Analysis of Residues Obtained on Treatment of Ancient Base Silver Alloys with Nitric Acid EARLE R. CALEY and CHARLES D. OVIATT' Department o f Chemistry, The O h i o State University, Columbus 10, O h i o
The usual treatment of samples of ancient base silver alloys with nitric acid often leaves residues composed of gold and impure metastannic acid. Silver chloride mag also be present in the residues if corroded alloys are being analyzed. Since procedures for the analysis of such complex residues were not included in the usual schemes for the complete analysis of base silver alloys, two such procedures were devised. These were tested on an alloy of known composition and on synthetic mixtures. The test analyses showed that the individual components of the complex residues could be determined with an absolute error not exceeding 0.2 mg. The addition of these procedures to the usual schemes for analyzing base silver alloys now makes possible the complete accurate analysis of ancient base silver alloys.
A-
\ ANCIEYT base silver alloy usually contains a small
proportion of gold and tin in addition to the silver, copper, and other metals soluble in nitric acid. Consequently the residue obtained on treatment of a sample with nitric acid contains gold as well as impure metastannic acid. If the alloy is corroded, silver chloride is usually present, and this also remains in the insoluble residue. The following procedures give accurate results for the analysis of these complex residues. Procedure I is for a residue from a n uncorroded alloy, and Procedure I1 for one from a corroded alloy that is known or suspected to contain silver chloride. These procedures presuppose that a sample of about 1 gram has already been treated with nitric acid in the usual way, and that the insoluble residue has been collected on paper and washed repeatedly with hot dilute nitric acid. PROCEDURE I
Method. Place the filter paper and residue in a weighed porcelain crucible and dry for 1 hour at 110" C. Burn off the paper a t the lowest possible temperature, and then ignite over a Meker burner or in an electric muffle a t 900" C. to constant weight. Thoroughly mix the residue in the crucible with about 15 times its weight of powdered ammonium iodide and place the crucible with its contents in a n electric muffle previously adjusted to 475" C. After 15 minutes remove and cool. Moisten the residue that now remains a-ith 2 drops of concentrated nitric acid, and evaporate to dryness. Ignite over a Meker burner or in an electric muffle a t 900" C. to constant weight. From the loss in weight due to the volatilization of the stannic oxide, calculate the percentage of tin. Treat the residue that remains in the crucible with 5 ml. of concentrated hvdrochloric acid and warm over a water bath until all action ceases. Dilute the solution with an equal volume of water, allow the residue to settle, and decant off the clear solution, preferably with.the aid of a filter stick. Wash the residue by decantation with a t least four 5-ml. portions of warm water. Add the decantate and washings to the original filtrate containing the metals soluble in nitric acid. Evaporate the w-ash water remaining in the crucible and ignite to constant weight. The weight of this final residue should be that of the gold in the sample. To confirm the result for gold, which may be slightly high because of the presence of other insoluble and nonvolatile matter, such as silica, first treat the final residue with 10 ml. of cold 10% aqua regia, filter the solution through paper, and wash w i 6 warm water. Add 2 ml. of concentrated sulfuric acid to the filtrate and evaporate until fumes of sulfur trioxide are abundantly evolved. Dilute to 50 ml., add 25 ml. of saturated
* Present address, Department of Chemistry, Tarkio College, TarLio, No.
oxalic acid solution, and warm and stir until the gold is coagulated. Filter through close-grained paper and wash with warm water. Ignite in a porcelain crucible to obtain the weight of the gold.
Discussion. The use of sublimed ammonium iodide is highly recommended because this avoids the necessity of correcting for the nonvolatile matter ordinarily present in the reagent grade salt in variable proportions. Furthermore, the sublimed salt is directly obtained in a desirable finely divided state. A suitable apparatus for the sublimation of ammonium iodide consists of a horizontal 100-em. length of 35-mm. borosilicate glass tubing fitted a t one end with a stopper containing an inlet tube for the introduction of tank nitrogen. I n use a convenient quantity of reagent grade salt is placed in the tube near the stoppered end, the air is swept out of the tube by a stream of nitrogen, and the tube is heated with a gas burner placed under the salt until i t has sublimed to the cool part of the tube. The stream of nitrogen should be so adjusted that no salt sublimes between the inlet tube and the heated salt and no salt is swept out the open end. For greater convenience in removing the sublimate, the tube may be provided with a few joints, though the use of a long glass scraper is almost equally convenient. Tests on samples of the salt so prepared showed that it does not contain weighable amounts of nonvolatile matter. When the sublimed salt is used, the additional step in the procedure that provides for the confirmation of the weight of the gold may usually be omitted. Experiments showed that gold, even when finely divided, ib not attacked when heated with ammonium iodide a t 475" C. I n these experiments, 30- to 40-mg. quantities of gold were dissolved in aqua regia in weighed crucibles, the solutions were evaporated to dryness, and the residues were ignited a t 900" C. to produce the finely divided gold. After weighing, about 1 gram of ammonium iodide was mixed with the gold in each of the crucibles and volatilized a t 478" C. The crucibles and their contents were again ignited a t 900' C. and weighed. No changes in weights of gold beyond the normal weighing errors could be detected. The leaching with hydrochloric acid serves to dissolve thr cupric oxide and the ferric oxide usually present as impuritiei in the stannic oxide. The use of cold dilute aqua regia instead of the usual hot concentrated mixture is more convenient from the standpoint of manipulation and shortens the time of evaporation. I t is prepared by diluting freshly made aqua regia with 9 times its volume of water. Experiments showed that the cold dilute solvent rapidly and completely dissolves metallic gold when it is in a finely divided state, as it is in these residues. The recommended method of reducing the gold to the metallic state usually produces a precipitate that may be filtered without the aid of macerated paper, though this may be used as an additional precaution. When less accurate results are sufficient, the whole procedure may be abbreviated and made much more rapid by omitting the treatment with ammonium iodide, leaching the original weighed residue with cold dilute aqua regia, and determining the weight of the residue remaining after this treatment. For convenience the original residue is treated in the crucible with 10 ml. of cold 10% aqua regia and washed by decantation using a filter stick. The weight of the residue that remains is then determined by evaporating the residual wash water and heating t o constant weight. The weight of the tin is calculated from the weight of
1602
1603
V O L U M E 2 7 , NO. 10, O C T O B E R 1 9 5 5 this second residue, which should consist of impure stannic oxide, and the weight of the gold is the difference between the weights of the two residues. Experiments showed that cold aqua regia of this concentration does not dissolve stannic oxide. When the tin content of the sample is very small-Le., not more than about 2 mg.-this shortened procedure yields accurate results. PROCEDURE I1
Method. Place the filter paper and residue in a weighed porcelain crucible and dry for 1 hour a t 110" C. Burn off the paper a t the lowest possible temperature and complete the removal of the carbon by placing the crucible in an electric muffle adjusted to 500" C. After the crucible has cooled, add 2 drops of concentrated nitric acid and evaporate to dryness. Add 10 ml. of a nearly saturated solution of ammonium iodide to the crucible and allow it to remain in contact with the residue for about 15 minutes with occasional agitation. Filter the solution through paper, transferring it with small portions of the concentrated ammonium iodide solution, preferably added from a dropper or small pipet, and catching the filtrate in a 250-ml. beaker. R a s h the residue on the paper with a t least six additional small portions of the concentrated ammonium iodide solution, and finally with sufficient water to remove all the ammonium iodide from the crucible and paper. Treat the paper and residue by Procedure I, using the original crucible for the drying and ignition. Add 10 ml. of concentrated sulfuric acid to the above filtrate and evaporate on a hot plate until fumes of sulfur trioxide have been evolved for an hour. Cool the beaker and contents, add 50 ml. of water, warm the solution. filter through paper, and wash thoroughly with warm water. Add dilute hydrochloric acid to the filtrate until precipitation is complete. Warm and stir the solution until the precipitate has coagulated. Collect the silver chloride in a weighed filter crucible, and wash, dry, and weigh in the usual way. -4dd a solubility correction of 0.5 mg. to the weight of the dried silver chloride to give the weight of silver chloride originally present in the alloy. le
15 -
-
S O ~ J O I I I o! f~ S i l v e : C n ' o r , d e in C m m o n t m
-
Iodlde SolJtions C c i c e n l r a t i o c s Expressed o s G r o - s o f S c l d t e per G r o n o f h ' o ! e r
14-
-
0 ii-
3
-
500" C. must be done with free access to air, and even under these conditions some silver may be formed. The purpose of adding nitric acid after ignition is to convert any such silver to eilver nitrate so that it can react with the ammonium iodide solution added in the next step. It is not practicable to collect the residue in a filter crucible and thus avoid the possibility of reduction by carbon, since the finely divided metastannic acid clogs the pores of the filter. That silver chloride is readily dissolved by ammonium iodide solutions of sufficiently high concentration is indicated by Figure 1, R-hich summarizes the results of a series of solubility determinations. For these determinations solutions were made up from accurately weighed quantities of ammonium iodide, silver chloride, and water, brought to 25" C. in a constant temperature bath, and slowly titrated with water to the first appearance of an opalescence as indicated by the Tyndall eff ect from a transverse light beam. The total weight of water for a saturated solution containing any of the various combinations of &,eights of ammonium iodide and silver chloride n a s the sum of the weight of water originally taken plus that added by titration. Closely reproducible results were obtained. An ammonium iodide solution nearly saturated a t room temperatures contains 1.7 grams of salt for each 1.0 ml. of water. Cold concentrated ammonium iodide solutions do not react with stannic oxide or partly dehydrated metastannic acid, at least during the short time of contact recommended in this procedure. I t is important not to wash v ith nater until all the silver solution is in the filtrate, for dilution of this solution may cause precipitation of silver iodide on or in the filter. Although the silver may be quantitatively precipitated by dilution of the filtrate with a very large volume of water, it is not feasible to end the determination by this means because the large volume and the very finely divided precipitate of silver iodide thus obtained make filtration very difficult. Furthermore, this precipitate will also contain a small proportion of silver chloride. The treatment with sulfuric acid oxidizes most of the iodide to free iodine, and the subsequent evaporation removes this iodine and any unconverted hydriodic acid. Filtration after the treatment with sulfuric acid is necessary to remove the sulfur formed from the reduction of the sulfate. -4s many as 3 hours of warming and occasional stirring may be needed to coagulate properly the silver chloride, which usually precipitates in a finely divided form. The reason for the solubility correction is that some silver chloride is dissolved by the nitric acid solution and washings that
Table I. Component Au Sn
Anal)-sis of Alloy Composition by Analysis. 70 Detn. Detn. I I1
Calcd. Composition, 70
0.50 4.78 46.91 47.81 100.00
.4 g
cu .411
0.53 4.79 46.70 47.85 99.87
0.51 4.74 46.75 47.93 99.93
1
0
OCB
'
1
1
"
l
Ci4
VI5 5
.
~
I
032
I
OW
1
1
1
048
1
0%
1
1
064
,
Table 11.
C
L~ g C5 l p e r G r o n of Nater
Figure 1
Discussion. Ignition before treatment with the ammonium iodide solution is necessary to dehydrate the metastannic acid to a considerable degree, so that it is not peptized by the salt solution during filtration. The temperature of ignition is restricted to 500" C., about the lowest a t which the carbon of the filter paper may be burned off, because the silver chloride is not only readily reduced to silver by contact with carbon a t higher temperatures but reacts with and fuses into the glaze of the crucible so that it cannot conveniently be recovered. The ignition a t
Component -4gC1
4nall sis of S>nthetic lMixtures Taken, 3Ig.
Found,
I
9 6 10 7
9 8 10 8
I
I1
21.9 15.9
22.0 16.1
io.1 +O.?
Sn
I I1
40 4 49.2
40.3 49.4
-0.1 +0.2
cu
I I1
71.3 97.9
71.2 97.7
-0.1 -0.2
I1
I
2.7 3.1
2.9 3.3
+0.2
I I1
145.9 176.8
146,2
-4 u
Fe .411
Mixture NO.
I1
RZg.
177.3
Error. llg. 1 0 2
+o
1
+o.a +0.3 +0.5
1604
ANALYTICAL CHEMISTRY
must be used when a sample of an alloy containing silver chloride is first dissolved for analysis. I n experiments in which pure silver chloride and synthetic mixtures containing silver chloride were treated with nitric acid under conditions simulating those that would be used for dissolving such an alloy, the loss of silver chloride was found to range from a minimum of 0.3 mg. t o a maximum of 0 8 mg., with an average of 0 5 mg., the recommended correction. TEST 4YALYSES
I n order to test Procedure I on an alloy of known composition, one &as prepared from 24-karat gold, fine silver of 99.99% purity, electrolytic copper, and reagent grade tin. These ingredients were weighed out on an analytical balance and fused in a graphite crucible under conditions that caused no loss of metal. Metallic lithium was used to deoxidize the melt which w:i* thoroughly stirred before being poured into a mold. Aftei
removal of the rough surface metal, the ingot was reduced to chips in a milling machine, and these were thoroughly mixed to obtain the analytical samples. The calculated composition of the alloy is shown in Table I along with the results of duplicate analysis in which the gold and tin were separated and determined by Procedure I and the other components by conventional procedures. It will be seen t.hat the results for gold and tin are very satisfactory and those for t'he ot'her two metals acceptable. Procedure I1 was test,ed on intimate mixtures containing silver chloride t'hat were prepared from accurately weighed quantities of the several components. I t will be seen from Table I1 t,hat satisfactory results were obtained from the silver chloride as well as for the other components. RECEIVEDfor review J a n u a r y 19, 1955. Accepted June 10, 1955. h b stracted from a dissertation submitted b y Charles D. Oviatt t o T h e Graduate School of T h e Ohio State University in partial fulfillment of thP reriuirementa for t h e Ph.D. degree, 1954.
Determination of Mixed Phthalic Acid Isomers in Alkyd Resins M. H. SWANN, M. L. ADAMS,
and
D. J. WElL
Paint and Chemical Laboratory, Aberdeen Proving Ground,
The meta and para isomers of phthalic acid ha\e recently attained commercial significance in alkyd resin manufacture. An anal?tical method for measuring each of the three phthalic acid isomers in mixture consists of a special saponification technique to recover the acids from resin solution, followed by hydrolysis in methanol solution and measurement of the absorptitity at three ultraviolet wave lengths. Anal) tical control can be exercised on the new compositions.
F
OR a number of years o-phthalic anhydride has been used
in the manufacture of alk?-d resins and these resins in paint. vehicles have been measured by determination of phthalic anhydride content. Recently, the nieta and para isomers, also known as isophthalic and terephthalic acid, have attained commercial significance and may be utilized in alkyd resin manufacture. Investigations were undertaken to determine the effect of these tbr-0 isomers on existing methods of analysis for o-phthalic acid and t,o devise methods of analysis for each isomer. Tivo quantitative methods for determining o-phthalic acid specifically in the presence of other dicarboxylic acids are found in the literature. I n these methods, the phthalic acid is measured by its absorption in the ultraviolet region a t 2 i 6 mp (2) or by the weight of the lead salt formed in glacial acetic acid ( 3 ) . These two methods with slight modification also appear as ASTII methods (1). The lat,ter is not affected by the presence of isoaud terephthalic acids and can he used to measure o-phthalic acid or anhydride in alkyd resins wit,hout int,erference. A l l phthalic acid isomers absorb strongly in the ultraviolet region a t 2 i 6 mp, so that the spectrophotonietric method in its present form is unsuit,able. Horn-ever, it was found that each isomer shorvs a distinctive absorbance curve throughout the ultraviolet range; the iso- and terephthalic acids show strong absorption at the shorter wave lengths and secondary absorption a t the longer wive lengths. Figure1 s h o w the absorptivities of 1to 1methanolwater solutions, made 0.1-Y in hydrochloric acid, of t,he three phthalic acids plotted against the wave length in millimicrons. Sirwe the isomers show different point,s of maximum absorption a t the longer wave lengths, it is possible to treat mixtures of the acid< as three-component R ems. The wave length? chosen for the analysis of the isoiiie phthalic acids, 275, 281, and 287
Md.
mp, are points at which the spread between the curves is large but not so large as to give absorbance readings which are either very high or low. The use of long wave lengths is considered advisable in order to minimize the interference that would he caused by the presence of any water-soluble organic contaminants that might be present in the saponification product of alkyd resins. PROCEDURES
Calibration. The absorbances of the three phthalic acids muit be determined at 275, 281, and 2 8 i mp. The isophthalic and terephthalic acids used were obtained from Eastman Kodnk Co., catalog numbers 3233 and 610, respectively. The Becknian spectrophotometer, Model DU, was used with 1-cm. rell*. Bepause of the low solubility in water of isophthalic and terephthalic acids, a 1 to 1 methanol-water mixture wap used as the solvent throughout. T o calibrate, 25 mg. of each acid are didsolved in 250 ml. of absolute methanol by reflusing. To the methanol solution are added 5 ml. of concentrated hydrochloric acid, and the solution is diluted to 500 ml. with distilled mater. thus giving a final concentration of 50 mg. of acid per liter of solution. The absorbance of each solution is determined a t the three wave lengths using a slit width of 0.6 mm. and a 1 to 1 methanolwater mixture, made 0 . l N in hydrochloric acid, as a blank, following the method of reversing the cells as proposed by Shreve and Heether (2) in the original spectrophotometric method for determining o-phthalic acid. The absorptivity of each acid a t each wave length is calculated, using the equation -4 bc
a = -
where u is the absorptivity a t the particular wave length measured; A is the average absorbance of the acid solution being measured a t the same wave length; b is the cell length in pentimeters: and c is the concentration expressed in grams of acid per liter.
Analysis of Acid Mixtures. The analytical data in Table I were obtained by applying the folloa ing procedure to knoxn mixtures of the isomeric phthalic acids. Slixtures totaling a maximum of 50 mg. of acid are refluxed with 50 ml. of absolute ethyl alcohol. When dissolved, 10 ml. of dry benzene are added, followed by 3 ml. of 21-\ alcoholic potassium hydroxide. After refluxing for 1 hour, 150 ml. of dry benzene are added, the flask is stoppered, and the contents are cooled with water and then filtered fhrough a Gooch crucible, benzene being used for tramferring and washing the precipitate. The residue is given a find ether wash and dried a t 105' C.
