into volume. 

The single most important application of statistical methods in science is the determination and propagation of experimental uncertainties. Quantitative experimental results are never perfectly reproducible. Common sources of error include apparatus imperfections, judgments involved in laboratory technique, and innumerable small fluctuations in the environment. Does the slight breeze in the lab affect a balance When a motor starts in the next building, does the slight power surge affect a voltmeter Was the calibrated volumetric flask perfectly clean ... 

A standard solution is a solution whose concentration is accurately known. Chemists often need standard solutions for chemical reactions. They are prepared by using a calibrated volumetric flask. 

The advantage of molarity is that it is generally easier to measure the volume of a solution, using precisely calibrated volumetric flasks, than to weigh the solvent, as we saw in Section 4.5. For this reason, molarity is often preferred over molality. On the other hand, molality is independent of temperature, since the concentration is expressed in number of moles of solute and mass of solvent. The volume of a solution typically increases with increasing temperature, so that a solution that is 1.0 M at 25°C may become 0.97 M at 45°C because of the increase in volume. This concentration dependence on temperature can significantly affect the accuracy of an experiment. Therefore it is sometimes preferable to use molality instead of molarity. 

Determinate errors can arise from uncalibrated balances, improperly calibrated volumetric flasks or pipettes, malfunctioning instrumentation, impure chemicals, incorrect analytical procedures or techniques, and analyst error. 

Bulk Density - Bulk density is the mass per unit volume of a fertilizer and includes the voids between particles. It is of interest in bag/package sizing, in calibrating volumetric feeders or applicators, and when considering the capacity of storage bins and transport vehicles. Bulk density is the most commonly used density value for fertilizers. 

Calibration Two solutions are required to produce a calibration curve of transmittance versus concentration of TDI. Approximately 200-255 mg of TDI is weighed accurately into 660 ml of glacial acetic acid. The solution is transferred and diluted to volume in a 1 litre volumetric flask. The solution should be used within 15 min after final dilution, to prepare a second solution by transferring a small quantity of the first solution containing 2200 /ig of TDI to a 1-litre calibrated volumetric analytical flask. A sufficient volume of 8-8n acetic acid is added so that when this second solution is diluted to 1000 ml it will have a normality of 0-6. The second solution is finally prepared by diluting to volume with water. To a series of eight graduated absorber cylinders, 5 ml of 1 -2n hydrochloric acid is added. Then 10-0,9-5,9-0,8-0,7-0,6-0,5 0, and 0-0 ml of 0-6n acetic acid are added and 0-0,... 

Gravimetric and volumetric feeders need periodic calibration. Even feeders with direct weight readout must be calibrated to ensure accurate feed is continuously maintained. The feeder, age, maintenance history, and operational experience influence the frequency of calibration. Volumetric feeders must be calibrated when a chemical batch is changed. This is because of a possible change in the consis-... 

Two types of flowmeters have been calibrated volumetrically in liquid hydrogen. Two turbine-type flowmeters (a in.-diameter 5 to 110 gpm, and a 1-in.-... 

For calculation of the volumetric flow rate only the cross section area of the pipe is to be known. In order to give flow under standard conditions the temperature and pressure must be measured, and for conversion to mass flow the composition or density of the gas must be determined. These process parameters are often monitored by calibrated instrumentation. 

Pipets and volumetric flasks provide a more accurate means for measuring volume. When filled to its calibration mark, a volumetric flask is designed to contain a specified volume of solution at a stated temperature, usually 20 °C. The actual vol-... 

Three important precautions are needed when working with pipets and volumetric flasks. First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Dirt and grease on the inner glass surface prevents liquids from draining evenly, leaving droplets of the liquid on the container s walls. For a pipet this means that the delivered volume is less than the calibrated volume, whereas drops of liquid above the calibration mark mean that a volumetric flask contains more than its calibrated volume. Commercially available cleaning solutions can be used to clean pipets and volumetric flasks. 

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

To prepare the solution we measure out exactly 0.1500 g of Cu into a small beaker. To dissolve the Cu we add a small portion of concentrated HNO3 and gently heat until it completely dissolves. The resulting solution is poured into a 1-L volumetric flask. The beaker is rinsed repeatedly with small portions of water, which are added to the volumetric flask. This process, which is called a quantitative transfer, ensures that the Cu is completely transferred to the volumetric flask. Finally, additional water is added to the volumetric flask s calibration mark. 

Calculate the molarity of a potassium dichromate solution prepared by placing 9.67 g of K2Cr207 in a 100-mF volumetric flask, dissolving, and diluting to the calibration mark. 

A 10-mL volumetric pipet was calibrated following the procedure just outlined, using a balance calibrated with brass weights having a density of 8.40 g/cm. At 25 °C the pipet was found to dispense 9.9736 g of water. What is the actual volume dispensed by the pipet ... 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

The capacity of a glass vessel varies with the temperature, and it is therefore necessary to define the temperature at which its capacity is intended to be correct in the UK a temperature of 20 °C has been adopted. A subsidiary standard temperature of 27 °C is accepted by the British Standards Institution, for use in tropical climates where the ambient temperature is consistently above 20 °C. The US Bureau of Standards, Washington, in compliance with the view held by some chemists that 25 °C more nearly approximates to the average laboratory temperature in the United States, will calibrate glass volumetric apparatus marked either 20 °C or 25 °C. 

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