Elements of qualitative and quantitative chemical analysis
<• • «« » ^ k « * • ^ V to w «» to • " ■'»» «. • ^ to >• k - tow PRESS OF WM. F. FELL & CO.. 1220-24 SANSOM ST., PHILADELPHIA. PREFACE. In this work the author has brought together what has already been published, separately, in the "Manual of Qualitative Chemical Analysis," by himself and Dr. S. M. Babcock, and the "Notes on Chemical Analysis," by himself alone; he has added thereto much matter on qualitative analysis, all of Farts III and IV on quantitative analysis, and nearly all of Part V, Of course no such work as this can be satisfactorily prepared without frequent reference to the standard treatises of Prescott and Johnson, and of Fresenius, in one of which the whole subject of qualitative analysis is very thoroughly treated, and in the other both qualitative and quantitative analysis. The author would acknowledge his indebtedness to these works, while claiming for himself at least some novelty in the arrange- ment of the matter, as well as in some of the matter itself, in this attempt to prepare a book that is much needed in his own laboratory. Also, for valuable suggestions in the revision of the work for this edition, he is indebted to Professor Dennis, and Instructors Chamot and Smith, of the University. Certain modifications in the spelling of chemical terms, adopted, after very careful consideration, by the Chemical Sec- tion of the American Association for the Advancement of Science, are used in this book. TABLE OF CONTENTS. INTRODUCTION. Chaftbr I. — Solution,. * Solution. The soWent agents used ; physical salutian; chemical soiution ; [able of Eolubililies; solution of metals; solubility and temperature; solubility of salts in acids; comparative range of solvent power of diSereni solvents; saturaled solutions; solution of gases or vapors; solids From solution, b; cooling of saturated solutions; by removal of the solvent agent. CHAt-i-ER II.^Aaus, Bases, Salts. The lerma detined; anbydrids; norma] salts; acid salts; basic salts. Chaptbk III. — Oxidation, Chlorination, Reduction. The terms defined ; conditions under which oiidation or chlorination may lake place; reduction defined; conditions under which reduction may take place; the reaclions with free oiiygen; with nitric acid; with aqna regia; with potasamm chlorate and hydrochloric acid; with broroin; with chromic acid ; wilh sulfuric acid; changes in valence accompany- ing oxidation or chlorination or their reverse. Chaptbr IV.— Metathesis. Term defined; conditions under which it talces place; important law of ; several kinds of metathesis made use of; on some of the products of melalhesia; metathesis requires time. Chapter V.— Writing Equations. Requirements for the correct writing of an equation ; structural or graphic formulas of compounds ; valence of elements nnd compounds; table of elements, acids and hypothetical compounds entering into the equations for the reactions of chemical analysis; special signs attached to the equation; additional directions; common errors in writing equalioni; writing the reaction-histary of a sobstance, in equations. 95: VI TABLE OF CONTENTS. Chapter VI. — The Manipulations of Analytical Chemistry in General. Making solutions; fluxing; evaporation, ignition, acidification, alkaliza- tion, neutralization; adding a reagent in excess; precipitation; how precipitates are described ; digesting ; boiling ; filtration ; washing precipitates; dissolving precipitates; cleanliness in the laboratory; rapidity of work. PART II. SYSTEMATIC COURSE OF QUALITATIVE ANALYSIS. Chapter VII. — The Mode of Procedure in General. Introduction; the grouping of the substances; the sulfids, how produced, and important characteristic properties, as to solubility, oxidation, chlo- rination, reduction ; the hydroxids, how produced, when the action of the reagents normal and when not, solubility ; the final tests, place of in the course of analysis, two classes of; order of arrangement of the work ; tabular view of the course of analysis. Chapter VIII — The Solution of Substances. Solution of non-metallic substances ; of metallic substances ; treatment of insoluble substances. Chapter IX. — The Acidigens and Acids. Solubility of their salts; reactions; systematic course for their detection. Chapter X. — The Systematic Course for the Separation and De- tection of the Basigens (or Metals). The silver group ; occurrence, reactions and detection of the members of the group, and explanations of the chemistry of the work; the tin group, ditto ; the copper group ; the iron group ; the aluminum group ; the calcium group ; the potassium group. PART III. THE OPERATIONS OF QUANTITATIVE ANALYSIS. Chapter XI. — Methods of Measuring, in Gravimetric Analysis. Introduction; the balance described; its proper care and use; the weigh- ing of the substance for analysis ; the manipulations of weighing ; the weighing of the products of the analysis. Chapter XII. — Measuring in Volumetric Analysis. Standard and normal solutions defined ; the instruments for measuring, measuring flask, pipette, burette; cleaning these instruments; chromic acid cleaning mixture ; calibration of graduated instruments ; special precautions for use of these instruments. TABLE OF CONTENTS. VU Chapter XIII. — Seven Important Oi'zrations of Quantitative Work. Preparation of the solutioD ; Ihe precipilalion of Ihe snbstBcce, for gravi- metric deterrainalion ; fillrotioB, and use of suction ; washing Ihe pre- cipitate ; the asbestos filler, in Gooch crucible ; preparation of precipitate for weighing, by ignition. Chapter XIV. — Miscellaneous Matters. Determination of specific gravity of liquids, by specific gravity pipette i calculations of results of analyses; the quanlitative notebook; specid maxims for the young analyst. EXAMPLES FOR PRACTICE IN QUANTITATIVE ANALYSTS. Chapter XV. — Iron and Sulfur Trioxid. Iron, gravimetric method; products of the analysis described; tbe process lor (he delermination; voluoietric method, process described; prepara- tion of standard solution of dicbromate; assay of limonite. Suliiir trioxid ; product of the analysis described ; process for the determination. Chapter XVI. — Acidimf.trv and Alkalimetry. The principles of these methods of determining free acid or base; the indicators used ; preparation of the standard solutions, and determina- tion of their relative strength; preparation of normal solutions; deter- n of absoliile strength of the acid ; actual determination of free I XVII. — ^Lead, Phosphdrib Pentoxid bv Gravimetric Meth- ), AND Calcium. Iodometry. Phosphorus Pentoxid by Volu- metric Method. Nitrogen in an Organic Substance. Lead, products of the analysis described ; the process for the determlnntlon. Calcium, by a gravimetric and a volumetric method, principles of the methods i product of the analysis described; preparation of the standard solution; the processes (or the delermi nation. Phosphorus penloxid, product of the analysis described ; the process for the deter- mination. Phosphorus penlosid, volumetric; principles of the method; preparation of standard solutions; the operation, lodometry; princi- ples of this method of volumetric analysis; preparation of the solu- tions; determination of antimony. Nitrogen; principlesof ihe method; apparatus and reagents required; Ihfe operation. Chapter XVIII. — Analysis by Electrolysis. SeparalioQ of copper and silver; determination of ibe copper by eleclriv Ijrsis; determinUion of silver as cblorid. vni . TABLE OF CONTENTS. PART V. MISCELLANEOUS MATTER. List of apparatus for qualitative work. Lists of apparatus for quantitative work, for shorter coarse, and for full course. Special directions concerning the use of apparatus. List of reagents used, arranged in the alphabetical order of their full names, and also in the alphabetical order of their formulas. Directions in regard to the preparation pf reagents. Table of atomic weights. Tables showing per cent, of NHj, SO,, HCl, NjOg and HNO3, *^ solutions of different specific gravities. Table of comparison of weights and measures. Course of laboratory work in quantitative analysis. Index and glossary. INTRODUCTION. I At Cornell University a large number of students in some of I- the technical courses are required to take an elementary course ■ of practice in chemical analysis, including both qualitative and l.quanlitative work, after having had the elements of general [chemistry in this University or elsewhere. There are many comprehensive and excellent works on Alchemical analysis in general, and on particular branches of chemical analysis, with which every special student of chemistry is acquainted ; but these works are too large and expensive for any but the professional chemist. There are also numerous smaller works on the general subject. But in such of these as tile author has met with no special chapters are devoted to the enlightenment of the student beginning this study, on the chemistry of the operations of qualitative and quantitative analysis in general ; he must, therefore, be referred to the larger works for this important information. Consequently every beginner, whether his proposed course be thorough or elementary, must possess the works of Fresenius, or some other treatise of equal scope, or else copies of ihese works must be placed in the laboratory, for reading or reference, — many copies, if the number of students is large. Each plan has its manifest disadvantages. ■ In ihese considerations the author finds the reason for the pub- lication of this little volume, for the convenience of his own students, — that they may have always at hand such full details as to the chemistry and manipulation of these operations that they are called upon to perform, many of which are entirely ne« them, that they need never be at a loss as to what to do, or how 1 to do it, or why they do it, if they will read with care what is here given. When the work laid out in this book is mastered, the student who wishes to go further in general or s[)ecial direc- tions will find himself prepared to use the general or special treatises on chemical analysis, for no one of which this introduc- tion can be taken as a substitute. 2 INTRODUCTION. Cheipaical analysis is studied usually because the student expects to make some application of it in his business or profession ; in so far it is a practically useful study. But beyond this it has, as does the study of every branch of science, its special disciplinary value. It may serve to enforce the importance of attention to even small details in manipulation ; the student may learn, if he is capable of learning anything by ejcperience, that the only sure way to success in any part of the work is by the closest following of the directions given him, at least till he has proved, by con- clusive results of his own experiments or observations, that there is a better way ; but this better way must in its turn be followed no less precisely. No one of the analytical methods in use is absolutely perfect ; it is this that makes accuracy and precision in the manipulation of so much the greater account. Such discipline as this practice affords of the hand, eye, and judgment is useful in many of the professions for which the first sf>ecial training is given in the University. Chemical analysis comprises two kinds : — Qualitative analysis, by which we ascertain what chemical ele- ments or compounds a substance contains. Quantitative analysis, by which we ascertain how much of one or more of its constituents the substance contains. The chemistry and manipulation common to both kinds of analysis is treated in Part I of this work. The systematic course of qualitative analysis follows in Part II. In Part III a descrip- tion is given of some of the special operations of quantitative analysis, and in part IV the special examples for practice com- prised in this short course are described and explained. Finally, in Part V, lists of apparatus and reagents are given, with some instructions concerning their proper use, and several Tables for reference. PART I. THE PROCESSES OF ANALYTICAL CHEMISTRY, AND THE PRODUCTS OF ITS OPERATIONS. SOLUTION, AND SOLIDS FROM SOLUTION. The getting of substances into solution, and getting ihera out of solution into the solid form, are fundamental operations of analytical chemistry with which the student has to deal more than with any others, especially in the earlier stages of his work. Some general acquaintance with the principles and methods of the solution of substances is essential at the very outset. 2. Solution : — The solvent agents used. The more Important solvent agents used in chemical analysis are, besides water, aqueous solutions of hydrochloric acid, nitric acid, and acetic acid, of the alkaline hydroxids, KOH, NaOH, or NH.OH, of alkaline sulfids, especially ammonium sulRd, and alcohol, ether, and carbon disullid. 3. Since all these acids and alkaline solvents consist largely of water, a substance that is insoluble in water cannot, so long as it is the same chemical substance, be dissolved by any one of them. Therefore when a substance that is insoluble in water is dissolved with the aid of a solution of-an acid, such as the hydrochloric acid of the laboratory, or of an alkaline hydroxid, or an alkaline salt, such as a sultid or a chiorid, some chemical change must precede such solution. Thus two kinds of solution roay be dis- tinguished, physical solution and chemUal solution. 4. Physical solution is the product solely of the mutual action of physical forces jtnnong the particles of the solid atid of the liquid, the final result of which is the disappearance of the solid 4 ELEMENTS OF CHEMICAL ANALYSIS. [|g 5-63 as a. separate substance, visibly distinct from llie liquid; : becomes completely merged in ihe liquid, but suffers nochemica change; on removing the solvent, in any way that does notj chemically affect the substance dissolved in it, as by evaporation,] this substance reappears possessing the same chemical propertied as before. This is the kind of solution that iscommonlyeffeclet by water. 5. Chemical solution }n\oh/GS a chemical aiteration of the sub* stance dissolved, and the production of a new substance that t soluble. It may take place with water alone, when the substar is an oxid that can form a soluble hydronid. CaO is probablj''« insoluble in water, but CaCOH),, formed as soon as Ibis oxid i brought in contact with water, is slightly soluble. We say that we know whether K,jO is soluble or insoluble in waterd for in the instant of contact between the two KOH is formed,] which is very soluble. Most commonly, it is a more active agent than water serves to bring about the solution by chemical change, such asM an acid or an alkaline solution ; and if the excess of acid orW alkali, remaining after the solution is completed, be whollj'j removed, leaving nothing but water, the solution still coi undisturbed, which is evidence that a new substance has beei formed, soluble in water; and it has been formed by chemicalj changes affecting the solvent agent as well as the substance dis- solved, as illustrated by the following equations; — CaCOj + 2HLi = CaCI, + II,0 + CO,. "^ajtPOJ, + 4HCI = CaH,(PO,), + aCaCI,. 3Pb + 8HNO, = 3"Fb"(Nb,)~ + 4H,0 + 2NO. AsjS, + 3(NH,),S = 2( NH.),Ai5,. The substances whose formulas are underlined are new sub-J stances, which aresoluble in water. 6. Table of solubililtts. Some acquaintance with the soIu-1 bility of the chemical compounds met with in the ordinary work J of chemical analysis is very helpful to the itudent. In the foK" lowing Table, If in ihe square corresponding to any salt i caies easy solubility in water, w sparing solubility, wa solubility in water only when at least slightly acidified, A insolubility in water but solution by moderately dilute mineral acids, and / ■insolubility in water or dilute acids. Slight^ trace of solubility in water or dilute acids, or solubility or insolubility in acetic ■ 1 §6 ] S0LUTIO>f AND SOLIDS FROM SOLUTION. 5 H § s ■™zl!s< ■.\<i<\>.%i\«<\,<i'^f« <S<< ; ■mniwojis S<:-«f|<S-« ss» 5?5 <<S 5« •e-S< ■siDuois &< : :s: : :s : !<■< S<S SS< <»- : ■snomiBis ^<!< <» : ■l<\:« s-<s *<-<: <:s- : ■IBIlipOS SS^ sss 5SSJSSS SSS sss SSSS: ' -JS'lfS *<<l-<'-'^ s-<l > > s<< < *M<  9<^\^<<]<^<'< ■3ijnDJ»K i« :s< ss. . : :|. .< S<;<j<; g«<: -snojnwjpi ^<<\ >< »-< \«-:i :< g-;< -u »-^ ■>s>u»3uEK S<< <:g:< &s * < "-i; |< < S |g <;<■<; S <: < ■mmsauSsw S<i<jsS<:3 5& S5S,<:<5i3: *< os< ■pB37|;g<<:j< *<;3 X ■<:*<:*<;*, 5 << <:-"<; ■3fU3j '^<;< <& :\^^* :&-,»<5|5.»< <S<:< ■^aui^\^«\<%<\^-S. : ■<"- s<s,5<< <s<< -prai: : : : ;& :\ :& : < '. '. '.<<[ '.'.<]'.'.'.'. ■i3ddo3||&<;-»; -i;S<^& » <»~\i<t\^<<\<^^< ■wallS-*-* <:3:<5^<:|<"- »'<:5'^<<|<?'-'< ■^ummoiqj II ^ < :'<5s&s<!<: : : »<;sss<'<s-< -mnpra>«;<5<^=:5*SS^<*S5«: <>s< ■uin,<np.3.|^^ -l^^^^j^^ ^„ ■ i^^.g^^ <$"< ■qinuisia ;^< ;{<?■< 5 g< * :-|9< "*"<<«: *-< ■mnma I^K i\<^<\^^< m\<^^\s< :,'<-5< ■•nouoiuiwv ]& < < : * : : s -a : ; : s : g | : < < < g - < sssssssssss lisss* ss : << ; 5<-:|<!S :S Ill III Iff 111 zoo ■ 1 a J ^^^1 ■ ^^1 ^H ^H 6 ELEMENTS OF CHEMICAL ANALYSIS. [§§ 7-S. acid is not taken account of, nor solubility or insolubility of ' other than narma/ sails (§ 15), except when, as in the case of n many carbonates for example, basic sails are those ordinarily I met with. J An examination of this Table shows at once that all □Dnnat..! nitrates and acetates, and all the salts of the alkaline metals I (potassium, sodium) are soluble. Other sireiilar general conclu- V sions can be deduced by the student himself, which, once 1 familiarly learned, will be very useful in working out equations. I Of solubility in strong acids, but not in dilute, the following 1 instances are specially important; nearly all insoluble sulfids are I soluble in concentrated nitric acid : many insoluble silicates are 'J decomposable by concentrated hydrochloric acid, with solution J of the bases. I 7. Solution of metah. As to the solubility of the commonly \ occurring metals, it is to be noticed that all are absolutely '\a-\ soluble in pure water; that those of the aluminum and ironl groups are soluble in hydrochloric or sulfuric acid, with evolu- | [tion of hydrogen, and with the formation of chlorids or sulfates; Ihey are soluble in nitric acid, with the formation of the usual products (§ 24 b); that the rest, except gold and platinum, are dissolved by nitric acid, forming solutions except in the cases of antimony and tin, that gold and platinum are attacked by aqua regia or any mixture evolving free chlorin. '8. Afiscellaneoui cotisiderations. a. Solubility and tempera- ture. The solubility of salts is in most cases much greater in J hot tlian in cold solvents. I b. Solubility of sails in acids. A salt that is perfectly solublaj in water may be insoluble in strong acid: the salt, if separated 1 out or precipitated in the acid, usually takes a somewhat crystal- fl line form, often to the confusion of the young analyst; on the 1 addition of sufficient water, solution takes place readily. For I instance, barium chlorid, which is easily soluble in water, is not H soluble in strong hydrochloric acid ; hence, if barium carbonate, I is dissolved in too strong a solution of the acid, the chlorid itj apt to appear as a crystalline deposit. 1 c. Comparative range of solvent power of different solvents. \ In the operations of chemical analysis, the solubility or insol- I ubility of substances is utilized in, more particularly, water, I alcohol, mineral acids, organic acids, and solutions of alkaline -' hydroxids or hydrates, or alkaline solutions as they are com- monly called. Of these, water has by far the widest range of I S9-] solvent power; it may, in fact, be called /Absolvent upon which we depend in nearly all cases for solution of substances : a chemically active agent does not dissolve, but, as has been already shown, makes new compounds that are soluble in water. For this preparatory work, hydrochloric and nitric acids have the widest range of action ; comparing these two acids, 'it may be observed that while nitric acid will dissolve nearly all sub- F stances that are dissolved by hydrochloric acid, it will, in virtue ^of being an oxidizing agent (§ 20), dissolve many substances that hydrochloric acid will not dissolve, such as some metals and their sullids. d. A saturated solution. For each substance and each neutral solvent in which it is soluble, there is a definite limit to the amount of the substance that can be dissolved at a given tem- perature ; when this limit is reached the solution is said to be . saturated. When the temperature of such a solution falls, or, in ta ft very few cases, is raised, or when any of the solvent is removed, la portion of the substance separates out in the solid form. When I thus separated it is more or less crystalline. Therefore, when a L crystalline deposit appears in a solution, for no other apparent f reason than a possible change in one or both of these conditions liinentioned above, it can be re-dissolved either by adding more lof the solvent, usually only water being required, or by raising 1 I the temperature. e. In the case of chemical solution, the quantity of the sub- fcetance dissolved is dependent on the quantity of water present to fcdissolve the new substance formed, and also, in accordance with the laws of stoichiometry, on the quantity of the chemically active, solvent agent present. So far as this active agent is con- cerned, the quantity dissolved is independent of the tempera- lure, except that, as chemical action is stronger at high than at low temperatures, the solution will proceed more rapidly if the mixture of substance and solvent is heated. g. Solution of gases or of vapors. Some of the reagents used in chemical analysis are solutions of gases or vapors in water, such as hydrochloric acid and ammonia; and some reactions that are made use of in the analytical work depend to some extent on such solution, As in the solution of solids in water, there is a definite limit to the amount of the gas dissolved for any given temperature; but the amount dissolved increases, instead of diminishes, as the temperature falls, and increases also as the pressure of the 8 ELEMENTS OF CHEMICAL ANALYSIS. L^^ lO-l particular gas dissolved increases on the surface of the liquid, i in other words, as the proportion of that gas increases in t air to which the dissolving liquid is exposed. From what has been stated above, except when a solution of a gas is very dilute, that is, has but a very little of the gaseous substance in it, if the solution is warmed a portion of the sub- stance will escape; or, if a strong solution is poured from a - closed vessel into an open dish, or even if the stopper of tho'l bottle containing it is left out, the gas will escape. Other reagents in common use are solutions of substances that'J are volatile only at moderately elevated temperatures, c vapors, such as bromin water. On heating such a solution the i substance dissolved will escape in part, and the more rapidly the 3 lower the temperature at which it assumes the vaporous form. ,■ In a solution containing a mixture of such substances, the more \ volatile one may be almost entirely expelled by heat, while "a large portion of the other slill remains ; thus nitric acid or hy- drochloric acid may be removed from a solution by heating it for a sufficient length of lime with a large excess of sulfuric ^acid. 10. Solids from Solution: — There are many ways by which he reverse of solution of solids can be accomplished; some of these have been referred to in the preceding sections. The cooling of saturated soluttotis. This is of little account i analytical work. In a few cases, when the cooling is slow,- the I crystalline form assumed by the solid may be a convenient means. 1 of identifying it or confirming results of other tests. 11. The removal of the solvent age?it. This may be accom' J . plished in three ways. a. By its slow escape, as it passes into vapor by simple ex-] posure to the air. This lakes place readily with the chemically^ 1 inactive solvents, water, alcohol, common ether, petroleum ether, \ and carbon disulfid. The process is of little use in analytical I work, except when, as also in the case of slow cooling of satu- rated solutions referred lo in the preceding section, the solid I obtained may exhibit some characteristic crystalline form that I will aid in its identification. Such an operation is best applied J to a small quantity of ihe liquid in a watch glass, and it is usually » necessary to examine the deposit with the microscope. b. By the rapid removal of the solvent with the aid of heat. ' This is a very important analytical process, for it often becomes necessary lo obtain the product of some operation, or scries § 12.] ACIDS, BASES, SALTS. 9 of Operations, in the solid form, expeditiously; see Evaporation, Chap. VI. c. By the removal of an active solvent through neutralization. Many precipitates are soluble in acids, and some are soluble in solutions of alkaline hydroxids or salts, forming new, soluble compounds ; on neutralizing the acid or the base, or decompos- ing the solvent salt by an acid, the original substance is precipi- tated again, in some cases; in other cases, another substance is formed, of the same qualitative composition, but difTerenl quan- titative composition. The solution of ammonium magnesium phosphate in acid and the effect of the neutralization of the acid by ammonia, also the solution of aluminum hydroxid in sodium hydroxid and the result of the neutralization of the alkali by acid, ate good illustrations of the first mentioned kind of action ; see aluminum and magnesium in the aluminum and calcium groups. Chap. X. The precipitation by acids of the sulfids of arsenic, tin, and antimony from their solutions in ammonium sulfid, illustrates the second kind of action; see the tin group. Chap. X. d. By the formation, through chemical action, of a new insol- uble compound containing at least one of the constituents of the substance dissolved. This method of obtaining solids from solu- tion, which is of fundamental importance in analytical chemistry, will be more fully considered in Chap. IV, on Metathesis. CHAPTER II. ACIDS, EASES, SALTS. 13. A solution made in any of the ways described in Chap. I may contain acids, basts, or salts. For our present purposes, an add may be defined to be a body always containing (i) hydrogen, (2) a so-called electro-negative or acidigenic (acid-generating) element, such as chlorin, sulfur, phosphorus, and (3) sometimes oxygen ; its hydrogen being replaceable by an electro- positive or basigenic (base-generating) element or compound, such as potassium, calcium, magnesium, or ammonium. An acid may, however, contain, in the place of one of the regular acidigenic elements, an element usually basi- lO ELEMENTS OF CHEMICAL ANALVSIS. [§§ I3-16. geniCj as iron, or manganese in manganic or permanganic acid. The presence of an acid in solution is indicated by the reddening of blue litmus paper brought in contact with it. 13. A base is a body containing (i) a basigenic or base-form- ing element or compound, such as potassium, calcium, iron, or ammonium, and (a) the hypothetical hydroxy!, HO, this hydroxyl being replaceable by an acid minus its basic hydrogen. The presence of a base in solution is indicated by the blue color that it gives lo red litmus, or the brown color given to turmeric. Those elements commonly known as metalloids are acidigenic ; those commonly known as metals are basigenic. By the abstraction of the elements of waier from one or two molecules of an acid or a base, the so-called anhydrids are pro- duced : these anhydrids are also called oxids, with ihe prefixes di, tri, ttira, pent, used to indicate the number of atoms of oxy- gen, two, three, four, five, when more than one atom is present. Thus :— zHNOj — H,0 = NjOj (nilrogen pentoxid). H^yO, — 11,0 ^ SOj (sulfur lrio»id), aKOll — H,0 = K,0. Ca(0tl)5 — H,0 = CaO. 14. A salt is the body formed when the hydrogen of an acid, replaceable by a basigenic element or compound, or the hydroxyl of a base, replaceable by an acid minus its basic hydrogen, is so replaced. Thus ;— It will be seen that in all cases where a salt is formed by the mutual action of a base and an acid, water is produced from the hydrogen of the hydroxyl and a part or all of the oxygen. Salts may also be formed by the direct union of acidigenic and basi- genic oxids. A most characteristic properly of an acid and a base, respect- 'ively, is this tendency to react with each other, and, each neutralizing the marked properties of the other, to form a salt. 15. Of salts there are several kinds. A normal salt is one in which the basigenic element or compomid is substituted for all the replaceable hydrogen of the acid; for example, K,SO„ NaNOa, A1,(S0.),- 16. An acid salt is one of an acid having more than one re- placeable atom of hydrogen, in which less than the whole num- S5 17-20.] OXIDATION, CHLORINATION, REDUCTION. II ber is replaced by a basigenic element ; for example, KHSO, NaH,PO., compared with Na,PO.. 17. A basic salt is one, following the same line of definition, in which only a part of the hydroxyl of a base having more than one molecule thereof is replaced by an acid minus its hydrogen ; or in which there is a smaller number of molecules of the acidi- genic part of the salt, as compared with the basigenic part, than in the normal salt; for example, PbOHNO,, Pb.O,OHNO„ compared with the normal salt PbCNOs).; Bi(0H),NO3, BiOCi, compared with the normal salt Bi{NO,),. Basic salts are often difficultly soluble, when normal salts of the same acid are soluble, and sometimes cause the analyst much trouble ; thus, while solubility in water is an eminent character- istic of all normal nitrates, many basic nitrates are insoluble in 18. A double salt is defined as one containing with one acidigenic element or oxid, two basigenic elements or oxids, as KPtClj, KNaSO,. CHAPTER III. OXtDATION, CHLORINATION, REDUCTION. 19. These processes play an important part in the operations L of chemical analysis, and a clear understanding of their nature, and of the conditions under which they take place, is essential to intelligent work. Oxidation is the addition of oxygen to a substance, in chem- ical combination : chlorination is the addition of chlorin to a subslance in chemical combination ; the reverse of cither of these operations is reduction. 20. Conditions under which oxidation or chlorination may take place. a. The substance must be one capable of taking oxygen or chlorin into chemical combination. Such substances are all the basigenic elements in the list in the Table, § 37, whether these elements are in the free stale, or combined with sulfur as sulfids. In the latter case the sulfur is displaced by the oxygen or chlorin. Moreover, several of these elements are met with in ordinary 12 ELEMENTS OF CHEMICAL ANALYSIS. [§ 20. analytical work in combination with more than one proportion of oxygen or chlorin, as shown in the following table : — FeO, FeClg; Fe^Og, FeClg NiO, NiCIj; Ni^Og, NiCl3 CoO, CoClj; Co.,03, C0CI3, CrA. CrClj; CrOg MnO, MnClg; MnjOg, MnCl3; MnOj; MnOj; Mn^O^ Cu^O, CuCl; CuO, CuClg Hg,0, HgCl; HgO, HgClj Aup, AuClj; AU2O3, AuClj A&jOj, AsClj; AsjOg, SbjOj, SbClj; SbPj, SbCIg SnO, SnClj-, SnOj, SnCl^ \ Therefore anyone of these elements, although already in com- bination with oxygen or chlorin, may possibly combine with an additional quantity. Further, in our analytical work, we may meet with compounds of certain acidigenic elements with oxygen in different propor- tions. Such elements are nitrogen, sulfur, and carbon. b. There must be an oxidizing or chlorinating agent. Such agents, as used in analytical work, are especially nitric acid and free chlorin. The free oxygen of the air, or the oxygen dis* solved in water, may oxidize. Hydrochloric acid alone may act as a chlorinating agent on the metallic elements, but without ■ ever carrying the chlorination higher than the first stage, if there be more than one with the metal in question ; when this acid chlorinates, in this sense, its hydrogen is set free. Fe + 2HCI = FeClj -f 2H. Also, when an oxid is dissolved in hydrochloric acid a chlorid is formed, but hydrogen is not set free. In compounds of bromin and chlorin with oxygen, the union is a feeble one, and these compounds may serve as active oxidiz- ing agents. Such are chlorates (KCIO3), and hypobromites (NaBrO). See § 24, d and e, for examples. Any element capable of combining with oxygen or chlorin in more than one proportion may yield compounds in these higher proportions, which themselves become oxidizing or chlorinating agents, when brought into contact, under the requisite condi- tions, with a substance capable of being oxidized or chlorinated. 4FeS04 -h HjMnO^ + sHjSO^ = 2Fe2(S04)3 -f- MnSO^ + 4H2O. § 21.] OXIDATION, CHLORINATION, REDUCTION. I3 Iron is changed from a ferrous to a ferric salt, and manganese is reduced from manganic acid to a manganous salt. c. In case the oxidizing agent is the acid constituent of a salt, it must usually be set free by an excess of another and a stronger acid before action will take place in solution. Hence, a nitrate in solution in water, as sodium nitrate or silver nitrate, is not an oxidizing agent as free nitric acid is; further, nitric acid, as formed free in an aqueous solution, is usually too itiuch di- luted to act as an oxidizing agent. Potassium chlorate in solu- tion does not oxidize till an excess of concentrated hydrochloric acid or other stronger acid is added. To get oxidizing action from potassium dichromate strong acid must be added in excess. These substances may, however, act as oxidizing agents with- out such assistance, when in a state of fusion. d. Usually, heat favors oxidation or chlorination, as it does chemical action generally. e. When an element can take more than one degree of oxida- tion or chlorination (see list in a of this section) the higher oxid or chlorid will commonly be formed (or a salt correspond- iogwith the higher oxid), if the oxidizing or chlorinating agent be in excess to a considerable extent, and especially if its action be aided by heat. If the element itself is in excess, so that, as in the case of a solution of a metal by nitric acid, and especially if the acid be not very strong and the operation be conducted without heat, and a portion of the metal remains undissolved when the action of the acid has ceased, the lower oxid or chlorid is more likely to be formed. These rules as to chlorination apply, however, only to cases where the work is done by free chlorin, and have nothing to do with the simple solution of metals in hydrochloric acid. In cases where an element, or one of its compounds, is attacked by free chlorin in the presence of water, and the chlorid of this element cannot exist under the conditions of the attack, but the oxid or a compound of the oxid can exist, this o.tygen compound may be formed, the oxygen being derived from il»e water present. AsjS, -I- loCl + 6H,0 = zHjAsO, -|- loHCl -|- 3.S. . 21. Reduction. This operation, the reverse of oxidation or chlorination, lakes place under conditions the reverse of those required for oxidation or chlorination. a. The compound must be one that is capable of giving up ELEMENTS C HEMICAL ANALYSIS. a oxygen or chlorin under the given conditions. Such compounds are any of those formed with oxygen or chlorin by the basigenic elements, and by certain acidigeuic elements. And as an element may combine first with one quantity of oxygen or chlorin, and then with an additional quantity (§ao a), so in reduction under certain conditions a part of the acidigenic element may be with- drawn, and under the same conditions continued in force, o^J under other more powerfully active conditions, another portioC may be withdrawn. aHgCl, + SnCl, = aHgCl + SnCl^ 2HgCl + SnCl, = sHg + .SnClj PiCl, + SnCI, = PtCl^ + SnCl, PtClj + 2H = I't + 2HCI b. There must be a reducing agent. This may be any element-J having under the given conditions a stronger attraction for thel oxygen or chlorin in combination, than is exerted by the otherj element in the compound ; or it may be a compound of one these elements with oxygen or chlorin in a lower proportion, that3 is capable ofcombining with more than one proportion (§ 20 a). 23. A reducing agent excelling all others in the scope anclfl power of its action is nascent hydrogen, or hydrogen acting justj at the moment when it is liberated from combination, as when zi is dissolved in dilute sulfuric acid, Further, hydrogen compound^ in which the h)drogen is rather loosely bound, and especially il associated with elements that are themselves capable of oxidationja are strong reducing agents; sometimes the other element of th( compound is set free. AsH, + 6AgN0, + 3H,0 = 6Ae + H^AsOj, + 6I1NO,. Oxalic acid, H.,C,0„ sulfurous acid, HjSOj, and hydrogen. I sulfid, often occur as agents of reduction in analytical workj J carbon dioxid and water, sulfuric acid and water, and free sulfur \ being the products from the reducing agents in the several, 1 As to the hydrogen, it may be observed in general that when-] ever any oxidation, chfbrination, or reduction is taking place, and J there is any hydrogen in the substances engaged, it is very likely' I to appear in H,0 or HCI in the products of the aclioi bond of union between hydrogen and oxygen, or that between ^ hydrogen and chlorin, is one of the specially strong ones ; con- sequently the tendency to form one or the other of these LORINATION, REDUCTION. IS compounds, whenever there is any opportunity to do so, is very powerful. 6HaC,0, (oxalic add) + iKMnO, + 2H,S0« = SH^O + liCO, -\- K,SO, + MnSO,. Free hydrogen not nascent is a strong reducing agent when aided by heat. Also at higher temperatures, a red heat or above, carbon is a powerful reducing agent, carbon monoxid or carbon dioxid being the product yielded. 23. Except when oxidation or chlorination is effected directly by free oxygen or chlorin, these processes and reduction go hand in hand; the simultaneous presence, and the co-operation, of both a substance capable of oxidation or chlorination, and a body capable of yielding oxygen or chlorin, are essential. Commonly, the existence together in a solution is impossible of a substance in the free state that is capable of acting as an oxid- izing or a chlorinating agent under the existing conditions, and of a substance capable of taking up oxygen or chlorin. Since, as is usually the case in analytical operations, oxidation or chlorination of one substance implies reduction of some other substance, the same general conditions that are favorable for one operation are favorable for the other. 34. The reactions of oxidation and reduction. 3. With free oxygen. When oxidation results from the action of free oxygen of the air, or the oxygen dissolved in water, as when moist and freshly precipitated ferrous sulfid is oxidized 10 the sulfate, the reaction needs no explanation. b. With nitric acid; products of the oxidation. Water is always formed by the union of the hydrogen of the acid with a part of its oxygen. When metals or their sulfids are treated, a nitrate of the metai is nearly always formed; in a very few cases, as with antimony and tin, for example, oxids that are insoluble in the excess of acid are produced. All such exceptions to the general rule that nitrates are formed are mentioned where they occur. In the treatment of a sulfid the sulfur is either set free or oxidized to sulfuric acid (see sulfids, Chap. VII), Products of the reduction. The nitrogen of the decomposed nitric acid may be evolved as ammonia, NHj, free nitrogen, nitrogen monoxid or nitrous oxid, NjO, nitric oxid, NO, nitrogen trioxid, N^Oj, or nitrt^en tetroxid, N,0„ or NO,, If the nitric acid is in excess and a boiling heat is used, the pro- l6 ELEMENTS OF CHEMICAL ANALYSIS, [§ 24. duct is nearly all NO ; but excess of reducing agent, a very dilute acid, and a low temperature favor the formation of NHj at the other extreme. Under certain conditions hydrogen may be set free, although, as the presence of nascent hydrogen and nitric acid together is incompatible, such conditions can occur but rarely. In general, NO, a colorless gas, is formed in analytical work, which in contact with the air takes up oxygen, forming NjOj and NO,, the latter appearing as brown fumes. The following diagram illustrates the manner in which HNO, may be supposed to be broken up in the oxidation of substances, or in the production of nitrates of the metals, with NO as the product of the reduction; — /» apodal, Formalion ef nili H _ I O _ O - X ^ O It will be seen that in simple oxidation z molecules of HNO3 yield 3 atoms of oxygen : and that, in order to get the product HjO, which is always formed, the number of molecules of HNO3 taken must always be 2 or some multiple thereof; and that the number of atoms of oxygen used for the oxidation must always be 3 or some multiple thereof. Observe, also, that just half of the oxygen of the HNOj consumed goes to oxidize the metal, and the other half appears in the H^O and NO. It is also shown that in the formation of nitrates, 4 molecules of HNO, yield 3 molecules of the hypothetical NO,; and that, in order to get the product H.,0, the number of molecules of HNOs used must be 4 or some multiple thereof, and the number of molecules of NO5 used directly in the production of, and appearing in, the nitrate in the right hand member of the equa- tion, must be 3 or some multiple thereof. 3Sd + 4HN0j = 3SnOj + 2H,0 + 4.NO. sPb + SHNO, =3Pb(N0,), + 411,0 + 2NO. c. With aqua regia (3 parts cone, HCl, These acids react with each other as follow I part cone. HNOj). 3HCI + HNO, = NOCli + CI ^- 2H,0, or 3Ha + HNOj = NOCl + 2CI + aH^O. The products vary according to the existing conditions. § 24-] OXIDATION, CHLORINATION, REDUCTION, I7 The reaction is aided by heat, and lakes place only very slowly between moderately dilute acids. These compounds of nitrogen, oxygen, and chlorin, or nilrosyl chlorids as they are called, are gaseous and very unstable ; they are dissolved by that part of the acid still undecomposed : when the solution is satu- rated their production ceases. If a metal is put into this solu- tion, action (chlorinalion) takes place, these compounds being decomposed and supplying chloriii, while oxids of nitrogen escape; the solution being then no longer saturated with these gases a fresh quantity is formed ; thus the operation goes on, as long as these chlorinating products are consumed by a body capable of chlorination. It is especially the production of this abundance of free and nascent chlorin that makes aqua regia such a powerful chlorinating agent. 