A compendious manual of qualitative chemical analysis

Front matter

rye v

A COMPENDIOUS MANUAL or

QUALITATIVE CHEMICAL ANALYSIS. —

Preface

Tue authors have endeavored to include in this short treatise enough of the theory and practice of qualitative analysis ‘‘ in the wet way,” to bring out all the reasoning involved in the subject, and to give the student a firm hold upon the general vrinciples and methods of the art. It has been their aim to give only so much of mechanical detail as is essential to an exact comprehension of the methods and to success in the actual experiments. Hence, é the multiplication of different tests or processes, having essentially the same object, has been purposely avoided. For the same reason none of the rare elements are alluded to. The manual is intended to meet the wants of the general student, to whom the study is chiefly valuable as a means of mental discipline and as a compact example of the scientific method of arriving at truth. To professional students who wish to make themselves expert analysts, this little book offers a logical introduction to the subject, an outline which is trustworthy as far as it goes, but which needs to be filled in and enlarged by the subsequent use of some more elaborate treatise as a book of reference. Prof. Johnson, of Yale, has supplied this need with his excellent edition of Fresenius’s comprehensive manual.

The authors believe that they have put into the following pages as much of inorganic qualitative analysis as is useful for training, and also as much as the engineer, physician, agriculturist, or liberally educated man needs to know. The book has been written - for the use of classes in the Institute of Technology, who haver already studied the authors’ ‘‘ Manual of Inorganic Chemistry.” It is simply an implement devised to facilitate the giving of thorough iv PREFACE.

instruction to large classes in the laboratory. It is the authors’ practice to examine their classes orally every four or five exercises, in order to secure close attention to the reasoning of the subject. With this exception, the subject is studied exclusively in the laboratory, tools in hand. Fifty laboratory exercises of two hours each have proved sufficient to give their classes a mastery of the subject as it is presented in this manual.

It is scarcely necessary to say that this little work is a compilation from well-known authorities, among which may be particularly mentioned the works of Galloway, Will, Fresenius, and Northcote & Church.

Boston, April, 1869.

Preface To The Revised Edition

In this revised edition, undertaken with the advice and consent of the authors, such alterations and additions have been made as have been suggested by the use of the book with a number of classes in the laboratory.

Boston, July, 1876.

Table Of Contents

Introduction, Qualitative analysis defined. Scope of this manual. Identifying compounds. Division ofthe subject. 2 .« «© » e

Part I

Chapter I. Example of the separation of two elements. Division of twenty-four metallic elements into seven classes. « «6 «© « « «

Chapter II. Class I. Chlorides insoluble in water and acids. Lead, Silver. Mercury. . e ° ° ° ° e . ° ° ° ° ° °

Chapter III. Class II. Sulphides insoluble in water, dilute acids and alkalies. Mercury. Lead. Bismuth. Copper. Cadmium. The precipitation of Classes II and III. . e ° ° ° . ° ° ° °

Chapter IV. Class III. Sulphides insoluble in water or dilute acids, but soluble in alkaline solutions, Arsenic, Antimony. Tin. [Gold and Platinum.] ° ° ° ° . ° ° e ° ° ° ° ° °

Chapter V, Class IV. Hydrates insoluble in water, ammonia-water, and solutions of ammonium salts. Simultaneous precipitation of some Salts which require an acid solvent. Treatment of the precipitate, produced by ammonia-water. Chromium. Aluminum. Manganese. Iron. Separation of Class IV from Classes II and III. The original condition of iron. The use of chloride of ammonium. Interference of organic matter. . ° ° ° ° . . ° ° . . e ° ry e

Whapter VI. Class V. Sulphides insoluble in water and in saline or alkaline solutions. -Manganese, Zinc. Nickel. Cobalt. Separation of Class V from ClassIV. . .- ° ° ° ° ° ° ° e e e

Chapter VII. Class VI. Carbonates insoluble in water, ammonia-water and saline solutions. Barium. Strontium. Calcium, Separation of Class VI from the preceding classes. . +« + +6 ce e © ©

Chapter VIII, Class VII. Three common metallic elements not comprised in the preceding classes. Magnesium. Sodium. Potassium, Table for the separation of the seven classes of the metallic elements. .

Chapter IX, General tests for the non-metallic elements. The classes of salts treated of. General reactions for acids. Metallic elements to be first detected. The barium test. The calcium test. The silver test.

Nitrates, chlorates and acetates, CO Ses oe. -26t) Bee Ww 6 ee

PAGE

59-63

64-73

BS ea at vi CONTENTS.

Chapter X, Special tests for the non-metallic elements. Effervescence. Carbonates. Cyanides. Sulphides. Sulphites. Hyposulphites. Chromates. Arsenites and Arseniates. Sulphates. Phosphates. Oxalates. Tartrates. Borates. Silicates. Fluorides. Chlorides. Bromides. Iodides. Nitrates. Chlorates. Acetates. . e« « Sie Were

Part II

Treatment of substances of unknown composition. General observations. Husbanding material. « « « «© «© «© e e «

Chapter XI. Treatment of a salt, mineral or other non-metallic solid. Order of procedure. . . . Preliminary examination in the dry way. Closed-tube test. Gases or vapors to be recognized. Sublimates. Reduction test. Metallic globules. . . . Dissolving a salt, mineral or other non-metallic solid, free from organic matter. Dissolving in water. Dissolving in acids. . . . Treatment of solutions obtained. An aqueous solution: neutral, acid, alkaline. An acid solution. . « . Examination of the solutions for the non-metallic elements. . . . Table of solubilities. . . . Treatment of insoluble substances. Fusions. Treatment of the fused mass. Decomposition by means of carbonate of calcium and chloride of ammonium. Fusion with acid sulphate of sodium. Deflagration, Vries ee, Yikes oe Jems. Pete ht

Chapter XII. Treatment of a pure metal or alloy. Action of nitric

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74-88

90-120 acid on metals. Gold test. Platinum test. “Sie ae ee ee ie a

Chapter XIII. Treatment of liquid substances, Evaporation test. Testing with litmus. Testing forammonia, .« ¢« e« «© e « eo e125, 126

Appendix

Reagents. Acids. Sulphuretted hydrogen. Ammonia-water. Ammonium salts. Caustic soda. Sodium salts. Potassium salts. Nitrate of silver. Calcium salts. Barium salts. Acetate of lead. Lead puper. Magnesium mixture. Ferric chloride, Nitrate of cobalt. Sulphate of copper. Chloride of tin. Oxide of manganese. Mercury salts. Bichloride of platinum. Zine. Solution of indigo. Litmus paper. Starch paste. Alcohol. Water. .°. . Solutions of known composition. - « e Utensils. Reagent-bottles. Test-tubes. Test-tube rack, Flasks. Beakers. Glass funnels. Filtering. Filter-stand. Rapid filtration, Porcelain dishes and crucibles. Lamps. Blast-lamps and blowers. Iron-stand. Tripod. Wire-gauze. Triangle. Water-bath. Sand-bath. Blowpipes. Platinum foil and wire. Pincers. Platinum crucibles. Wash-bottle. Glass tubing. Stirring-rods. Cutting and cracking glass. Bending and closing glass tubes. Blowing bulbs, Caoutchouc. Corks. Gas-bottle. Self-regulating generator. Mortars. Spatule, e ° e e e e e e e e . e e e e e i-xly&

Tes yy

PaaS

QUALITATIVE ANALYSIS.

Introduction

1, Qualitative Analysis, in the widest sense of the term, is the art of finding out the elements contained in compound substances. The general definition has important limitations in practice. In the first place, the art, as commonly taught, applies almost exclusively to mineral, or inorganic, substances, and touches only incidentally upon the multifarious compounds. of carbon with hydrogen, oxygen, nitrogen and a few other elements, which form the subject-matter of that branch of chemical science called organic chemistry. Again, the » analysis of gases constitutes a distinct branch of analysis, requiring methods and apparatus of its own, and therefore to be most advantageously studied by itself. These deductions made, there remains the analysis of inorganic solids and liquids, which is in fact the main subject of qualitative analysis in the present technical sense of the term.

Of the sixty-four recognized chemical elements, only the thirty-four most important are embraced in the systematic course of this manual. Means of detecting a few other less common elements are incidentally given; but most of the elements which are so rare as to be at present of little interest except to the professional chemist or mineralogist are not alluded to.

2. Some previous knowledge of general chemistry is essential to the successful study of qualitative analysis. It is assumed that the student knows something of the common

9 IDENTIFYING COMPOUNDS. § 3 elements and of their most important combinations, that he is familiar with the principal laws which govern chemical changes, and that he possesses a certain skill in the simplest manipulations. The tools and operations employed in qualitative analysis are few and simple; but neatness, method in working and a vigilant attention even to the minutest details, are absolutely essential. As the various substances used or produced in the operations of analysis will not be particularly described, the careful student will keep at hand some text-book on general chemistry, to which he can constantly refer to refresh his recollection of the formulz and physical and chemical properties of the substances referred to. It should be observed that it is often very difficult — in fact, impossible in the present state of knowledge — to express in exact equations the involved or obscure reactions which occur in complex mixtures during the operations of analysis. Itisa useful exercise for students to write out in equations the simpler chemical changes which occur in analysis ; but when the attempt is made to put a complex reaction into numerical symbols, the equations are apt to express either more than we know, or less.

3. Although the detection of the elements contained in _ compound substances is the ultimate object of analysis, it is only by exception that the elements themselves are isolated, and recognized in their uncombined condition. An element is generally recognized through some familiar compound, whose apparition proves the presence of all the elements it contains, just as the presence of any word upon this page makes it sure that the letters with which it is spelt are imprinted there. If, as the result of a definite series of operations upon some unknown body, the hydrated oxide of iron _ be produced, no iron having been added during any stage of the process, the proof of the presence of iron in the original - body is quite as certain as if the gray meval itself had been extracted from it. If some well-known sulphate, like sulphate of lead, or of barium, for example, result from a series

§ 4 IDENTIFYING COMPOUNDS. 3 of experiments upon some unknown mineral, it is certain that the mineral contained sulphur ; provided only that no sulphur has been introduced in any of the chemical agents to whose action the mineral has been submitted.

The compounds through which the elements are recognized are necessarily bodies of known appearance, deportment and properties. They are, in fact, bodies of various, though always definite, composition; oxides, sulphides, chlorides, sulphates and many other salts, are thus made the means of identifying one or more of the elements which they contain. The object of the analyst is to bring out from the unknown substance, by expeditious processes and under conditions which admit of no doubt as to their testimony, these identifying compounds, with whose appearance and qualities he has previously made himself acquainted. As he follows the course of experiments laid down in this manual, the student will gradually acquire, with the aid of frequent references to a text-book of general chemistry, that stock of information concerning the identifying compounds which must be always ready for use in his mind, and at the same time he will be made familiar’with the character of the methodical processes which secure a prompt and sure testimony to the elementary composition of the substances he examines.

4. The subject is treated in two parts or divisions, of which the first contains a series of experiments to illustrate a systematic course of examination for substances in solution, — when once that solution has been made; while the second treats chiefly of the preliminary examination of solids and the means of bringing them into solution, and indicates the general methods to be pursued in the actual analysis of a substance of unknown composition.

Chapter I. Division Of The Metallic Elements Into Classes

5, Example of the Separation of two Elements. — Put a small crystal of nitrate of silver and a small crystal of sulphate of copper into a test-tube (Appendix, § 65), and dissolve them in two teaspoonfuls of water, warming the water at the lamp to facilitate the solution. Add to this solution a few drops of dilute chlorhydric acid (App., § 3). Shake the contents of the tube violently, wait until the curdy precipitate, which the acid produces, has separated from the liquid, and then add one more drop of chlorhydric acid. If this drop produces an additional precipitate, repeat the operation until the new drop of acid produces no change in the partially clarified liquid. Then, and not till then, has all the silver which the original solution contained been precipitated in the form of chloride of silver, an unemployed balance or excess of the reagent, chlorhydric acid, remaining in the clear liquid ; this liquid can be readily separated by filtration from the curdy chloride.

Shake the contents of the test-tube, and transfer them as completely as possible to a filter (App., § 70), supported in a very small glass funnel (App., § 69), which has been placed in the mouth of a test-tube. With a wash-bottle (App., § 81) rinse into the filter that portion of the precipitate which has adhered to the sides of the first test-tube. When the

§ 6 THE TERM CLMBS.— CLASS I. 5 filtrate has drained completely from the precipitate, set the test-tube which has received it aside. Wash the precipitate together into the apex of the filter by means of a wash-bottle with a fine outlet; and, in order to wash out the soluble sulphate of copper which adheres to the precipitate, fill the filter full of water two or three times, throwing away this wash-water when it has passed through the filter.

The complete separation of the silver and copper which were mixed in the original solution is already accomplished ; the silver is on the filter in the form of chloride; the copper is in the clear, bluish filtrate. This speedy and effectual separation of the two elements is based upon the fact that chloride of silver is insoluble in water and acid@iquids, and is, therefore, formed when chlorhydric acid is added to a solution containing a salt of silver; chloride of copper, however, is read@ly soluble in water and acid liquids, and, even if formed by the addition of chlorhydric acid to a solution of a compound of copper, would fail to manifest itself by appearing as a precipitate. It is, in general, true that whenever, by the addition of a reagent, there can be formed in any solution a compound insoluble in the liquids present, this compound always separates as a precipitate. Such differences of solubility as are illustrated by the case of the chlorides of silver and copper are the chief reliance of the analyst.

6. Definition of the term ‘‘ Class.”? Class I. — In the foregoing experiment only two elements have been separated. It might obviously be very difficult, if not impossible, to find a special reagent for every element, which would always precipitate that single element and never an er. Chlorhydric acid, for example, which precipitates silver so admirably from any solution containing that element, is capable of eliminating two other elements under like conditions. The lower chloride of mercury (mercurous chloride or calomel) is insoluble in water and weak acids. Chloride of lead is sparingly soluble in cold ‘water, and is still less soluble in water acidulated with chlorhydric acid.. The chlorides of the ot:

6 DIVISION INTO CLASSES. § 6 other metallic elements are all soluble in water. and acids under the conditions of the analytical process.

There are embraced in the scope of this manual twenty-two of the so-called metallic elements,—elements whose hydrates or oxides are said to be basic in their character, and are collectively designated as bases: if chlorhydric acid were added in proper quantity to a solution imagined to contain all these elements, three, and only three, of the twenty-two elements would be precipitated as chlorides. After filtration and washing, a mixture of chloride of silver, chloride of lead and mercurous chloride would remain upon the filter, and all the other elements would have passed as soluble compounds into the filtrate. Silver, lead and mercury, the three elements thus separated from the rest by this well-marked reaction with chlorhydric acid, constitute a class, the first of several classes into which the metallic elements are divided for the ends of qualitative analysis. Each class is characterized by some clear reaction which suffices, when intelligently applied, to separate the members of any one class from the other classes. The chemical agent, by means of which this distinctive reaction is exhibited, is called the general reagent of the class. Thus, chlorhydric acid is the general reagent of the first class.

This division of the elements into classes renders it unnecessary to find means of separating each individual element from all the others. In the systematic course of an analysis, the classes are first sought for and separated; afterwards each class is treated by itself for the detection of its individual members. ,It is an incidental advantage of this division of the elements into classes that, when the absence of any whole class has been proved by the failure of its peculiar general reagent to produce a precipitate in a solution under examination, it is unnecessary to search further for any member of that class. Much time is thus saved, for it is as easy to prove the absence of a class as of a single element. The full treatment of the first class of elements, comprising, as

F e

§ §7, 8 DIVISION INTO CLASSES. 3 we have seen, silver, lead and m y, is the subject of Chapter II.

7. Experiment to Illustrate the Division of the Metallic Elements into Classes. — We proceed to demonstrate experimentally the chemical facts upon which rests the division of the-other metallic elements into convenient classes.

Prepare a complex solution, by mixing together in a small beaker (App., § 68) a small teaspoonful of each of the following solutions (App., § 62), viz.: — chloride of copper, arsenious acid in chlorhydric acid, ferrous chloride, chloride of zinc, chloride of calcium, chloride of magnesium and chloride of sodium. Dilute the mixture thus prepared with its own bulk of water. Should any turbidity or precipitate appear, add chlorhydric acid, little by little, until the solution becomes clear. This solution is representative ; it contains at least one member of each of the classes of elements which remain to be defined. It contains no member of the first class; but we may consistently suppose that the members of this class have been previously precipitated, as in the foregoing experiment (§ 5), and that an excess of chlorhydric acid remains in the liquid.

8. Definition of Classes II and III. — Pass aslow current of sulphuretted hydrogen (App., § 14) from a gas-bottle or self-regulating generator through the acid liquid in the beaker. This operation must be performed either out of doors or in a current of air sufficient to carry the excess of the gas away from the operator. A dense, dark colored precipitate will immediately appear, and gradually increase in bulk. When the gas iieed continually for five or ten minutes through the liquid, remove the beaker from the source of the gas (or interrupt the stream of gas if a self-regulating generator be employed), stir the liquid well, and blow out the sulphuretted hydrogen which lies in the beaker. If after the lapse of two or three minutes the liquid smells distinctly of sulphuretted hydrogen, it is saturated with the gas, and it is sure that the reagent has done its work. If the liquid does not retain the

8 DEFINITION OF CLASSES II AND III. § 8 characteristic odor, the gas must be again passed through it until saturation is certainly attained.

In order to obtain still further assurance of the saturation of the liquid, it is often well to take the first portions of the filtrate of the succeeding paragraph and add a small quantity of sulphuretted hydrogen water (App., § 15). Ifa sufficient amount of the gas had not been passed into the liquid, the addition of the sulphuretted hydrogen water would cause the appearance of a precipitate. In such a case the filtered portion must be returned to the beaker and the stream of gas again passed through the liquid.

Pour the contents of the beaker, well stirred up, upon 4 filter which is supported over a test-tube or second beaker. Rinse the first beaker once with a teaspoonful of water, and transfer this rinsing water to the filter, allowing the filtered liquid to mix with the original filtrate. Label* this filtrate ‘¢ Filtrate from II and III” (classes), and preserve it for later study.

If any considerable quantity of precipitate has adhered to the sides of the original beaker, it may be detached and washed on to the filter by means of a sharp jet of water from the wash-bottle. The precipitate, as it lies upon the filter, must then be washed once or twice with water ; the wash-water

* The student should at once make it a rule to label every filtrate or precipitate which he has occasion to set aside, even for a fewmoments. A bit of paper large enough to carry a descriptive symbol or abbreviation should be attached to the vessel which contains the liquid or precipitate. Paper gummed on the back or the small labels which are sold already gummed are convenient for this use.

This habit, once acquired, will enable the student to carry on simultaneously without error or confusion, several operations. He may be ‘throwing down one precipitate, washing another, filtering a third. and dissolving a fourth at the same time, and the four processes may belong to as many different stages of the analysis. There will be no danger of error if labels are faithfully used ; and a great deal of time will be sayed. The unaided memory is incapab‘e of doing such work with that full certainty, admitting of no suspicion or after-qualms of doubt, which is alone satisfying, or indeed admissible, in scientific research.

§8 CLASSES II AND III. 9 is thrown away. The washed precipitate consists of a mix ture of sulphide of copper (CuS) and tersulphide of arsenic (As.8;). The fact that these sulphides are precipitated under the conditions of this experiment proves that they are both insoluble in weak acid liquors. They are also both insoluble in water. But an important difference between the two sulphides nevertheless exists, a difference which affords a trustworthy means of separating one from the other.

When the water has drained away from the precipitate, open the filter upon a plate of glass, and gently scrape the precipitate off the paper with a spatula of wood or horn. Place the precipitate in a small porcelain dish (App., § 73), pour over it enough of a solution of sulphydrate of sodium (App., § 23) to somewhat more than cover it, and heat the mixture cautiously to boiling, stirring it all the time with a glass rod. The quantity of sulphydrate of sodium to be employed varies, of course, with the bulk of the precipitate ; in this case two or three teaspoonfuls will probably suffice. It is very undesirable to use an unnecessarily large quantity of the reagent for reasons that will hereafter appear. A portion of the original precipitate remains undissolved; but a portion has passed into solution. Filter the hot liquid again. The black residue on the filter is sulphide of copper, which is insoluble, not only in water and weak acids, but also in alkaline liquids. To the filtrate, collected in a test-tube, add gradually chlorliydric acid, until the liquid has an acid reaction on litmus paper (App., § 58). A yellow precipitate of sulphide of arsenic will appear as soon as the alkaline solvent which kept. it in solution is destroyed. The sulphide of arsenic differs from the sulphide of copper in that it is soluble in alkaline liquids.

Tn this series of experiments copper and arsenic stand, not as isolated elements, but as representatives of classes. The following common elements have sulphides which are insoluble in water, weak acids and alkaline liquids : — Lead, mercury, bismuth, cadmium and copper. ‘These elements constitute é Class II in our system of analysis. The following elements _

10 MERCURY AND LEAD. §§ 8,9 have sulphides which are insoluble in water and weak acids, but soluble in alkaline liquids : — Arsenic, antimony, tin (and the precious metals gold and platinum). These elements constitute Class III. If all the elements of both groups had been present in the original solution, one class might have been separated from the other by the same process employed in the case of the representative elements, arsenic and copper.

The question: may naturally suggest itself, how it happens that lead and mercury are included in Class II, when they were both precipitated in Class I. The chloride of lead, which is thrown down by chlorhydric acid, is not wholly insoluble in water ; hence it happens that the lead is not completely precipitated in Class I. That portion of the lead which has escaped precipitation as chloride in Class I, will be thrown down as sulphide in Class LI, for the sulphide of lead is insoluble in water, weak acids and alkalies. In regard to mercury, it will be remembered that there are two sorts of mercury salts, mercurous salts and mercuric salts. The mer- _ eurous chloride, Hg,Cl:, (calomel) is insoluble in water; but ~ the mercuric chloride, HgCl,, (corrosive sublimate) is soluble in water. If, therefore, mercury be present in the form of some mercurous salt, it will be separated as mercurous chloride in Class I. If, on the contrary, it be present in the form of some mercuric salt, it will be separated in Class II as mercuric sulphide (HgS), for this sulphide is insoluble in water, weak acids and alkaline liquids. If a mixture of mercurous and mercuric salts be contained in the original solution, mercury will appear both in Class I and in Class II.

The treatment of Class II is fully discussed in Chapter ITI. The separation of Class III and the means of separating the members of the class, each from the others, form the subject of Chapter IV.

9. Definition of Class IV.— We now return to the study of the filtrate from Classes II and III. Pour the liquid into a small evaporating dish, and boil it gently for five or six minutes to expel the sulphuretted hydrogen with which the

§ 9 DEFINITION OF CLASS IV. 11 fluid is still charged. To make sure that all the gas is expelled, hold a bit of white paper moistened with a solution of acetate of lead (App., § 46) over the boiling liquid; when the paper remains white, all the sulphuretted hydrogen is gone. Next, add to the liquid in the dish ten or twelve drops of strong nitric acid (App., § 4), and again gently boil the liquid for three or four minutes, in order that all the iron present may be converted into ferric salts. Then pour the liquid into a test-tube, add to it about one third its bulk of chloride of ammonium (App., § 19), and finally add ammonia-water (App., § 16), little by little, until the mixture, after being well shaken, smells decidedly of ammonia. A brownish-red precipitate of hydrated sesquioxide of iron will separate from the liquid. Pour the contents of the test-tube upon a filter, rinse the tube and the precipitate once with a little water, and preserve the whole filtrate for subsequent operations.

Two other metals, aluminum and chromium, are precipitated, as iron has here been, by ammonia-water under the same conditions and in the same form, viz., as hydrates. These three elements, therefore, constitute the fourth class, whose treatment forms the subject of Chapter V.. The hydrates of these elements are insoluble in water, even in the presence of salts of ammonium, such as the chloride of ammonium which has been expressly added, and the nitrate of ammonium which has been formed during the neutralization of the acid liquid. The student may be curious to know why the presence of ammonium salts is insisted upon before — the elements of this class are thrown down by ammonia-water. The ammonium salts have nothing to do with the precipitation of iron, aluminum and chromium ; but by their presence they prevent the precipitation, as will be hereafter explained, of certain other elements whose hydrates, though but slightly ‘soluble in water, are dissolved by solutions of ammonium salts. The salts of ammonium are therefore added to keep in solution certain other elements which otherwise would encumber Class IV.

12 DEFINITION OF CLASS V. § 10

10. Definition of Class V.— We now proceed to the examination of the filtrate from the precipitate of Class IV. Bring this liquid to boiling in a test-tube, and add sulphydrate of ammonium (App., § 17), little by little, to the boiling liquid as long as a precipitate continues to be formed. To make sure that the precipitation is complete, shake the hot contents of the test-tube strongly, and then allow the mixture to settle until the upper portion of the liquid becomes clear. Into this clear portion let fall a drop of sulphydrate of am- . monium ; when this drop produces no additional precipitate, the precipitation is complete. Filter off the whitish precipitate of sulphide of zinc, and preserve the filtrate for further treatment. It sometimes happens that this precipitate refuses to settle and leave the upper portion of the liquid sufficiently clear to test in the manner described above; in this case a small portion of the mixture may be filtered and a drop of sulphydrate of ammonium added to the clear filtrate in order to determine whether the precipitation is complete. If not, the filtered liquid must be returned to the flask and more of the reagent added.