Potassium chromate, reaction with

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

In actual practice, however, such titrations are more or less restricted to those involving precipitation of Ag+ with anions, for instance halogens (Cl , Br, I-) and thiocyanate (SCN-). Generally, it is quite difficult and tedious to locate the exact point at which further addition of reagent affords no more precipitation. Therefore, the choice and wisdom of a chemical reaction is preferably sought so as to result in either a coloured solution or a coloured precipitate at the end point. A typical instance may be cited by application of potassium chromate (K2Cr04) solution in the above case whereby any extra drop of silver nitrate, after all the chloride has been precipitated, immediately causes precipitation of red chromate showing that the end point has been duly achieved. 

Bi(N03)3 + 3NaOH Bi(OH)3 + 3NaN03 Reaction with potassium chromate produces bismuth chromate ... 

Preparation of Potassium Chromate. (Perform one experiment on each table.) Melt a mixture of 1 g of potassium carbonate, 1 gof potassium hydroxide, and 2 g of potassium nitrate in an iron crucible by heating with the flame of a burner. While stirring the melt with an iron wire, introduce 1 g of finely comminuted chromite or -0.8 g of chromium oxide into the crucible. Roast the mixture for 5-10 minutes on a blowpipe. Treat the cooled melt with water. Filter the solution and evaporate it until a crystalline film appears. What is the composition of the formed crystals Why was potassium carbonate introduced into the reaction Write the equation of the reaction. 

Pour an ammonium sulphide solution into a potassium chromate or dichromate solution and heat the mixture. How does the solution s colour change What is the composition of the precipitate Write the equation of the reaction. What properties does chromium (VI) exhibit in this reaction How does an acidified potassium chromate or dichromate solution react with hydrogen sulphide, sulphur dioxide, and an iron(II) salt Write the equations of the reactions. 

Pour potassium chromate and dichromate solutions into separate test tubes and add a solution of a barium salt to each of them. What is the composition of the precipitates See how the obtained salts react with dilute acids. Perform similar experiments with a lead salt. Write the equations of the reactions. 

Methyl 3-Carbethoxyoxy-7-keto-12a-hydroxycholanate.79 A solution of 4 g (8.6 mmoles) of crude methyl 3-carbethoxyoxycholate in 50 ml of acetic acid containing 10 g of sodium acetate trihydrate is treated gradually with a solution of 3 g (0.015 mole) of potassium chromate in 8 ml of water with shaking. After 10 hr at 25° the solution is diluted with water and the product collected by filtration to afford 3.9 g (97%) of methyl 3-carbethoxyoxy-7-keto-12a-hy-droxycholanate, mp 150-158°. Crystallization from methanol gives hard, compact needles or rods mp 157-158° [a]D25 41°. The same substance is also obtained (93 % yield) by conducting the above oxidation in the absence of sodium acetate but stopping the reaction after 6-7 hr. 

Chromate. Potassium chromate. [CAS 7789-00-6], ICj-CrO, yellow solid, soluble, formed by reaction of potassium carbonate and chromite at a high temperature in a current of air, and then extracting with water and evaporating the solution. Used (1) as a source of chromate. (2) in leather tanning, (3) in textile dyeing, (4) in inks. 

PbCr04 (c). Goldblum and Stoffella1 measured the heat of reaction of aqueous potassium chromate with aqueous lead nitrate to be 10.6. For the heat of the reaction of aqueous lead chloride with aqueous potassium chromate, Roth, Schwrartz, and Buchner1 found Q = 11.16. 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

Neutral solutions of cobalt salts produce with potassium chromate a reddish brown precipitate of hydrated basic cobalt chromate, Co0.CoGrO4.Aq. This reaction may be utilised in detecting small quantities of cobalt.4... 

Spot test place a drop of the test solution on a watch glass or on a spot plate, add a drop of ammonium carbonate solution and stir (this renders any mercury(I) or lead ions unreactive by precipitation as the highly insoluble carbonates). Remove one drop of the clear liquid and place it on drop-reaction paper together with a drop of the potassium chromate reagent. A red ring of silver chromate is obtained. 

If barium is present, proceed as follows. Impregnate some quantitative filter paper or drop-reaction paper with a saturated solution of potassium chromate, and dry it. Place a drop of the test solution on this paper and, after a minute, place 1 drop of the reagent on the moistened spot. A brownish-red spot or ring is formed. 

Lead acetate solution white precipitate of lead sulphate, PbS04, soluble in hot concentrated sulphuric acid, in solutions of ammonium acetate and of ammonium tartrate (see under Lead, Section III.4, reaction 5), and in sodium hydroxide solution. In the last case sodium tetrahydroxoplumbate(II) is formed, and on acidification with hydrochloric acid, the lead crystallizes out as the chloride. If any of the aqueous solutions of the precipitate are acidified with acetic acid and potassium chromate solution added, yellow lead chromate is precipitated (see under Lead, Section III.4, reaction 6). 

When two solutions are mixed, an insoluble substance sometimes forms that is, a solid forms and separates from the solution. Such a reaction is called a precipitation reaction and the solid that forms is called a precipitate. For example, a precipitation reaction occurs when an aqueous solution of potassium chromate, K2Cr04(aq), which is yellow, is mixed with a colorless aqueous solution containing barium nitrate, Ba(N03)2(aq). As shown in Fig. 4.12, when these solutions are mixed, a yellow solid forms. What is the equation that describes this chemical change To write the equation we must know the identities of the reactants and products. The reactants have already been described K2Cr04(aq) and Ba(N03)2(tf<7). Is there some way we can predict the identities of the products In particular, what is the yellow solid ... 

In this section we will consider the types of equations used to represent reactions in solution. For example, when we mix aqueous potassium chromate with aqueous barium nitrate, a reaction occurs to form a precipitate (BaCr04) and dissolved potassium nitrate. So far we have written the molecular equation for this reaction ... 

It has been involved in many industrial explosions. Explodes on contact with aluminum + barium nitrate + potassium nitrate + water. Forms explosive mixtures with aluminum powder + titanium dioxide, ethylene glycol (240°C), cotton lint (245°C), furfural (270°C), lactose, metal powders (e.g., aluminum, iron, magnesium, molybdenum, nickel, tantalum, titanium), sulfur, titanium hydride. Reaction with ethanol + heat forms the explosive ethyl perchlorate. Violent reaction or ignition under the proper conditions with aluminum + aluminum fluoride, barium chromate + mngsten or titanium, boron + magnesium + silicone rubber, ferrocenium diammine-tetrakis(thiocyanato-N) chromate(l —), potassium hexacyanocobaltate(3—), A1 +... 

If an excess of hydrogen peroxide be added to a solution containing a chromate, made slightly acid with sulphuric acid, and the mixture then shaken up with a few c.c. of ether, a blue ethereal layer is formed above the aqueous solution. The colour is due to the formation of perehromie acid (see p. 73), and the reaction aSords a usefiil test for identifying chromates in presence of sulphuric acid. If the ether used is free from alcohol, 1 part of potassium chromate in 40,000 parts of water can be detected. Another sensitive test for chromates and di-ehromates is to add a trace of a-naphthylamine to the solution and acidify with tartaric acid, when an intense blue colour results. ... 

Mohr titration, which involves the formation of a coloured precipitate by reaction with the indicator. For example, in the determination of chloride concentration with silver nitrate a small amount of potassium chromate solution is added as an indicator. This results in the formation of a red silver chromate (Ag2Cr04) precipitate at the end-point ... 

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