Accuracy volumetric flask

Accuracy tolerances for volumetric flasks at 20°C are given by ASTM standard E288. 

A standard sample of 57.22% w/w CE was analyzed by placing 0.1011 g in a 100-mL volumetric flask and diluting to volume. Three unknowns were prepared by pipeting 0.250 mL, 0.500 mL, and 0.750 mL of the bulk unknown into separate 50-mL volumetric flasks and diluting to volume. Analysis of the three unknowns gave areas of 15310, 31546, and 47582, respectively. Evaluate the accuracy of this analysis. 

In the laboratory, a solution of known concentration is often prepared in a glass vessel called a volumetric flask. Volumetric flasks typically allow volumes to be measured with an accuracy of four significant figures. Figure 3-17 summarizes the general procedure for making a solution in a volumetric flask. This procedure is further described in Example 3-17. 

The British Standards Institution (BSI) and the National Physical Loaboratory (NPL) have laid down the tolerances in the capacity of volumetric flasks (i.e., measuring flask) at 20°C by two sets of tolerances viz., Grade A and Grade B respectively, evidently to indicate the class of accuracy to which the flask has been subjected to for graduation, followed by the manufacturer s name and finally the BS standard number. However, the permitted tolerances for volumetric flasks commonly used in analytical laboratories are depicted in Table 2.8. 

Pump Flow accuracy Run pump at 0.3 and 1.5 mL/min (65% acetonitrile/water) and collect 5 mL from detector into a volumetric flask. Measure time. < 5%... 

Flow accuracy is checked at 0.3 and 1.5mL/min with the column in place by measnring the time required to fill a 5-mL volumetric flask from... 

Flow Rate Accuracy. One of the key performance requirements for the pump module is the ability to maintain accurate and consistent flow of the mobile phase. This is necessary to provide stable and repeatable interactions between the analytes and the stationary phase [8,9]. Poor flow rate accuracy will affect the retention time and resolution of the separation. The flow-rate accuracy of the pump can be evaluated simply by calculating the time required to collect a predetermined volume of mobile phase at different flow rate settings. For example, the flow-rate accuracy at 2 mL/min can be verified by using a calibrated stopwatch to measure the time it takes to collect 25 mL of effluent from the pump into a 25-mL volumetric flask. A calibrated flow meter can be used to determine the flow rate as well. The typical acceptance of the flow rate accuracy is listed in Table 11.1. A steady backpressure may be required, depending on the requirement of the system. 

Once the density of a liquid, such as water, is known with great accuracy as a function of temperature, it provides a very useful means of determining the accurate volumes of vessels. Volumetric flasks are purchased with a nominal (approximate) value ofthe volume printed on their walls. Th accurate volume can be determined by calibration with water, as illustrated in the next problem. Once calibrated, the flask can be used Over and over again for other accurate measurements. 

The accuracy of the results will be greatly affected by the accuracy of the volumetric measurements. Make sure that any volumetric flasks or pipets used for obtaining the appropriate dilutions are calibrated correctly. 

Solid-state samples can be prepared by accurately weighing the drug substance into a container that can be stored under the appropriate condition. Suitable containers include volumetric flasks, scintillation vials, etc. The amount of drug substance used is usually dictated by the availability of material, accuracy of balances, and the final concentration desired for analysis. Typical amounts used for solid-state samples would be between 2 and... 

Start with 1000.0 mL of pure water (measured with a volumetric flask to 0.5 mL accuracy). Add 10.000 mg silver chloride (weighed to 0.01 mg accuracy), then stir to produce a saturated solution. 

The quality of any given volumetric ware is based on how accurate any given calibration line will deliver the amount it claims. For example, say that a one-liter flask is accurate to 5%, meaning that the flask is likely to contain anywhere from 950 mL to 1050 mL of liquid. For comparison, a one-liter Class B volumetric flask is accurate to 0.60 mL, or 0.06% accuracy, and a one-liter Class A is accurate to 0.30 mL, or 0.03% accuracy. Needless to say, it costs more for greater accuracy. 

Standards of Accuracy The capacity tolerances for volumetric flasks, transfer pipets, and burets are those accepted by the National Institute of Standards and Technology (Class A),1 as indicated in the accompanying tables. Use Class A volumetric apparatus unless otherwise specified in the individual monograph. For plastic volumetric apparatus, the accepted capacity tolerances are Class B.2... 

The volumetric flask has a ring engraved around its neck when the bottom of the liquid meniscus is level with this ring, at the designated temperature, the volume of liquid contained is that indicated by the engraved label. Frequently the letters TC, meaning to contain, are present on the label. When such a flask is used to deliver, it should be allowed to drain for at least 1 min even so, its accuracy is not as high when used to deliver as when used to contain. 

Before measuring a volume it is important to choose the most appropriate equipment in order to achieve the greatest accuracy. The volume and level of accuracy will help determine which piece of equipment should be used. The most common equipment includes various pieces of glassware (e.g. volumetric flask, measuring cylinder, burette, pipette), mechanical micropipette (or pipettor) and syringes (Figure 2.3). 

While both a measuring cylinder and volumetric flask are useful in the measurement and dispensing of larger volumes (typically 5-5000 ml), a volumetric flask has a higher level of accuracy. Conical flasks or beakers are particularly convenient for dispensing liquids, but as they have a much lower accuracy they should not be used for measurement of volume. 

Subtract the weight of the empty weighing container from p that of the weighing container plus sarnple and you now know the mass of cherqical, to an accuracy of four decimal places, f > which has Seen transferred into-your volumetric. flask/ Beaker.. -- oncbnical flask. - - I... 

For maximum accuracy, the batch method (separate solutions prepared in volumetric flasks) is preferred, but a titration method (aliquots of ligand solution added to a single solution in a spectrophotometer cell) may be necessary in some situations. Spectrophotometry permits the use of much lower concentrations of metal ion and ligand than are feasible in the pH titration method, if the molar absorptivity of the complex is high. 

The accuracy and stability of the pump flow rate can be checked at 1 mL/min, with the column in place, by measuring the time required to fill a 10-mL volumetric flask... 

Eor total metal solubilization a representative unfrozen sample of 0.5 g, dried at 110°C and finely powdered, is dissolved in a Teflon bomb by an acid mixture of 5 ml of 40% HE, 1 ml of aqua regia (HNO3-HCI 1 3 v/v) and 5 ml of MQ water, heated at 180°C for 90 min and then cooled to room temperature. 5 ml of saturated H3BO3 solution are then added to the Teflon bomb and, after dilution to 50 ml in a volumetric flask, the solution is analyzed by ICP-AES. To test the accuracy of the total solubilization method several certified reference materials in sediment matrices are always analyzed along with the unknowns, obtaining results in a good agreement with certified values, as reported in Table 8.2. 

Highest Accuracy. The volumetric glassware items with the highest accuracy are bulb pipettes and volumetric flasks (see Figure 4.4). 

The most important part of any spectroscopic assay is not the performance of the spectrophotometer (although the accuracy of the instrument is checked periodically). The crucial part of any experiment is the accurate preparation of the test and standard solutions. This often involves the accurate dilution of a stock solution using the volumetric glassware introduced in Chapter 6, namely the pipette and the volumetric flask. 

The method is based on the addition of a standard reference (internal standard) that is detected at a different wavelength from the analyte. The reference standard is added at the same concentration to samples and standards and diluted to mark in a volumetric flask. This technique uses the signal from the internal standard to correct for matrix interferences and is used with respect to precision and accuracy as well as eliminating the viscosity and matrix effects of the sample. 

Determinate error usually results from experimental equipment which is faulty. Students usually first meet this concept in the analytical chemistry laboratory in determinations of weight and volume. The quality of the equipment used is reflected in the accuracy of the results obtained. Accuracy is a measure of how close the experimental result is to the truth. For example, if one wishes to make up a solution of accurately known concentration in a volumetric flask of 100 mL, both the flask and the balance used must be carefully calibrated. The flask is calibrated by weighing it empty and then filled with distilled water at a known temperature. On the basis of the weight of water, and the known density of the water, one may calculate an accurate volume for the flask when it is properly filled to the mark. Calibration of the balance is based on the use of standard weights which do not corrode and which cover the range in mass used in the experiment. The accuracy of the standard weights and the quality of the volumetric flask determine the accuracy of the concentration of the solution which is made. 

For liquids at ambient conditions, one can obtain a reasonably accurate density simply by weighing a known volume of the liquid. For rough work, one can use something as simple as a volumetric flask for more precise work, calibrated volumes (known as pycnometers) are used. One can easily obtain 1% accuracy, and careful pycnometry can obtain 0.1%. 

The levels of accuracy of solution preparation required are usually indicated in the protocol or by the nature of the experiment. Look for phrases such as accurately weighed which means to four decimal places on an analytical balance, together with quantitative transfer. Volumes quoted as 250.00 mL, 100.00 mL, 25.00 mL imply the use of volumetric flasks and pipettes. 

A standard solution is one in which the solute is weighed out to an accuracy of 4 decimal places and is made up in a volumetric flask. 

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