The Analytical Balance

To weigh a chemical, first place a clean receiving vessel or weighing paper (which has a smooth surface to which powders do not easily stick) on the balance pan. The mass of the empty vessel is called the tare. On most balances, you can press a button to reset the tare to 0. Add the chemical to the vessel and read its mass. If there is no automatic tare operation, subtract the tare mass from that of the filled vessel. To protect the balance from corrosion, chemicals should never be placed 

Balances are delicate and expensive. Be gentle when you place objects on the pan and when you adjust the knobs. A balance should be calibrated by measuringa set of standard weights at least once a year. 

A mechanical balance should be in its arrested position when you load or unload the pan and in the half-arrested position when you are dialing weights. This practice prevents abrupt forces that would wear down the knife edges and decrease sensitivity. 

When you swim, your weight in the water is nearly zero, which is why you can float. Buoyancy is the upward force exerted on an object in a liquid or gaseous fluid. An object weighed in air appears lighter than its actual mass by an amount equal to the 

If mass m is read from a balance, the true mass m is 

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The addition of chemicals to the receptacle must be done outside the balance case. It is good practice to weigh the chosen receptacle on the analytical balance, to transfer it to a rough balance, to add approximately the required amount of the necessary chemical, and then to return the receptacle to the analytical balance for re-weighing, thus giving the exact weight of substance taken. 

The company had declared surplus two secretary chairs because they did not match the decor of the new offices. They were in excellent condition and very comfortable. One was placed at the analytical balance, the other at the desk in the main room. 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

Note This experiment assumes that a permanent log and a quality control chart are constantly maintained for each analytical balance in use in the laboratory. Each day you use a given analytical balance and log in with your name and date. The following calibration check should be performed weekly on all balances. If, according to the log, the calibration of the balance you want to use has not been checked in over a week, perform this procedure. Review Section 3.3 for basic information concerning the analytical balance. 

Check the calibration of the analytical balance you have chosen to use by weighing this standard weight on this balance. When finished, store the standard weight in the specified protected location. 

An analyst determines that the analytical balance he used in a given analytical test is wrongly calibrated. Is this a determinate or an indeterminate error Explain. 

If there is one, record the unknown number of your cement material. Using the analytical balance, weigh approximately 1.0 g of the cement (do not dry first) into the crucible, transferring with a spatula. If the balance has a tare feature, you may use it, but make sure you have the weight of the crucible with lid recorded. 

Place the crucible with lid back into the muffle furnace, with lid ajar, for 30 min. Remove from the oven and allow to cool, as before, for a total of 20 min. Weigh on the analytical balance. Repeat this step, but for just 5 min in the muffle furnace. 

At the end of the 30 min, allow the crucibles to cool in place for 1 min and then in your desiccator for an additional 15 min. Handle them with tongs until after you have weighed them. Weigh each, with lids, on the analytical balance. Record the weights in your notebook. 

The analytical balance is a much more sensitive weighing device than an ordinary balance. Name three things that are important to remember when using the analytical balance that are not important when using an ordinary balance (Refer to Appendix 2). 

Prepare three solutions of KHP for titration. To do this, again clean three 250-mL Erlenmeyer flasks and give each a final thorough rinse with distilled water. Now, weigh into each flask, by difference, on the analytical balance, a sample of KHP weighing between 0.7 and 0.9 g. Add approximately 50 mL of distilled water to each and swirl to dissolve. 

Weigh between 0.10 and 0.13 g of the macaroni sample on the analytical balance using weighing paper. Add this sample to a clean, dry 100-mL Kjeldahl flask to avoid having sample particles stick to the neck of the flask. Set aside for a few minutes (inside a beaker so that it stays upright) while you do step 4. 

This is an indeterminate error because it is inherent in the use of the analytical balance. 

A number of considerations are important. For example, the analytical balance must be level, the pan must be protected from air currents, the object to be weighed must be at room temperature, the object must be protected from fingerprints, etc. 

In some cases, the exam question will request a list of the equipment needed, while in other cases you will get a list from which to choose the items you need. Certain items appear in many experiments. These include the analytical balance, beakers, support stands, pipets, test tubes, and Erlenmeyer flasks. Burets, graduated cylinders, clamps, desiccators, drying ovens, pH meters, volumetric flasks, and thermometers are also commonly used. If you are not sure what equipment to choose, these serve as good guesses. Most of the remaining equipment appears in three or fewer experiments. 

Figure 6.14. Scheme of the experimental apparatus for gravimetric measurements described by Zavrazhnov et al. (2003) in their investigation of the phase composition control in chemical transport reactions. (1) quartz ampoule, (2) thermocouples, (3) two-zone furnace, (4) quartz rods, (5) wire for suspending the ampoule, (6) support, (7) weighing beam of the analytical balance,... 

Let the setup cool to room temperature. Bring the pressure in it to atmospheric by lifting or lowering the beaker with water or the flask. Close the clamp. Measure the volume of the water in the beaker by pouring it into a measuring cylinder. Weigh the test tube on the analytical balance with the same accuracy as before. 

A complimentary foreword must invoke the possibility of future editions. In future versions of this book, I hope that F. Rouessac and his wife will include a few additions on the analytical balance, as I have mentioned above, but also on electrochemical methods, on sampling - so rarely discussed and on the treatment of numerical data (archiving of primary data, statistical treatment, graphical representation, etc.). Or perhaps, in a volume 2 ... 

Figure 2-9 (a) Class A glass volumetric flask. [Courtesy A. H. Thomas Co., Philadelphia, PA.] (fc>) Class B polypropylene plastic volumetric flask for trace analysis. [Courtesy Fisher Scientific, Pittsburgh, PA.] Class A flasks meet tolerances of Table 2-3. Class B tolerances are twice as big as Class A tolerances, (c) Short-form volumetric flask with Teflon-lined screw cap fits in the analytical balance in Figure 2-3a. Teflon protects the cap from chemical attack. 

A chemical formula tells the numbers and the kinds of atoms that make up a molecule of a compound. Because each atom is an entity with a characteristic mass, a formula also provides a means for computing the relative weights of each kind of atom in a compound. Calculations based on the numbers and masses of atoms in a compound, or the numbers and masses of molecules participating in a reaction, are designated stoichiometric calculations. These weight relationships are important because, although we may think of atoms and molecules in terms of their interactions as structural units, we often must deal with them in the lab in terms of their masses—with the analytical balance. In this chapter, we consider the Stoichiometry of chemical formulas. In following chapters, we look at the stoichiometric relations involved in reactions and in solutions. 

The average accuracy of the data was 4.9% for volume, 2.5% for mass and 5.6% for the particulate matter levels. Future improvements in our technique of measurement of Ah will permit a decrease in the uncertainty of PMi0 levels to about 4%. Further decreases in the final error are limited by the quality of mass measurements. For example, a decrease of 0.001 g in the uncertainty of mass measurements will reduce the PM10 final error to a value as low as 0.5%. Thus, we are now focusing efforts in the improvement of our weighting procedure, particularly in what concerns ambient conditions for the operation of the analytical balance. 

After the sealed bulb has been weighed it is to be filled with water and weighed on the platform (not the analytical) balance. Water from which dissolved air has been expelled must be used... 

There are only two occasions where measured weight equals true mass, when k = 1. This occasion occurs when measurements are made in a vacuum or the density of a sample is equal to the density of the mass standard. Fortunately, the greatest differences only occur when an object s density is particularly low (0.1% for density 1.0 g/cm3 and about 0.3% for density = 0.4 g/cm3). In most situations, the effect of air buoyancy is significantly smaller than the tolerance of the analytical balance. The effects of varying densities (of objects being weighed) and varying air densities are shown in Table 2.22. 

Immediately reweigh the sample tube on the analytical balance say 2.1564g. This is the mass of the container together with a few crystals of the chemical which have remained in the container. However, you now know exactly the mass of the chemical in the beaker 11.9726 — 2.1564 = 9.8162 g of (NH4)2FeS04.6H20. 

C3. iCheck the-zdro reading pn the analytical balance by-pressing ithe.bar/Buttdn with the balartce-doors closed. ".i.. t--... 

Analytical Chemistry. For over two decades spectrophotometers have found increasing use as a means of making certain kinds of analytical measurements. Twenty years ago chemical abstractors apparently did not consider this instrument of much analytical importance and consequently one could not depend upon finding articles abstracted for equipment of this kind. The obvious answer in this case is that the subject was considered to be physics and the searcher should have turned to that subject. One might reply that all measuring instruments, such as the analytical balance, belong in the strict sense just as much to physics. 

Although chemical reactions have been observed over the course of human history, it was only in the late eighteenth century that scientists began to use quantitative tools to monitor chemical changes. The revolutionary quantitative tool developed at this time was the analytical balance, which was capable of measuring very small changes in mass. 

The analytical balance has undergone a dramatic evolution over the past several decades. The traditional analytical balance had two pans attached to either end of a lightweight beam that pivoted about a knife edge located in the center of the beam. The object to be weighed was placed on one pan sufficient standard masses were then added to the other pan to restore the beam to its original position. Weighing with such an equal-arm balance was tedious and time consuming. 

After a bit, you can scrape your crystals into the appropriate tared container. (See Tare to the Analytical Balance in Chapter 6, Microscale Jointware. ) Don t try to get all of them it s usually not worth the effort. 

The Conical Vial As Vial Packaging Oops Tare to the Analytical Balance Electronic Analytical Balance Heating These Vials... 

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