7s. T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y Vol. 11, No. 8

igiioring it. Although the author did not obtain the same figure with the smallest quantity of dextrose as he did with the larger ones, it is interesting t o note that the average of the results with 1.5 mg. of dextrose in Tables I and I1 gives the value taken as the correct constant per cubic centimeter of thiosulfate.

Various modifications can be made in this method, such as adding the acetic acid, then the hydrochloric acid, and finally running in a known excess of iodine and titrating back with thiosulfate. Under carefully controlled conditions good duplicate results can be obtained in this way, but the dextrose-thiosulfate value is not constant.

While the method described may be modified, as outlined by Clark, and used for much larger quantities of sugar than those employed in the present work, still very accurate results may be obtained by diluting the sugar solution so tha t i t will come somewhere near the limit employed in this investigation.

The same procedure may be used for reducing sugars other than dextrose.

S U M M A R Y

A method is described for the determination of, reduced copper by iodimetry in a modified Benedict’s solution.

The method is simpler than any previously reported. The ratio of reducing sugar t o thiosulfate is constant. The results of duplicate determinations differ by

The method may be used for any reducing sugars. less than 0.1 mg. of dextrose.

B U R E A U OF PLANT INDUSTRY DEPARTMFNT OF AGRICULTURE

WASHINGTON, D. C .

THE ANALYSIS OF ALLOYS OF TIN By ARCHIBALD CRAIQ

Received August 15, 1918

Though the gravimetric method for t in has been largely superceded by volumetric methods, the analyst occasionally finds the older method of advantage, as, for instance, when the sample is. very small. A dis- cussion of the conditions of greatest accuracy of the analysis based on the nitric acid separation will there- fore be of interest t o some.

When an alloy of t in is treated with nitric acid, the precipitation is never complete, no matter what the precautions, and the precipitate is always impure. The elements which may be found with the tin in ordinary alloys are antimony, arsenic, phosphorus, copper, iron and magnesium, If sulfur is present in the alloy the treatment with strong nitric acid will cause the formation of lead sulfate, which may be avoided. if i t is known in time, by the use of dilute acid. Sulfur will then separate without precipitating lead, and may be burned off without causing error. Lead is not held by tin t o an appreciable extent, even in solders, though i t may be found in the precipitate if not enough acid is used in washing to prevent hy- drolysis. Coarse drillings of bronze are not easily decomposed, and may retain some of the original alloy. This difficulty may be overcome by boiling the strong nitric acid toward the end of the reaction, so

that the lumps are broken up and the acid can reach all of the alloy.

As most varieties of brass and bronze contain the same elements, differing, from the analytical point of view, mainly in the proportions of t in and zinc, the schemes for analysis differ only in the quantities used for the determination of those elements. I n the bronze group as large a sample is taken as the tin determina- tion will permit, and for brass, copper is the limiting element, the solution being finally divided to obtain a suitable quantity of zinc for titration. After the methods of decomposing the different varieties have been disposed of, therefore, the scheme of analysis is the same for all, until the zinc determination is reached.

The ignited tin precipitate should weigh less than 0.5 g. The copper content may be 2 g. or more. Samples often contain copper and lead segregations so marked that it is necessary to sample the sawings or drillings with great care, using the entire fraction de- livered by the sampler instead of an even weight, or to sift the entire sample and use proportionate parts of fine and coarse. Mixed drillings may be sampled down t o from I O g. t o 2 0 g., treated with nitric acid and filtered, the precipitate ignited and weighed, and an aliquot taken to correspond t o the fraction of the solution taken with the pipette.

For ordinary bronze, sample about 3 g. and weigh into a 250 cc. beaker, add cautiously 2 0 cc. nitric acid (1.4 sp. gr.), and after violent action has ceased, boil gently until the alloy is entirely decomposed. This is easily seen by observing the disappearance of froth on removing from the hot plate. The residue should be fine and dense. Dilute t o 150 cc. with hot water, stir well, warm half an hour, and filter.

White bronze, which is high in tin, is run on 0.5 g. using 5 cc. nitric acid (1.4 sp. gr.) and diluting to 7 j to IOO cc. before filtering.

If sulfur is known t o be present, treat 3 g. of the sample with 40 cc. nitric acid (1.2 sp. gr.) on the water bath, crushing the lumps if necessary to assist the decomposition and separation.

Brass may be run on as much as 5 g. if special at- tention is to be paid to small percentages of impuri- ties. It should be treated with slightly dilute acid and not boiled. Generally tin, if present, has a ten- dency t o run through, and evaporation t o dryness before filtering may make a better separation; but as some tin goes through the filter in any case, and is recovered afterward, the writer does not consider that evaporation is ever worth while, except in the case of the nitric acid residues of large samples, which may clog the filter unless previously dried.

Select a funnel with a narrow stem and without internal enlargements, particularly near the top, so that the column will not break from below, and air bubbles accidentally forming will be carried down. Fold a 9 cm. filter a t a little wider angle than tha t of the funnel, but not so wide as to form wrinkles a t the top. Tear off the corner (single) of the paper which comes next t o the glass. Set in this way the filter

Aug.. 1919 T H E J O U R N A L O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 75s

will give the maximum stream, and the column will hardly ever break. Pour on a little pulp made by shaking up ashless paper with water. This will make the stream slower a t the start, but i t will pre- vent clogging by the slimy precipitate, and the fil- tration will be more rapid toward the end than with- out it.

Filter and bring the precipitate onto the paper, using cold or warm water, as hot water has a tendency to harden some of the precipitate on the sides of the beaker before i t can be rubbed out. Wash the paper once with hot water, once with hot j per cent nitric acid, and 4 or 5 times with hot water.

When the filtration is finished, heat the filtrate. uncovered, almost to boiling until the solution equals about 7 cc. for each gram of the sample. Then add I O cc. of sulfuric acid for the larger samples, or 5 cc. for the smaller, mix the solution well, and continue the evaporation on the water bath until copper sulfate separates and then carry, uncovered, a t a low heat on the hot plate, to fumes of sulfur trioxide. After %he acid fumes slightly, cool the beaker and add water equal to 5 cc. for each cubic centimeter of sulfuric acid, dissolving the copper sulfate, and set the beaker away for the lead sulfate t o separate.

Meanwhile ignite the t in precipitate without loss of time. The writer prefers a broad round-bottomed crucible, of the kind variously known as annealing cups or combustion crucibles, of 2 5 cc. capacity. If a muffle, in which the heat can be accurately controlled, is not available, a fair substitute may be made for use with a blast lamp. Knock the bottoms out of two Denver 3 0 g. crucibles, and set horizontally in the middle of one of them a ring of heavy iron wire. Put a metal triangle without legs on the ring, and on i t set a porcelain crucible whose top is about as large as the bottom of the one used for analysis. Set the whole in a ring s tand, put on the other clay crucible for a chimney, and direct a moderate blast, with an oxidizing flame, vertically through the bottom.

I n the meantime, set the weighed crucible contain- ing the moist precipitate in another crucible to avoid cracking, and heat gradually over a Bunsen flame un- til the paper has been burned off. Then transfer t o the hot muffle and blast for 5 min. uncovered. If the residue is not thoroughly calcined before the crucible is covered, some tin and much of the antimony will be volatilized. The Bunsen flame is not hot enough to do this. If the crucible is cooled after the first ignition and the residue moistened with nitric acid, dried, and ignited again, the oxidation will be more certain than by blasting uncovered, though the latter method is safe if carefully done, and much quicker.

After the oxidation, cover the crucible and blast to constant weight. 15 min. heating should do this, and less if the residue weighs less than 0. I g. When the temperature is properly regulated the crucibles will be so softened that they will stick together slightly, and care should be taken in removing. Heat the tongs so as not t o crack the crucible and lift i t with- out removing the flame. Set the crucible on a clay

triangle to cool somewhat before putting i t in the desiccator. Weigh, and blast again, if the method is not familiar, t o make sure t h a t constant weight has been obtained. The blast flame, once adjusted, is not changed, and any number of precipitates can be treated under identical conditions. The precipi- ta te should be burned t o the same color throughout and should not stick to the bottom. When a blast flame is directed on the bottom or side of a crucible in the ordinary way the heat is not uniform, and part will be heated too much and part not enough, while the Bunsen flame will not bring the stannic oxide (SnOs) to theoretical weight. Perhaps some of the newer burners, which the writer has not tried with this method, may be found to give the required tem- perature more automatically than the blast lamp, which is capable of giving too high a temperature.

After weighing, cover the residue in the crucible with about twenty times its weight of sulfur fusion mixture. This mixture is made of 14 parts potassium carbonate, I O parts sodium carbonate, and I O parts sulfur. Cover the crucible with an inverted lid from which the handle has been removed, set i t in another crucible, and apply full Bunsen heat for a few min- utes, until the fusion is quiet. Leave the crucible covered until cool. Cover with water in a small beaker, and leach. If iron colors the solution green, it may be settled by the addition of ammonium hy- dmxide and ammonium sulfate, and i t should then be filtered as soon as the solution shows yellow. Fil- ter on ashless paper and wash well with ammonium hydrogen sulfide solution. Ignite in the original crucible, if i t is not cracked. Brush out the residue and weigh it, return i t to the crucible, cover with a little hydrochloric acid, and warm for a few minutes until the copper and iron are dissolved. A little t in will dissolve, in proportion t o the amount present, but not enough to affect the result. Dilute in the crucible, filter on a very small paper, ignite the resi- due, and weigh as before. The first residue contains some stannic oxide (SnOz) and silica (SiOz). and possi- bly fragments of the crucible, which remain in the second residue. The difference between these is, therefore, the proper correction for oxides insoluble in the fusion.

Add a little sulfuric acid to the hydrochloric acid filtrate from the first residue, evaporate i t to fumes, dilute, and filter it with the lead sulfate separation, thus combining all the lead, copper and iron before the electrolysis. Ordinarily, lead will not be found in the tin precipitate. I ts presence may be due to sulfur in the sample.

The alkaline filtrate from the sulfur fusion contains all the antimony, if i t is less than ' / 6 as much as the tin. Some arsenic will be lost in the ignition, but not all, so tha t the correction for arsenic in the tin residue must be made from the fusion of the residue itself. The total arsenic should be determined on a separate portion.

Add sodium chlorate in slight excess over the sulfur present, and then hydrochloric acid equal to half the volume of the solution, and boil down t o small

752 T H E J O U R N A L OF I N D U S T R I A L

bulk, just enough to keep all in solution. Dilute slightly, make just alkaline with ammonium hydroxide, add oxalic acid and ammonium oxalate each equal t o 5 per cent of the solution, and pass in hydrogen sulfide until the arsenic is precipitated. A good alkali solution to dissolve antimony sulfide (Sb&) is 5 per cent potassium hydroxide into which some hydrogen sulfide has been passed. Sodium hydrogen sulfide is likely to cause the precipitation of antimony as antimonate.

Filter the precipitate of antimony and arsenic, dissolve in the least amount of the alkali solution, without dissolving much sulfur, oxidize the solution in a flask with sodium chlorate and hydrochloric acid, boil out the excess of chlorine, and titrate with thiosulfate. Success in this method depends on at- tention to details. The alkaline solution should be about 7 j cc. and warm. Add z or 3 g. of sodium chlo- rate, just enough to give an excess of chlorine, and then add the acid a little at a time, with gentle shaking, until the free chlorine slowly develops, much of the sulfur is oxidized, and the rest forms whitish flakes which do not occlude antimony. Then add more acid, 125 cc. in all, and boil vigorously. The color of chlorine should be gone in 1 5 min. if the temperature is right. Then boil I O min. more. Titrate as soon as the solution has cooled to 40' C. This titration can be considered accurate only if a standard is run under identical conditions, often enough to confirm the constancy of the operator.

After the titration, add IOO cc. of hydrochloric acid and pass in hydrogen sulfide, which will precipi- tate the arsenic a t once. Make a plug of glass wool in a funnel, pour on asbestos pulp with plenty of water, so that a column will form and stay, wash out the water with a mixture of 3 parts hydrochloric acid and 2 parts water, filter, wash with the acid solution and then with water, dissolve through the filter with ammonium hydroxide, acidulate and reprecipitate, and weigh as arsenious sulfide (As&).

If one of the other volumetric methods for antimony is preferred, the arsenic may be separated as above described after the first oxidation with sodium chlorate and the antimony recovered from the filtrate, or the arsenic may be separated by distillation.

Phosphorus is best determined on a separate por- tion, but if it is necessary to use the main portion, there is a choice of two methods. Ammonium hy- droxide may be added to the oxalate filtrate from the antimony, and the phosphorus precipitated with magnesia mixture As phosphorus is generally either absent or present in considerable quantity, in bronze, this method will do for ordinary work.

If it is necessary to identify a trace in the main portion, the alkaline filtrate from the sulfur fusion may be acidulated, the mixed sulfides of tin, anti- mony and arsenic filtered, dissolved, and examined for arsenic and antimony, and the filtrate boiled free from hydrogen sulfide. About 0. I g. of iron in ferric solution is added, an ammonia precipitation made, the ferric hydroxide dissolved in nitric acid, and the

A N D E N G I N E E R I N G C H E M I S T R Y Vol. 11, N o 8

phosphorus precipitated as molybdate in the ordinary way for phosphorus in steel.

The ignited tin precipitate may now be corrected. The oxides insoluble in the fusion are deducted with- out separating them from each other, as described above. Phosphorus is calculated to Pz05, antimony t o Sbz04, and arsenic to AszOj. A further addition may have t o be made of tin recovered from the elec- trolyte.

In the meantime, the filtrate from the tin precipi- tate has been converted to sulfate, boiled, and settled. Filter the lead sulfate with paper pulp, as under these conditions i t is likely t o be very fine, and run through the filter. Wash with cold water. This is safe when the lead is t o be converted to chromate, as, while the lead sulfate is decomposed to some extent, the lead becomes basic and does not dissolve, while the washings show acid. Wash the lead sulfate back into the beaker, and fill the paper with ammo- nium acetate solution. This is conveniently made by mixing 80 cc. ammonium hydroxide, IOO cc. water, and 7 0 cc. glacial acetic acid. 2 j cc. of this mixture will dissolve 0. 5 g. of lead.

Boil the lead sulfate with the acetate until it dis- solves, filter, wash once with water, once with about 5 cc. of acetic acid mixed with water t o fill the filter, and several times more with water. Dilute the fil- trate with hot water t o a volume between 400 cc. for 0 . 5 g. of lead and 5 0 cc. for a trace. To the warm solution add from z t o I O cc. of cold saturated potas- sium dichromate solution. The precipitant should be in excess over the lead to the extent of about 2 cc. per IOO cc. of solution. Heat strongly on the water bath, stirring two or three times until the precipitate subsides rapidly after stirring, and cool somewhat. Fold close-fibered filters in pairs, let them stand in funnels for a few minutes to recover from the mois- ture of the hands, and cut them to equal weight, cutting the heavier one of each pair through the double corner, and the other at one of the single corners. I n setting the filters, leave the first paper folded and open the other with one uncut thickness next t o and slightly above the first. When set in the funnel, the folded filter will be entirely covered by one thickness of the other, and the two uncut sides will make the un- broken top of the filter. After moistening, the filters should be pulled up and loosened if necessary, so tha t the stream will be rapid without a column.

I n filtering, pour off nearly all of the solution with- out disturbing the precipitate. Then take the beak- ers one a t a time, if thero are several, and transfer the precipitate to the filter, using cold water. This will prevent the formation of a dry pulverized portion which may run through the filter. Fill each filter with hot water from a beaker, and when tha t has run through, wash all visible colored solution out of the paper, being careful t o avoid washing the precipitate over the top. When the filter looks clean from below, fill again with hot water and wash as much as before. Wash the top of the filter with alcohol. Allow the precipitate t o keep its natural form on the filter, with- out stirring it or washing it down. Dry in an oven

Aug., 1919 T H E J O U R N A L O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 7 53

surrounded by boiling water for z hrs. Remove, loosen the papers in the funnels, let stand for half an hour. in the weighing room, and weigh. The em- pirical factor t o lead is 0.6375. If any variation of this method is used, i t should be standardized against pure lead and a factor calculated. The theoretical factor will give results too high when paper filters are used. If preferred, the lead sulfate may be separa- ted by the use of alcohol and weighed as such.

The filtrate from the lead is electrolyzed for copper a t about zoo cc., after adding 5 cc. nitric acid. This can be done best with a coil and cylinder, so that the electrolyte can be saved without dilution. I n remov- ing the electrodes more care should be used t o avoid diluting or spilling the solution than t o avoid dissolv- ing the copper, for the copper can be recovered.

Pass hydrogen sulfide into the electrolyte and filter on an ashless paper, with a little pulp. I n some cases it may be necessary t o add sulfuric acid t o prevent the precipitation of zinc. The precipitate will con- sist of copper.and tin. I n most cases i t is accurate enough t o burn and weigh, and guess a t the propor- tion of each. Or the cupric oxide may be dissolved away from the stannic oxide in the crucible after igni- tion with a little hydrochloric acid, as described above. The weights of t in and copper are added t o the major amounts found.

In some determinations, particularly when the t in is low and is found largely in the electrolyte, it is best to reserve the t in precipitate until this second por- tion is recovered, before ignition. The t in will then be weighed all a t once, and the copper found with i t can be determined separately. The separation is conveniently made with hydrogen sulfide, leaving the iron in condition t o add to the main portion.

Wher the separation has proceeded thus far, the filtrates will contain all the manganese, iron, aluminum, nickel, and zinc. I t is convenient t o oxidize the iron by adding hydrochloric acid while boiling out the hydrogen sulfide, as some nitric acid is present. As the amount of nickel is small, the basic acetate separa- tion can be made almost a t neutrality. Bring the solu- tion just t o the acid side with ammonium hydroxide and hydrochloric acid, and then add 2 0 per cent am- monium hydroxide, using a wash bottle, until a slight cloudiness of iron remains after stirring. Add a gram or so of sodium acetate, boil, settle, filter, and wash well with water containing ammonium sulfate. Dis- solve, and precipitate the iron and aluminum with ammonium hydroxide, mixing some ashless paper pulp with thc precipitate before filtering. This makes the ignited oxide soft and easily fusible. As the alu- minum is generally present only in traces, i t is not safe t o take i t by difference. After weighing the combined oxides they may be fused two or three times with sodium carbonate and the aluminum precipitated by nearly acidulating the filtrate with hydrochloric acid, or the fusion may be made with acid potassium sul- fate, the solution made just acid with sulfuric acid, using ammonium hydroxide t o neutralize i t , and the iron removed electrolytically, using a mercury cathode. The precipitate of iron and aluminum is likely t o con-

tain silica, and expulsion before weighing, using a drop of sulfuric acid with the hydrofluoric acid, is a wise precaution.

Precipitate the manganese in the filtrate from the iron by the use of ammonium persulfate, and determine i t as usual.

The nickel may now be separated from the zinc by dimethyl glyoxime, and filtered and weighed on a Gooch filter. As the solution by this time contains too much interfering acid to titrate the zinc directly, i t should be made just acid with acetic acid and the. zinc precipitated with hydrogen sulfide. Filter, but do not wash. Put paper and all back into the beaker, pour in I O cc. water and 8 cc. hydrochloric acid, warm for a few minutes, dilute, and titrate with ferrocyanide. I n the case of brass it will be necessary t o take a suit- able aliquot before precipitating the zinc. Instead of the glyoxime separation, the zinc may be precipitated first by Treadwell’s salting out method.

This method is laborious and tedious, but i t makes possible a practically complete analysis of one weighed portion, and the results are very accurate, in spite of the multiplicity of corrections. Complete analyses generally foot up within a few hundredths of IOO per cent.

Titration of the iron is still better.

48 TONNELB AVENUE JERSEY CITY, N E W JERSSY

THE ALKALIMETRIC DETERMINATION OF SMALL AMOUNTS OF MAGNESIUM

By P. L. HIBBARD Received January 9, 1919

A method for determining magnesium by titration of ammonium magnesium phosphate was published by Bruckmiller in 1917.‘ The principles of his method had been known for some time, but he improved the technique considerably. While endeavoring t o use the method for determining magnesium in soil extracts, the writer has further changed the procedure so that it is now very convenient and exact for the estimation of quantities of j mg. or less, down t o 0.1 mg.

The principal changes introduced are: 1-Use of the Gooch crucible for filtration whereby it is pos-

sible to wash the precipitate in the most efficient manner, with the least quantity of wash solution.

z-Use of neutralized alcohol followed by water solution of ammonium magnesium phosphate for washing.

3-Use methyl red instead of methyl orange as indicator, giving a much sharper end-point.

Nearly all of the experimental work was in connec- tion with the second point, finding a suitable wash liquid. Among those tried were water, alcohol, ether, in various concentrations and mixtures, I per cent water solution of ammonium sodium hydrogen phosphate, and a water solution of the salt which was to be purified, ammonium magnesium phosphate. The last named and pure alcohol were the only ones permitting good results, so it is not necessary t o give the results with others. Ordinary alcohol is not neutral, but when properly neutralized i t has very little solvent effect on ammonium magnesium phos- phate. However, if left in contact with the salt for some time ammonia is taken up by the alcohol so tha t

1 “Titration of Magnesium,” J A m Chem. Soc., 39 (1917), 610