Dichromate, solution preparation

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. 

Potassium Dichromate Solutions. Preparation of the potassium chromate solution differs for the NIST and BP/EP procedures. For the NIST procedure, a solution of potassium dichromate (0.06 g of dichromate per kilogram of solvent) is prepared in 0.001 N perchloric acid. A 0.001 N perchloric acid solution as the reference is scanned and then the potassium dichromate solutions at 235, 257, 313, and... 

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). 

Benzoquinone ( quinone ) is obtained as the end product of the oxidation of aniline by acid dichromate solution. Industrially, the crude product is reduced with sulphur dioxide to hydroquinone, and the latter is oxidised either with dichromate mixture or in very dilute sulphuric acid solution with sodium chlorate in the presence of a little vanadium pentoxide as catalyst. For the preparation in the laboratory, it is best to oxidise the inexpensive hydroquinone with chromic acid or with sodium chlorate in the presence of vanadium pent-oxide. Naphthalene may be converted into 1 4-naphthoquinone by oxidation with chromic acid. 

Describe how you would prepare 465 mL of 0.3550 M potassium dichromate solution starting with... 

Prepare 250 mL of 0.02 M potassium dichromate solution and an equal volume of ca 0.1 M ammonium iron(II) sulphate solution the latter must contain sufficient dilute sulphuric acid to produce a clear solution, and the exact weight of ammonium iron(II) sulphate employed should be noted. Place 25 mL of the ammonium iron(II) sulphate solution in the beaker, add 25 mL of ca 2.5M sulphuric acid and 50 mL of water. Charge the burette with the 0.02 M potassium dichromate solution, and add a capillary extension tube. Use a bright platinum electrode as indicator electrode and an S.C.E. reference electrode. Set the stirrer in motion. Proceed with the titration as directed in Experiment 1. After each addition of the dichromate solution measure the e.m.f. of the cell. Determine the end point (1) from the potential-volume curve and (2) by the derivative method. Calculate the molarity of the ammonium iron(II) sulphate solution, and compare this with the value calculated from the actual weight of solid employed in preparing the solution. 

Procedure. Prepare a 0.25 per cent solution of diphenylcarbazide in 50 per cent acetone as required. The test solution may contain from 0.2 to 0.5 part per million of chromate. To about 15 mL of this solution add sufficient 3M sulphuric acid to make the concentration about 0.1M when subsequently diluted to 25 mL, add 1 mL of the diphenylcarbazide reagent and make up to 25 mL with water. Match the colour produced against standards prepared from 0.0002M potassium dichromate solution. A green filter having the transmission maximum at about 540 nm may be used. 

Standardisation may be carried out by the use of solutions prepared from a chromium-free standard steel and standard potassium dichromate solution. After dissolution of the standard steel, the solution is boiled with perchloric... 

Potassium chloride (nitrate) bridge 583, 582 Potassium chromate as indicator, 343, 349 Potassium cyanoferrate(II) D. of, (ti) 384 Potassium cyanoferrate(III) D. of, (ti) 399 Potassium cyanonickelate(II) prepn., 328 Potassium dichromate solution analyses involving, 375 oxidising properties of, 375 internal indicators for, 377 preparation of, 0.02M, 375 redox indicators for, 377 standardisation of, by iron, (cm) 546, (ti) 376... 

The detector was calibrated by pxm ing solutions of sodium dichromate of known absorbance through the sample port of the detector. The solutions were prepared in the carrier fluid which served as reference. The recorder response was measured as the ultimate height reached on the chart paper above the baseline when the sample fluid was switched to a sodiiam dichromate solution of known absorbance. The calibration was insensitive to flow-rate variations. 

An established school preparation of 2-propanone (acetone) involves the small-scale (and rather exothermic) oxidation of the alcohol with dichromate(VI). It was observed in several laboratories that when the acidified dichromate solution was added to the alcohol in small portions (1-2 cc) rather than dropwise as specified, small sparks or incandescent particles were produced which sometimes survived long enough to escape from the neck of the flask. This also happened if the alcohol and/or the oxidant solution were diluted with extra water, with old or new samples of alcohol, and if air were displaced from the flask by carbon dioxide. It is therefore important not to exceed the specified dropwise rate of addition of oxidant solution. It is very unusual for glowing particles to be produced from a homogeneous liquid reaction system. 

In some cases in which the Caro s acid oxidation of amines was not satisfactory, the corresponding hydroxylamines have been oxidized with acidified dichromate solutions [42], Both aliphatic and aromatic nitroso compounds have been prepared by this method [17, 42, 82, 90]. Frequently the reaction mixture from the reduction of a nitro compound is treated directly with the oxidizing medium without the isolation of the intermediate hydroxylamine. The method has been called the nitro reduction oxidation technique, [82] a terminology we cannot condone. 

To alleviate the burden of trying to prepare a potassium dichromate solution of exact concentration, the specific absorbance value cm can be used to compare the measured and standard values. The specific absorbance value normalizes the absorbance value obtained from using a solution of approximate 0.006% w/v to the equivalence of 1% concentration and a 1-cm path length measurement ... 

Determining the Solubility of Potassium Dichromate. Prepare a potassium dichromate solution that is saturated at room temperature. Using the table of solubility (see Appendix 1, Table 1), calculate the amount of potassium dichromate needed for the saturation of 50 ml of distilled water, and take an excess of it (10%). Place the amount of salt weighed on a technical chemical balance in a 100-ml flask and add 50 ml of distilled water to it. Close the flask with a stopper and stir its contents during 10-15 min (better in a shaker) while holding the flask by its neck. Prior to filtration, measure the temperature of the solution. Filter off the remaining insoluble salt and gather the filtrate in a dry flask. 

Thoroughly wash a burette with a chromium mixture and water, and then rinse it two or three times with the prepared saturated potassium dichromate solution. Why is this done Fasten the burette in a stand and pour the transparent solution into it. Fill the tip of the burette with the solution and note the level of the liquid in the burette. 

Assemble an apparatus for preparing acetylene (see Fig. 54). Put about 5 g of calcium carbide into 25-ml round-bottomed flask 1. Fill dropping funnel 2 and cylinder 3 with a 25% sodium chloride solution. Pour a little of a 1-2% sodium hydroxide solution into wash bottle 4, and a 2% potassium dichromate solution acidified with a dilute sulphuric acid solution into test tube 5. By adding the sodium chloride solution in small portions to the calcium carbide, achieve a uniform stream of gas. When the apparatus is filled with pure acetylene, test the gas for the absence of air in it. How can this be done ... 

The solution thus prepared is compared in the Duboscq colorimeter with the 8 mm. layer of the dichromate solution. 

According to the experiments at various authors, this solution exhibits the same colour as a liquid with a creatinine basis prepared as follows 0-010 gram of creatinine is dissolved In a little water and the liquid treated with 15 c.c. of picric acid solution (a) and 5 c.c. of caustic soda it is then left at rest for five minutes and made up to 500 c.c. As regards intensity of coloration, a layer 8-i mm. thick of this solution corresponds with one 8 mm. thick of N /2-potassium dichromate. In the colorimetric determination of creatinine, the dichromate solution is sufficient. 

Reagents. A solution prepared by dissolving 30 grams of pure potassium dichromate, free from organic matter, in 500 c.c. of water and adding 50... 

In a standardization procedure, 13.76mL of iron(II) sulfate solution were required to reduce 25.00mL of potassium dichromate solution, which was prepared by dissolving 1.692 g of K2Cr207 in water and diluting to 500.0 mL. (See reaction in Problem 13.38.) Calculate the molarity and the normality of both the potassium dichromate and the iron(II) sulfate solution. 

One of the filtered water samples was acidified on the same day as collection by the addition of 1.0 ml of concentrated HN03 acid using a droplet bottle. The sample for mercury analysis was acidified and preserved by adding 5 ml of a prepared solution of concentrated HN03 acid and potassium dichromate solution. Bottles were then stored in a refrigerator and sent to the laboratory soon after sampling. A blank water was collected, filtered, and preserved in the same manner as the actual samples after every 20th sample. 

The weights and volumes given in the suggested confirmatory tests are very approximate and serve to indicate a reasonable scale for the various operations. They are given solely for the guidance of the student satisfactory results can, however, be obtained on an appreciably smaller scale, t For the preparation of K2Cr207 paper, the dichromate solution should be almost saturated. 

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