Concentrated solution, preparing dilute

Preparation. Acids can be used as straight concentrated solutions or diluted to some degree with water. Some can be mixed (i.e., aqua regia three parts hydrochloric and one part nitric acid). Another mixed solution is peroxysulfuric acid. It is made by mixing a few milliliters (i.e., 5 mL) of concentrated H2SO4 with an equal amount of 30% H2O2. Warming (by steam) can often increase the effectiveness of an acid or oxidizer. 

To save time and space in the laboratory, routinely used solutions are often purchased or prepared in concentrated form (these are called stock solutions). In a process called dilution, water is then added to achieve the molarity desired for a particular solution. For example, the common acids are purchased as concentrated solutions and diluted as needed. A typical dilution calculation involves determining how much water must be added to an amount of stock solution to achieve a solution of the desired concentration. The key to doing these calculations is to remember that since only water is added in the dilution, all of the solute in the final dilute solution must come from the concentrated stock solution. That is,... 

Diluting molar solutions In the laboratory, you might use concentrated solutions of standard molarities, called stock solutions. For example, concentrated hydrochloric acid (HCl) is 12M. Recall that a concentrated solution has a large amount of solute. You can prepare a less-concentrated solution by diluting the stock solution with additional solvent. When you add solvent, you increase the number of solvent particles among which the solute particles move, as shown in Figure 14.7, thereby decreasing the solutions concentration. 

To prepare for instance, a liter of a 1 X 10 Af solution, one is faced with the need to weigh out 1x10 mol of the solute, a delicate task at best. However, there is a more practical way to accomplish the same thing, and that is to prepare a more concentrated solution and dilute it to the desired concentration. To carry out this process of dilution, we first note the required amount of solute in the final dilute solution, and then calculate the volume of the concentrated solution which will contain that amount of solute. 

It is difficult to measure out 0.1 mL with accuracy, so in this case it is best to carry out the dilution process in two steps. First prepare a lxl0" Af solution by diluting the concentrated solution, then dilute that solution to 1 X 10" A/. The mathematical procedure describing the dilution procedure can be greatly simplified by noting that, whatever the concentration of the final solution, the amount of solute in the final solution must be the same as the amount of solute taken from the more concentrated solution. So that... 

Applications for coolers include caustic being transferred to Storage, low-concentration solutions prepared by exothermic dilution, circulating caustic in vent scrubber systems, and full-concentration process applications. Stainless steel plates are used frequently at temperatures up to about 60°C. Nickel or one of its alloys is the... 

Acids are supplied as concentrated acids. The solutions required in the laboratory are prepared by diluting the concentrated solutions with water. For safety reasons the dilution is carried out by slowly adding the concentrated acid to the water. Water should never be added to concentrated acids. When a concentrated solution is diluted with water, the amount of solute in the solution remains unchanged. 

Often it is necessary to make a less concentrated solution from a more concentrated solution this process is called dilution. This situation usually occurs in the laboratory because it is more convenient to store more-concentrated solutions in order to save shelf space. Also, laboratory acid solutions, such as those of hydrochloric acid, sulfuric acid, and phosphoric acid, are almost always purchased as highly concentrated solutions these are more economical because there is more of the active ingredient per bottle. For standard solutions to use in chemical analysis, it is more accurate to weigh out a relatively large quantity of solute to make a relatively concentrated solution, llien dilute the solution quantitatively to prepare a more dilute standard solution. 

Repeating this process four more times, each time using the most recently prepared solution as the concentrated solution and diluting 10.00 mL to 100.00 mL, we get five KMnOd solutions with concentrations 4.00 X 10 A/, 4.00 X 10 A< 4.00 X 10 A/, 4.00 X 10 A/, and 4.00 X 10 A/ [Figure 4.12(b)]. This type of serial dilution is commonly used to prepare standard solutions with precisely known concentrations, for quantitative analysis. 

Freeze the solution till required. Do not re-freeze once the solution has thawed. Once the solubility characteristics of a standard are known, prepare concentrated solutions and dilute to working strength. Freeze multiple samples (1 —5 ml), but do not re-freeze once they have thawed. 

Absolute diethyl ether. The chief impurities in commercial ether (sp. gr. 0- 720) are water, ethyl alcohol, and, in samples which have been exposed to the air and light for some time, ethyl peroxide. The presence of peroxides may be detected either by the liberation of iodine (brown colouration or blue colouration with starch solution) when a small sample is shaken with an equal volume of 2 per cent, potassium iodide solution and a few drops of dilute hydrochloric acid, or by carrying out the perchromio acid test of inorganic analysis with potassium dichromate solution acidified with dilute sulphuric acid. The peroxides may be removed by shaking with a concentrated solution of a ferrous salt, say, 6-10 g. of ferrous salt (s 10-20 ml. of the prepared concentrated solution) to 1 litre of ether. The concentrated solution of ferrous salt is prepared either from 60 g. of crystallised ferrous sulphate, 6 ml. of concentrated sulphuric acid and 110 ml. of water or from 100 g. of crystallised ferrous chloride, 42 ml. of concentrated hydiochloric acid and 85 ml. of water. Peroxides may also be removed by shaking with an aqueous solution of sodium sulphite (for the removal with stannous chloride, see Section VI,12). 

IsoValeric acid. Prepare dilute sulphuric acid by adding 140 ml. of concentrated sulphuric acid cautiously and with stirring to 85 ml. of water cool and add 80 g. (99 ml.) of redistilled woamyl alcohol. Place a solution of 200 g. of crystallised sodium dicliromate in 400 ml. of water in a 1-litre (or 1-5 litre) round-bottomed flask and attach an efficient reflux condenser. Add the sulphuric acid solution of the isoamyl alcohol in amaU portions through the top of the condenser shake the apparatus vigorously after each addition. No heating is required as the heat of the reaction will suffice to keep the mixture hot. It is important to shake the flask well immediately after each addition and not to add a further portion of alcohol until the previous one has reacted if the reaction should become violent, immerse the flask momentarily in ice water. The addition occupies 2-2-5 hours. When all the isoamyl alcohol has been introduced, reflux the mixture gently for 30 minutes, and then allow to cool. Arrange the flask for distillation (compare Fig. II, 13, 3, but with the thermometer omitted) and collect about 350 ml. of distillate. The latter consists of a mixture of water, isovaleric acid and isoamyl isovalerate. Add 30 g. of potassium not sodium) hydroxide pellets to the distillate and shake until dissolved. Transfer to a separatory funnel and remove the upper layer of ester (16 g.). Treat the aqueous layer contained in a beaker with 30 ml. of dilute sulphuric acid (1 1 by volume) and extract the liberated isovaleric acid with two... 

Dissolve 46-5 g. (45-5 ml.) of aniUne in a mixture of 126 ml. of concentrated hydrochloric acid and 126 ml. of water contained in a 1-htre beaker. Cool to 0-5° in a bath of ice and salt, and add a solution of 36-5 g. of sodium nitrite in 75 ml. of water in small portions stir vigorously with a thermometer (1) and maintain the temperature below 10°, but preferably at about 5° by the addition of a httle crushed ice if necessary. The diazotisation is complete when a drop of the solution diluted with 3-4 drops of water gives an immediate blue colouration with potassium iodide - starch paper the test should be performed 3-4 minutes after the last addition of the nitrite solution. Prepare a solution of 76 g. of sodium fluoborate (2) in 150 ml. of water, cool, and add the chilled solution slowly to the diazonium salt solution the latter must be kept well stirred (1) and the temperature controlled so that it is below 10°. Allow to stand for 10 minutes with frequent stirring. Filter... 

I) An alternative procedure is to cool the solution containing the sodium sul. phanilate and sodium nitrite in a bath of crushed ice to about 5° and then add 10-5 ml. of concentrated hydrochloric acid diluted with an equal volume of water slowly and with stirring the temperature must not be allowed to rise above 10 and an excess of nitrous acid should be present (the solution is tested after standing for 5 minutes). The subsequent stages in the preparation—addition of dimethyl-aniline solution, etc.—are as above. 

The acetone test reagent consists of a 0 1 per cent, solution of 2 4-dinitro-phenylhydrazine and is prepared as follows Dissolve 0-25 g. of 2 4-dinitrophenyl-hydrazine in 60 ml. of water and 42 ml. of concentrated hydrochloric acid by warming on a water bath cool the clear yellow solution and dilute to 250 ml. with water. The acetone test is considered negative when 5 ml. of the reagent and 4-5 drops of the distillate give no cloudiness or precipitate of acetone 2 4-dinitro-phenylhydrazone within 30 seconds. After a negative test is obtained, it is stron y recommended that the mixture in the flask be refluxed for 5-10 minutes with complete condensation and then to collect a few drops of distillate for another test. If no acetone is now detected, the reduction is complete. 

Method B. For some purposes a shghtly more active catalyst is obtained when it is prepared in more concentrated solutions. The procedure is the same as above, but the volumes of solution for 5 g. of metal are dilute acid, 25 ml. formaldehyde, 35 ml. potassium hydroxide, 32 g. in 32 ml. of water. 

Freshly opened bottles of these reagents are generally of the concentrations indicated in the table. This may not be true of bottles long opened and this is especially true of ammonium hydroxide, which rapidly loses its strength. In preparing volumetric solutions, it is well to be on the safe side and take a little more than the calculated volume of the concentrated reagent, since it is much easier to dilute a concentrated solution than to strengthen one that is too weak. 

After adding p-rosaniline and formaldehyde, the colored solution was diluted to 25 ml in a volumetric flask. The absorbance was measured at 569 nm in a 1-cm cell, yielding a value of 0.485. A standard sample was prepared by substituting a 1.00-mL sample of a standard solution containing the equivalent of 15.00 ppm SO2 for the air sample. The absorbance of the standard was found to be 0.181. Report the concentration of SO2 in the air in parts per million. The density of air maybe taken as 1.18 g/L. 

Rea.ctlons, The chemistry of butanediol is deterrnined by the two primary hydroxyls. Esterification is normal. It is advisable to use nonacidic catalysts for esterification and transesterification (122) to avoid cycHc dehydration. When carbonate esters are prepared at high dilutions, some cycHc ester is formed more concentrated solutions give a polymeric product (123). With excess phosgene the usefiil bischloroformate can be prepared (124). 

Analysis for Poly(Ethylene Oxide). Another special analytical method takes advantage of the fact that poly(ethylene oxide) forms a water-insoluble association compound with poly(acryhc acid). This reaction can be used in the analysis of the concentration of poly(ethylene oxide) in a dilute aqueous solution. Ereshly prepared poly(acryhc acid) is added to a solution of unknown poly(ethylene oxide) concentration. A precipitate forms, and its concentration can be measured turbidimetricaHy. Using appropriate caUbration standards, the precipitate concentration can then be converted to concentration of poly(ethylene oxide). The optimum resin concentration in the unknown sample is 0.2—0.4 ppm. Therefore, it is necessary to dilute more concentrated solutions to this range before analysis (97). Low concentrations of poly(ethylene oxide) in water may also be determined by viscometry (98) or by complexation with KI and then titration with Na2S202 (99). 

Monobasic aluminum acetate is dispensed as a 7% aqueous solution for the topical treatment of certain dermatological conditions, where a combination of detergent, antiseptic, astringent, and heat-dispersant effects are needed (12). The solution, diluted with 20—40 parts water, is appHed topically to the skin and mucous membranes as a wet dressing (13). Burrow s solution, prepared from aluminum subacetate solution by the addition of a specific amount of acetic acid, is also used as a topical wet dressing. Standards of purity and concentration have been estabHshed for both pharmaceutical aluminum acetate solutions (13). Each 100 mL of aluminum subacetate solution yields 2.30—2.60 g of aluminum oxide and 5.43—6.13 g of acetic acid upon hydrolysis. For the Burow s solution, each 100 mL yields 1.20—1.45 g of aluminum oxide and 4.25—5.12 g of acetic acid. Both solutions may be stabilized to hydrolysis by the addition of boric acid in amounts not to exceed 0.9% and 0.6% for the subacetate and Burow s solutions, respectively (13). 

Solutions of anhydrous stannous chloride are strongly reducing and thus are widely used as reducing agents. Dilute aqueous solutions tend to hydrolyze and oxidize in air, but addition of dilute hydrochloric acid prevents this hydrolysis concentrated solutions resist both hydrolysis and oxidation. Neutralization of tin(II) chloride solutions with caustic causes the precipitation of stannous oxide or its metastable hydrate. Excess addition of caustic causes the formation of stannites. Numerous complex salts of stannous chloride, known as chlorostannites, have been reported (3). They are generally prepared by the evaporation of a solution containing the complexing salts. 

Stannous Chloride Dihydrate. A white crystalline soHd, stannous chloride dihydrate is prepared either by treatment of granulated tin with hydrochloric acid followed by evaporation and crystallisation or by reduction of a stannic chloride solution with a cathode or tin metal followed by crystallisation. It is soluble in methanol, ethyl acetate, glacial acetic acid, sodium hydroxide solution, and dilute or concentrated hydrochloric acid. It is soluble in less than its own weight of water, but with much water it forms an insoluble basic salt. 

C. Thymoquinone.—The wet aminothymol thus prepared is immediately dissolved in no cc. of concentrated sulfuric acid diluted to 4 1. and contained in a 12-I. flask. To this solution is added 150 g. of sodium nitrite (2.18 moles), in 5-10-g. portions, with shaking after each addition. The resulting mixture is heated to 60° on a steam bath, with occasional shaking, for half an hour (Note 5), and is then distilled in a current of steam, by means of the apparatus described in Org. Syn. 2, 80 (Note 6). All the thymoquinone passes over with the first 3 1. of distillate it solidifies on cooling, and is filtered with suction (Note 7), washed, and dried at room temperature. The yield is 80-87 g. (73-80 per cent of the theoretical amount) of bright yellow crystals, melting at 43-45° (Note 8). 

C. Palladium chloride on carhon (5% Pd). A solution of 8.2 g. (0.046 mole) of palladium chloride in 20 ml. (0.24 mole) of concentrated hydrochloric acid and 50 ml. of water is prepared (Note 2). The solution is diluted with 140 ml. of water and poured over 92 g. of nitric acid-washed Darco G-60 (Note 10) in an 8-in. evaporating dish (Note 3). After the palladium chloride solution has been thoroughly mixed with the carbon, the whole mixture is dried, first on a steam bath and then in an oven at 100°, with occasional mixing until completely dry. The mass (98-100 g.) is powdered and stored in a closed bottle. 

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