Laboratory solution preparation

C04-0157. As a final examination in the general chemistry laboratory, a student was asked to determine the mass of Ca (0H)2 that dissolves in 1.000 L water. Using a published procedure, the student did the following (1) About 1.5 mL of concentrated HCl (12 M) was added to 750 mL of distilled water. (2) A solution of KOH was prepared by adding approximately 1.37 g KOH to 1.0 L distilled water. (3) A sample of potassium hydrogen phthalate (185.9 mg) was dissolved in 100 mL of distilled water. Titration with the KOH solution required 25.67 mL to reach the stoichiometric point. (4) A 50.00-mL sample of the HCl solution prepared in step 1 was titrated with the KOH solution. The titration required 34.02 mL of titrant to reach the stoichiometric point. (5) The student was given a 25.00-mL sample of a saturated solution of Ca (0H)2 for analysis. Titration with the HCl solution required 29.28 mL to reach the stoichiometric point. How many grams of calcium hydroxide dissolve in 1.00 L of water ... 

In the laboratory, chemists prepare buffer solutions in three different ways. Each results in a solution containing a weak acid and its conjugate base as major species. The most straightforward way to produce a buffer solution is by dissolving a salt of a weak acid in a solution of the same weak acid, as described in Example. ... 

Samples gathered and solutions prepared by laboratory personnel must be properly labeled at the time of sampling or preparation. In addition, a complete record of the sampling or preparation should be maintained. Sound quality assurance practices include a notebook record where one can find the source and concentration of the material used, the identity and concentration of the standard being prepared, the name of the analyst who prepared it, the specific procedure used, the date it was sampled or prepared, and the expiration date for any stored solutions. The reagent label should have a clear connection to the notebook record. A good label includes an ID number that matches the notebook record, the name of the material and its concentration, the date, the name of the analyst, and the expiration date. See Workplace Scene 2.5. 

Another example of the preparation of parts per million solutions is by dilution. It is a very common practice to purchase solutions of metals that are fairly concentrated (1000 ppm) and then dilute them to obtain the desired concentration. This is done to save solution preparation time in the laboratory. As per the discussion in Chapter 4, (see Equation (4.14) and the accompanying discussion), the concentration is multiplied by the volume both before and after dilution. Using the parts per million unit, we have... 

Since certified reference materials for seawater nutrient analysis are currently unavailable, individual laboratories must prepare their own standard solutions for instrument calibration. Standard stock solutions are prepared at high concentrations (mM) so that they can be used for months without significant alterations in concentration. Working low-concentration standard solutions are unstable and need to be prepared daily by diluting stock solutions with distilled water or low-nutrient seawater. In this case, the accuracy of nutrient analysis at a given laboratory is highly dependent upon the accuracy of the daily preparation of the calibration solutions. 

Commercially available kits are more stable and not as light-sensitive as enhancing solutions prepared in the laboratory. 

The polymer should be dissolved at room temperature [28]. Magnetic stirring devices or laboratory shakers are recommended to aid dissolution. Excessive temperature or ultrasonic devices may cause the polymer to degrade. Polystyrene solutions prepared with solvents such as THF are very stable, as long as MW < 500,000 g mol 1. However, it is a good practice to analyze polymer solutions within 24 hr of their preparation [28]. 

Problem A careful laboratory technician prepares an ethanol/water solution of precisely known concentration by measuring the number of moles of pure ethanol (net), mass of pure water (Mwa, in kilograms) and volume of the resulting solution (VSoin)- From these, he calculates the precise molarity and molality of the solution,... 

It is common practice in biochemical laboratories to prepare concentrated stock solutions and buffers. These are then diluted to the proper concentration when needed. Because of the concentration effects described above, it is important to adjust the pH of these solutions after dilution. 

Electrochemistry may be exploited for the analysis of extremely low concentrations of elec-trochemically active species, for laboratory scale preparations and in manufacturing processes on a massive scale [ 17]. In the context of the investigation of reaction mechanisms in solution, the present focus is on electroanalytical techniques these are included in Chapter 6, but there are obvious connections with Chapter 5 (see above) and Chapter 10 on free radicals. 

Small Quantities. Wear nitrile rubber gloves, laboratory coat, and eye protection. Work in the fume hood. To decompose 5 mL (5.4 g) of acetic anhydride, place 60 mL of a 2.5 M sodium hydroxide solution (prepared by dissolving 6.0 g of NaOH in 60 mL of water) in a 250-mL, three-necked, round-bottom flask equipped with a stirrer, dropping funnel, and thermometer. Add the acetic anhydride to the dropping funnel and run it dropwise into stirred solution at such a rate that the temperature does not rise above 35°C. Allow to stir at room temperature overnight. Neutralize solution to pH 7 with 2 M hydrochloric acid (slowly add 16 mL of concentrated acid to 80 mL of cold water) and pour into the drain.23... 

Gram Quantities. Work in the fume hood. Wear nitrile rubber gloves, eye protection, and laboratory coat. Prepare a solution of 0.05 mol of the dialkylnitrosamine in 500 mL of water in a 2-L, three-necked, round-bottom flask equipped with a stirrer and an outlet tube leading to the back of the fume hood. Cool in an ice bath. Add 500 mL of 1 M sodium hydroxide solution (20 g of NaOH dissolved in 500 mL of water). Gradually add... 

Notice that this is not the same as saying per liter of water. This distinction is accomplished in the laboratory by using a volumetric flask (Figure 7.1), which is specially designed for solution preparation. Volumetric flasks are manufactured in a variety of sizes, and so the preparer chooses the flask to correspond to the volume of solution desired. First, the solute is added to the flask. Then, water or another solvent is added until the liquid level rises to the etched line on the neck of the flask. 

The heat stabilities of both ovotransferrin and its iron complex were greatly increased under a pressure of 1000 atm. (8). Metal-free ovotransferrin was completely stable under 1000 atm. at 64° C. and pH 8.2 for 7 minutes. At the same temperature and pH the control at atmospheric pressure was completely denatured within five minutes. Temperatures as high as 90° C. for 30 minutes caused no change in iron ovotransferrin at pH 8.2 in 0.05 M sodium bicarbonate under 1000 atm. (37). Dilution of solutions of iron ovotransferrin with an equal volume of one of several different alcohols caused no demonstrable changes after 15 minutes incubation at room temperature while the metal-free ovotransferrins were completely and irreversibly denatured. (Advantage has been taken of this property to prepare crystalline iron ovotransferrin by incipient crystallization in the author s laboratory. Solutions are prepared at room temperature and then are chilled slowly in the refrigerator.)... 

WHO should draw to the attention of organizations such as CIPAC and AOAC International the fact that the quantities of analytical standards required to prepare calibration solutions are generally excessive in many published methods. The use of a more accurate balance and correspondingly smaller volumes of solvent for dilution will enable laboratories to prepare standards freshly and more economically. 

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