ASALYTICAL EDITIOS
276
titration is generally about 2 per cent higher than that calculated from the alkali titration, and it is probable that the experimental errors of the method are within this limit. While the above degree of precision is ample for the purpose intended, we believe that the method is capable of further refinement and consequently greater usefulness. We are now engaged in experiments with this end in view.
Yol. 2 . No. 3
(3) Bougault, Compt. r e n d . , 164, 1008 (1917). (4) B u n d a n d Xathews, J . A m . Chem. Soc., 31, 464 (1909). ( 5 ) Cajori, J . B i d . Chem , 64, 617 (1922). (6) Colin and Lievin, J . SOL. Chem. I n d . , 38, 381A (1919); Bull. assocn. chim. sucr. d i s l . , 36, 107 (1919); Bull. SOC. chim., 141 23, 403 (1918). (7) deBruyn and van Ekenstein, Rec. trav. chim., 16, 92 (1896). (8) Goebel, J . B d . Chem. J . , 72, SO1 (1927). (9) Hinton and hfacara, .AnaL>sL, 49, 2 (1924). (10, Judd, aiochem. J . , 14, 255 (1920). (11) Sichols, I N D .ERG.CHEM.,20, 553 (1928). (12) Romijn, Z . a n a l . Chem., 36, 349 (1897). (13) Voorhies a n d Xlvarado, I N D . E K G . CHEX.,19, 848 (1927). (14) Wllstaetter and Schudel, Ber., 51, 780 (1918).
Literature Cited (1) Baker a n d Hulton, Bzochem J , 14, 754 (1920) ( 2 ) Bland and Lloyd, J Soc Chem I n d , 33, 949 (1914).
Colorimetric Determination of Silica in the Presence of Phosphates and Iron’ Lewis A. ThayerZ JACQUES
D
LOEBLABORATORY, HOPKINS hf ARIKE
U R I S G the course of a n investigation of the rate of intake of silica by marine diatoms, in which the culture medium contained, besides silicates, considerable q u a n t i t i e s of phosphates and iron, it was found that the Di6nert and Wandenbulcke m e t h o d of d e t e r m i n i n g silica is not satisfactory under these conditions.
STATION, STANFORD USIYERSITY, P A C I F I C GROVE,C A L I F .
tion markedly increased the silica values obtained upon determination. The method of Diknert and Wandenbulcke for the determination of silica, as given by Atkins (3, 4 , 5 ) . is as follows: To 100 cc. of the solution, the silica content of which is to be determined, are added 2 cc. of 10 per cent ammonium molybdate and 4 drops of 50 per cent [by volume) sulfuric acid to bring the p H to approximately 2.0. (In this paper 17.5 drops equal 1 cc.) The mixture is shaken and compared after 10 minutes in Hehner cylinders or a colorimeter with a picric acid standard, 25.6 mg. of picric acid per liter being equivalent to 50 mg. of Si02 per liter, according to the findings of King and Lucas (10). Solutions stronger than 30 mg. of SiO2 per liter must be diluted before determining, because the amount of molybdate contained in 2 cc. of 10 per cent solution is only sufficient, according to King (9),for 3.5 mg. of silica. This does not meail that it is impossible to obtain a deeper color intensity with 2 cc. of ammonium molybdate solution than that equivalent to 30 mg. of SiOz per liter, but that above 30 mg. of silica per liter the color developed is no longer proportional to the concentration of silica. This is shown in Table I and Figure 1. Curve A shows the color intensity developed in the presence of 2 cc. of ammonium molybdate solution b y yarious amounts of silica. Although Di6nert and Wandenbulcke ( 7 ) in their original paper mention the influence of phosphates, Atkins (5) states that phosphates and arsenates do not interfere, since a solution of K2HPOI containing 0.1916 gram per liter analyzed as silicate gave a color equivalent to 0.31 mg. of SiOn per liter; and again (4)that a solution of KHzPOI,equivalent to 100 mg. of P205per liter, gave a color equivalent to 0.3 mg. of SiOz per liter, and that 0.5 mg. of PzOs per liter were equiralent to 0.08 mg. of SiOz per liter ( 3 ) . Similar experiments by the writer, on more than one occasion, hare seemed to coiifirrn this result, qualitatively at least. Also King ( 9 ) states that the influence of phosphate is very small, phosphate, reckoned
The colorimetric method of Didnert and Wandenbulcke for the determination of silica is shown to be inaccurate (a) in the presence of phosphates, owing to the formation of yellow ammonium phosphomolybdate, or of a colorless phospho compound with a stronger affinity for molybdate than silica possesses; ( b ) in the presence of iron, owing to the formation of an iron silicomolybdate, which is more intensely colored than the ammonium salt; and (c) especially in the presence of both phosphates and iron, owing to the formation of an iron phosphomolybdate, which is even more intensely colored. Details of a method to remove the interfering substances, without loss of silica, are given. Iron is precipitated in acetic acid solution as ferric phosphate and removed by filtration; excess phosphate is then precipitated as calcium triphosphate, and filtered off. Experimental results for solutions of silica in distilled water and sea water are given.
Sole-Although this method is attributed t o Dienert a n d Wandenbulcke in t h e modern literature, it has been called to t h e writer’s attention, since t h e manuscript was prepared, t h a t t h e method was used by Schreiner [ J . A m . Chem. Soc., 26, 1056 11903) :, 26.. 808 (1904) . . . .1 a n d bv Lincoln and Barker [Ibid., 26, 975 (1904)l for determining both phosphates a n d silicates in soil waters. Schreiner found t h a t if t h e ammonium molybd a t e was added t o a solution containing silica a n hour before t h e solution was acidified, t h e resulting color was only half a s intense a s when t h e solution was acidified immediately after t h e addition of molybdate. On the other hand, t h e time of acidifying had no effect on t h e intensity of color produced by phosphates. When t h e acid was added immediately, t h e color d u e t o silica was found t o be twice a s great a s t h a t due t o t h e same weight of P206. B y using two samples of a solution containing both phosphates and silicates a n d acidifying one immediately a n d t h e other after one hour, t w o values were obtained, t h e silica content of t h e solution being equal t o twice t h e difference between them. Phosphates could then be determined b y difference.
The solutions were turbid and, on determining the silica, gave values which were many times too high. Filtration improved the results to a considerable degree, though they were inconsistent and still too high. Thus it’ was considered that the precipitate, consisting chiefly of ferric phosphate, was the primary cause of the difficulty. This conclusion was further supported by the observation that the addition of the nutrient solution, containing phosphates and iron, to a silica solu1 Received April 10, 1930. This paper contains results obtained in a n investigation on “Diatoms a s a Source of Oil” listed a s Project No. 5 of American Petroleum Institute Research. Financial assistance in this work has been received from a research fund of t h e American Petroleum Institute donated by t h e Universal Oil Products Company. This fund is being administered by t h e Institute with t h e cooperation of t h e Central Petroleum Committee of t h e I\-ational Research Council. 2 American Petroleum Institute Research Fellow.
INDUSTRIAL A N D E.VGISEERING CHEMISTRY
July 15, 1930
as P205,giving less than one-tenth the color given by an equal weight of silica. of P h o s p h a t e s on D e t e r m i n a t i o n of Silica
T a b l e I-Influence P 2 0 6
ADDED
I
COLORINTESSITY EQUIYALEXTTO LITER
I Solution
Aa
..
0 1
5 10 20 25 30 40 50 60 70 80 90 100 110 120 130 230
..
Solution B
Solution C
. .
4.2 5.0 6.1 7 7 10.1
, . . , . .
10'6
19 9
MILLIGRAMS
3.06 5 85 , . .
29 5 36 3 45 0 50 0 50.3 54.5 49 5 51 3 52 1 50 0 50.5 50 0
8 5 12 2 14 3 15 4 14.8 12.2 6.2 1 05 0 94 0.83 0.78 0.56
13:9
Si02
PER
Solution D
0.1, 0 2 0.75, 0.78 1.90, 1 . 9 6
. . . . . .
9 ' 3 5 , 9'70
. . . . . .
15.6
18.2 20.5 19.6 16.7 11 0 4.6 1.32 1.08 0.95
...
17:2, 1 7 ' . i
. . . . . .
. . . . . . . . . . . .
i '92, 1: 96 ,.. ,.. . . . . . .
0.'72. 0 : 7 6
Si02 added instead of P2Oi Solution A-Distilled water plus sodium silicate; 2 cc. of 10 per cent ammonium molybdate a n d 4 drops acid. Solution B-Distilled water plus disodium phosphate; 2 cc. of 10 per cent ammonium molybdate and 4 drops acid. Solution C-Sodium silicate solution containing 4.2 mg. of Si02 per liter plus disodium phosphate. Solution D-Same as Solution B, b u t with 5 drops of acid used in determining.
Yet, as above stated, when phosphates and iron were both present, the method was certainly not accurate. Therefore, a inore detailed investigation was carried out' as to the possible interference of phosphates and iron, separately as well as combined, in silica determinations. The influence of iron is mentioned b y Di6nert and Wandenbulcke (?), who state that the solution should not contain inore than 20 mg. of Fe20a per liter. To avoid any possible interference of iron in studying the influence of phosphates alone, La hIotte's buffer disodium phosphate was used, and t.he ainmonium molybdate solution was niade very alkaline with S a O H and filtered to be certain that it contained no iron. All chemicals used other than the disodium phosphate were Baker's c. P. Analyzed. Since all determinations were made in comparison with blanks in which only the substance in question was omitted, and since picric acid was used as a standard, further tests as to purity of chemicals were not deemed necessary.
277
colorimetric determination of silica, this behavior is easily explained. The use of ammoniuni molybdate as a colorimetric reagent is based on the formation of colored complex salts of molybdic acid. If the ammonium molybdate is added in excess, the amount of colored substance formed is proportional to the concentration of the substance to be determined. Suinerous investigators have described a vast array of complex phospho- and tungsto-molybdic acids with Carious P 2 0 5 : ~ ~ o(or o 3 K O a ) ratios. Arnfeld ( I ) , however, on the basis of his preparation and analysis of their salts, found that there are but two groups of definite compouncls; and concluded that all compounds with €',Os: MOOSratios varying froin these are not true compounds, but mixtures. The type formulas of Arnfeld's salts are as follows: Group l-3R20. P20j.5MoOI or R20.2P205. 10Mo03. These salts are colorless and are formed in alkaline solution; the corresponding acids cannot be isolated. Group 2-3R20, P 2 0 j 185100~ , and 3R20. PnOj, 24hfoO3. These salts are colored and formed in acid solutions; the corresponding acids can be isolated.
dtkins' very natural mistake in assuming that if a large amount of phosphate did not interfere, the effect of a smaller amount certainly would be negligible (for his purposes the assumption has: very probably, not led to a significant error) can be explained on the basis of the proportions of P?05to 11003. Two cubic centimeters of a 10 per cent solution of ainmoniuni molybdate contain about 180 mg. of Mooa, which is, theoretically, sufficient to t'ransform i.4 mg. of P?05to the complex 3R20.PzOs.2411003, the colored compound. When more P205is present', the colorless compound, 3R20.P20a.51100a, will be formed. This agrees fairly well with the
Influence of Phosphates
The results obtained by analyzing a series of silica solutions with varying phosphate content are given under Solution C of Table I. These figures show that the amount of silica originally present was found only in those solutions which contained (1) no phosphates and ( 2 ) 100 mg. of P205per liter. The other solutions. however: gave discrepancies from the actual silica concentration which, up to 60 mg. of PzOj per liter, appear to be a linear function of the phosphate concentration. Higher phosphat'e concentrations cause a rapid fall in the values found, even to values below the actual amount of silica (Figure 1, curve C). It is evident, then, that Atkins' statement concerning the negligible effect of phosphates on silica determinations according to D i h e r t and TTandenbulcke's method is due to the coincidence that Atkins used phosphate concentrations which, on the one hand, were too low (0.5 mg. of P20jper liter) and, on the other hand, too high (100 mg. of P205per liter) to have any appreciable influence. At first it may seein strange that higher phosphate concentrations have less effect than lower concentrations. However, if TTe consider the chemical reaction which is the basis of the
F i g u r e 1-Influence of P h o s p h a t e s o n D e t e r m i n a t i o n of Silica A-Sodium silicate solutions B-Disodium phosphate solutions C-Sodium silicate equivalent to 4 2 mz, Si02 per liter with varying amounts of disodium phosphate
maxima of the curves B and C of Figure 1, which lie between 60 and 70 mg. of P20jper liter. The correctness of this view is further attested by the higher value obtained when the amount of amnionium molybdate is increased, though the increase in the determined value is no longer strictly proportional to the phosphate concentration (Table 11). This is due partly to the precipitation of ammonium phosphomolybdate a t these high phosphate concentrations. -inother factor, however, is the concentration of acid in the solution, higher concentrations leading to a marked deepening of the color. As a n example, the following figures are given, which were obtained by determining the color produced by a phosphate
AAITALYTICALEDITION
278
solution containing 250 mg. of PzOi per liter. With 10 cc. of molybdate and 6 drops of 50 per cent HrSOl (pH 1.5)the color was equivalent to that produced by 33 to 35 mg. of SiOz per liter. With 10 drops of 50 per cent HSO, (pH 1.0) the apparent value n-as 61 to 66 ing. of Si02 per liter. Although in this case the value does not quite reach the value calculated on the assumption of a linear relationship (equivalent to 80 mg. of &On per liter). it indicates that this relationship exists, especially since here, too, precipitation was apparent immediately after the readings were made. T a b l e 11-Influence of A m o u n t of M o l y b d a t e on Color of Phosphomolybdate Complex ( T h e intensity of t h e color is expressed as equivalent t o mg. of Si02 per liter. T h e values recorded represent t h e spread of t h e three readings taken on each determination, and thus give some indication of the accuracy of t h e readings.) AMMONIUMMOLYBDATE
P20: ~
M e-. fler . liter 100 250
II
2 cc.
6 cc.
1 92-1 96 0 72-0 76
31 6-32 7 22 4-22 9
10 cc.
27 8-28 5 Precipitate formed
From these experiments i t follows that phosphates alone do interfere with the silica determination. Influence of Iron
With the object of estimating the possible iiifluence of iron alone on the silica determinations, two series of experiments were carried out. I n the first, various amounts of ferrous and ferric iron were dissolved in distilled water; in the second, the same amounts were dissolved in a silicate solution containing 12.2 mg. of SiOnper liter. Results of the first series showed that neither ferrous nor ferric iron alone, in concentrations u p to 100 mg. of iron per liter, gave a significant color with the silica reagents (Table 111). There was this difference between the two ions, however. When the ammonium molybdate reagent was added to solutions of ferrous iron, a brown color was produced, due to the reduction of the molybdenum, which disappeared coinpletely when the sulfuric acid was added. When silica also was present, however, the brown color faded to blue with the addition of the acid, so that the results could not be determined directly. By the addition of one drop of hydrogen dioxide the blue complex was oxidized to a determinable yellow compound. Since this is the same as the color produced with ferric iron, only the results of solutions in which ferric iron was used are given in Table 111.
F e PRESZXT (1) Distilled Hz0 plus ferrous or no ferric ammonium sulfate; silica Fe+* Fe+++
(2) Silicate containing 12.2 mg. Si02 per liter plus ferric ammonium sulfate (Fe++-)
1
T'ol. 2,
so. 3
interference of other substances cannot be e?tablishetl ewept in the presence of the &stance to be determined. Combined Influence of Phosphates and Iron
The separate influences of iron and phosphates having been established by the foregoing experiments, there is yet to be determined the combined influence of the two. K i t h this in view, two series of experiments were planned-the one with a fixed quantity of phosphates (250 mg. of P205per liter) and varying amounts of ferric iron, the other with a fixed quantity of ferric iron (50 mg. per liter) and varying amounts of phosphates. But 50 mg. of iron per liter were used to avoid the formation of an iron molybdate precipitate a t higher concentrations. The solutions did not contain silica. The results of these two series are shown in Table IV. Again it is very evident that the combined influence of phosphates and iron is much greater than the sum of their separate effects, which suggests that' the formation of a definite compound is responsib!e for the increase. Arnfeld (1) has already described this compound, Fez03.P205.24Mo03, which he prepared by the action of X o 0 3 on ferric phosphate, and which has a yellow color. Similar series with ferrous iron were not made hecause, as in the case of the silicat'es. bhe phosphomolybdate complex is reduced by the ferrous ion, this time with the formation of a green compound which, like the reduced iron-silica complex, is easily oxidized. On t,he basis of the results of Truog and Meyer (13)this is to be explained by the formation of a ferric phosphoniolybdate brought about by the ferric ion always present in the ferrous solutions, and the subsequent reduction of this compound by the unoxidized iron. I n studying the Den& method for det'erniining phosphates by the blue color produced by the reduction of phosphomolybdate in t'he presence of stannous chloride, Truog arid 1Ieyer found that in the presence of ferric iron the color produced mas green instead of blue. The foregoing experiments shorn clearly that a close parallelism exist's between t.he reaction of silicates and phosphates with respect to the reagents used in the Dienert and Wandenbulcke method for determining silica. It is well known that both phosphates and silicates form complexes with molybdates in which the proportion of phosphates or silicates to MoOs is 1:12 ( 2 , 12). The writer's experiments also show that the ferric salts of both of these complex acids are more intensely colored than the ammonium salts. It became necessary, in working out the details of the rnodification of the method, t o examine this pwallelisn sornewhat more closely. We may first note that the behsvior of the two substances t'omards a fixed quantity of ammonium inolyh-
T a b l e 111-Influence of I r o n on D e t e r m i n a t i o n of Silica (Apparent values expressed as equivalent t o milligrams of Si02 per liter.) 0
1XG.PER
5MG.PER
LITER
LITER
by which he claims to be able to distinguish between silicates and phosphates in the presence of each other. This. method is based on Issacs' observation that in the presence of acetic acid silico-molybdates are reduced by sodium sulfite, whereas under these conditions phosphoniolybdates are not. Bertrand (6) has criticized this method and cites experimental evidence in support of his statement that Isaacs' method does not give accurate results for the determination of silica in the presence of phosphates. -kccording to Bertrand, the reduction of phosphoinolybdates by sodium sulfite in a solution containing acetic acid is not completely inhibited, but takes place a t a slower rate, so that the results obtained will represent the total amount of reduced silicoinolybdate plus part of the reduced phosphomolybdate-the quantity of the latter depending upon the time elapsed after the addition of the sulfite, the temperature of the solution, etc. I n repeating Bertrand's experiment except that disodium phosphate was used instead of phosphorus pentoxide a n d
AN.4 L Y TICAL EDITION
280
metallic sodium, it was found that, under the conditions specified by Isaacs, the phosphomolybdate was not reduced by the sodium sulfite to give a blue color. The only change that could be observed was the disappearance of the yellow color after some time. A blue discoloration, however, could not be observed even after 15 hours; the solution remained perfectly colorless. T a b l e VI-Amounts Initial p H
Vol. 2, s o . 3
Using 0.5 cc. of 10 per cent amnionium molybdate in 100 cc. of water he found that a sulfuric acid concentration of 0.3 S was sufficient to prevent reduction of molybdate alone. Restated in ternis of the Denigi.s reagents, his findings were that a t sulfuric acid concentrations of 0.3 -V or greater, when the volume ratio 50 per cent (by volume) H2S04to 10 per cent amrnoniuin molybdate is greater than 2.5:1, animonium
of Silica P h o s p h a t e s a n d Iron R e m a i n i n g i n S o l u t i o n 16 H o u r s a f t e r 100 mg. per Liter of E a c h S u b s t a n c e Were Added t; E a c h S a m p i e , a n d t h e pH A d j u s t e d ; Precipitated S u b s t a n c e s R e m o v e d b y F i l t r a t i o n 4
6
a
7
8
8
DISTILLED WATER
pH after filtering Silica, mg. per liter PzOj, mg. per liter Iron, mg. per liter State of iron Solution
145 Ferric Clear
2 8-3 6 64 5-66.5 23.8-24,2 *40 Ferric Clear
p H after filtering Silica, mg. per liter PzO;,mg. per liter Iron, mg. per liter State of iron Solution
4.2 32.8-34.0 32.2-32.8 *l5 Ferric Clear
4.9 42.5-43.0 22.8-23.6 115 Ferric Clear
2.6 63.5-66.5 26.2-27.4
5.2 58.8-60.7 9.3-9.4 115 Largely ferrous Clear
8.2 59 7-62.6
6 8 54 8-55 32.@-32. 125 Ferrous Colloidal
Ferrous Colloidal
6.7 45 0-45. 0.4 *20 Ferrous Clear
6.8 40.4-40.9 0.4 *I5 Ferrous Clear
37.2 -50
9.8
45.4-46.5 44.2-45. 3 *50 Ferrous Colloidal
SEA WATER
6 6 4 5 . 4-47.6 1.54-1.56 120 Ferrous Clear
It is impossible to determine the reason for this disagreement without further experiments. These were not carried out because the only aim of this investigation was to x o r k out a method which would allow of an accurate determination of silica in the presence of phosphate and iron. Special experiments would also be needed to settle the question as to the possible influence of iron on the results obtained by Isaacs’ method, since Isaacs does not consider this possibility a t all. I n the following paragraphs it will be shown that the D i h e r t Kandenbulcke method can give satisfactory results. -is for the possibilities of Isaacs’ method, it remains an open question. Inasmuch as this method requires standard solutions of silica, which are difficult to prepare and to keep unchanged, no further attempts were made in this direction. ?\.Inreover,the shade of the reduced substance is very sensitive to the presence of other substances. I n an unknown solution the worker, therefore, would be troubled constantly with “whimsical” variations in shade rrhich would vitiate his determinations. [cf. Wu ( I 4 ) and, especially, King ( q ) . ] Severtheless, because Isaacs’ method is similar to the DenigPs method for phosphate determinations, which the present writer had frequent occasion to use. and because of the known interference of phosphates with silica determinations, supgesting that the reverse might also be true, some preliminary experiments on the DenigBs phosphate method were also made. Isaacs’ method differs essentially from the Denigbs method for phosphate determination (5, 6 ) only in the use of acetic acid instead of sulfuric acid, and of sodium sulfite instead of stannous chloride. I n htkins’ procedure for the Denigbs determination two reagents are used, conqisting of ( A ) 1 part by volume 10 per cent ammonium molybdate solution and 3 parts b y volume 50 por cent sulfuric acid; and ( B ) stannous chloride solution, freshly prepared by dissolving 0.1 gram tin in 2 cc. concentrated hydrochloric acid \pith 1 drop of 3 per cent copper sulfate and diluting to 10 cc. For 100 cc. of solution containing 0.005-0.1 mg. P20s,use 2 cc. -4, mix, add one drop of solution B , and compare with a standard similarly treated. Truog and Meyer (13), who have recently published the results of more detailed experiments on the Denigks reaction, found that a concentration of acid equivalent to 0.35 W would prevent the reduction of molybdate alone. They used a molybdate concentration of 2 cc. of 5 per cent ammonium molybdate in 100 cc. of water. The writer’s experiments agreed well with this result.
8.6 16 4-16 8
3.6-3.8 0
Clear
molybdate alone is not reduced b y stannous chloride. This relationship was tested for molybdate concentrations up to 2 cc. of 10 per cent solution in 100 cc. of water. In the case of silica solut’ions the same relative concentrat’ion of acid to molybdate (2.5:l) was found sufficient to prevent the reduction of siliconiolyhdates xhen only 0.5 cc. of 10 per cent ammonium molybdate was used. When the quantity of molybdate solution was increased to 2 cc., however, 15 cc. of 50 per cent H2SOd (7.5 t’imes as inuchacidas molybdate) was not sufficient to prevent the reduction of the silicomolybdate. The non-interference of silica with the DenigBs phosphate reaction is thus shown to be strictly dependent upon the concentration of molybdate and the concentration of acid relative to molybdate. But xhen the reagents are used as recornniended b y I t k i n s , silicates in concentration up t o a t least 250 mg. of SiO? per liter do not interfere with the determination of phosphates. K i t h respect to the silica determinations, the main conclusion to be drawn from the foregoing experiments is that no one of the three methods considered above-Di6nert and Wandenbulcke’s, Isaacs’, and ?;(tmec‘s-can safely be used in the presence of iron and phosphates. I n experimenting upon methods for the removal of phosphates and iron, it v a s found that in solutions near neutrality ferric silicate as well as ferric phosphate is insoluble. Thus was raised the question in regard to the extent to which the three substances, iron, phosphates, and silicates, might be simultaneously present in the same solution. T o settle this point, two series of solut’ions,one with distilled water and the other with natural sea water, were prepared. To these were added sodium silicate solution, ferrous ammonium sulfate, and disodium phosphate in quantities equivalent to 100 mg. of SiO?, iron, or P20jper liter. By means of 0.2 N HC1 or S a O H the members of each series were then approximately adjusted to various pH’s ranging from 4.0 to 9.0. They were then put aside and allowed to stand overnight (16 hours). After filtering, the pH of the filtrate was determined colorimetrically. The iron concentration was determined colorimetrically with thiocyanate after oxidizing with hydrogen peroxide, and the phosphates lvere determined b y the Denig&smethod. After the phosphates and iron had been removed, by a method which mill be given below, the silica was also determined colorimetrically. The results, given in Table VI, show that, within the range of hydrogen-ion concentrations ordinarily found in natural waters, all three substances may be expected.
I S D U S T R I A L S X D EiYGINEERING CHEMISTRY
July 15, 1930
Although the iron was not present in all cases in true solution, it had passed through two filters. It is therefore evident that its influence cannot be eliminated by filtering. In the case of natural sea water, of course, quantities of phosphates and iron sufficient to interfere with the silica determination will be encountered only in exceptional areas subject to pollution or large dilution by fresh waters.
tate with dilute ?JH40H somewhat increased the amounts of Si02 found. T o eliininat'e possible error from this source, 1 drop of concentrated NHIOH or S a O H was substituted for the sodium acetate, but results were no better. Moreover, blank determinations showed that very considerable amounts of silica might be derived from the glassivare (Pyrex) if alkaline solutions were heated to boiling. The use of ferric chloride and annnoniuni hydroxide to remove phosphates is suggested by Atkins and TTilson ( 5 ) ,but they do not cite data showing that they tried the method.
Elimination of Interference of Phosphates and Iron
The influence of the phosphate and iron compounds could be eliminated if it were possible to remove these substances without altering the silica content. A second possibility is the use of another coloriinetric method, the reactions of which are not subject to the interference of phosphates and iron. I n this case it was considered practicable to use the first method, and the following experiments will show how the inethod of Dienert and Randenbulcke may be modified so as to give accurate results in samples which contain appreciable amounts of phosphates and iron.
Precipitation of Phosphates as Calcium Phosphate
Boiling with calcium carbonate was suggested early in the course of the experiments as a means of precipitating both phosphates and iron. But here again considerable amounts of silica are dissolved from the glass. Two cubic centimeters of a 10 per cent solution of calcium chloride were then tried as a reagent, 1 drop of concentrated ",OH being added to prevent re-solution of the precipitate in the acid formed. Heating is unnecessary. This method gave good results, as Table TI11 shows, with solut'ions containing 0 to 100 mg. of Si02 per liter with the addition of 0 to 250 mg. of PzOj per liter. Since stronger solutions will have to be diluted before determining, t'liey can as well be diluted. before treatment as after. Likewise in solutions containing only phosphates and iron the inet'hod gave satisfactory results, but when a silicate solution containing also phosphates and iron was used, silica results were lorn (Table IX). Since this precipitation was carried out a t a p H of about 8.0, the loss of silica could not be in the form of silicic acid. Investigation showed it to be due to the low solubility of iron silicate. The experiments thus far carried out, with a few inore simple tests, showed that both phosphates and silicates of iron are insoluble near the neutral point. Therefore, any method for reinoving phosphates without loss of silica in the presence of iron mu3t be carried out a t some other pH. On the alkaline side i t was found that, although ferric phosphate is rather readily hydrolyzed by an excess of hydroxide, ferric silicate is hydrolyzed only in solutions so strongly alkaline that they could not be filtered through paper. Furthermore, such a procedure would involve the use of large ainounts of strong alkali, difficult to keep free of silica, and equally large amounts of acid to neutralize i t later. Plainly the solution of the problem did not lie in that direction. On the acid side of iieutralit'y it was found that, although bot'h silicates and phosphates are soluble in mineral acids, ferric silicate only is soluble in acetic acid. This made it possible to remove the iron first-with an excess of phosphate in a solution containing acetic acid-so that it could not cause precipitation of silicic acid during the removal of the phosphate, which subsequently was carried out with a n excess of calcium chloride in a n-eakly alkaline solution. Xft'er the writer had developed this method, his attention x a s called to the work of King (9), who determined silica in
Precipitation of Phosphates as Ferric Phosphate
=Ilthough it has already been stated that iron interferes with the determination of silica this fact was not discovered until late in the euperiinents. It was therefore assumed that if the phosphates were ;eniored conipletely such iron as remained would cause no error. Table VII-Effect of P r e c i p i t a t i o n of P h o s p h a t e S o l u t i o n C o n t a i n i n g 10 m g . of SiOr per Liter
J l g . p e r liter 0" 0
J l g . fie? liter 9 9 to 10.1 5 0 to 5 . 4 5 4to.5 5 5 . 3 to 5 . 6 6 . 1 to g . 4 6.9 t o , 1 4 . 9 to 5 3 3 4to3 5
L .> j
30 100 2 50
Untreated. all others treated
The method used in the first experiments mas to add an escess of iron as ferric chloride (I cc. of a solution containing 25 mg. of iron per cc.) t'ogether with an excess of sodium acetate (5 cc. 16 per cent solution) to solutions containing P,Oj to the amount of 250 mg. per liter or less. By this procedure the excess iron is precipitated as acetate. The solution was heated just to boiling, to promote flocculation of the precipitate, filtered, and determined. Results mere satisfactory in distilled-mater solutions of phosphates, but when a solution containing 10 mg. of SiO, per liter was taken, the results shown in Table S'II were obtained. When tested after filtering, the pHof the solution was found to be about 5.3. It was thought possible, therefore, that part of the silica was lost as silicic acid. Washing the precipiTable VIII-Removal
28 1
of Interference of P h o s p h a t e s Alone w i t h C a l c i u m Chloride
( v a l u e s in milligrams of SiO? per liter
Values for treated samples have not been corrected for reagent blank error) Si02 SOLUTION
~
Untreated Reagent blank Treated: N g . PzOj added per liter.
?
,
I
5
25 50 100 250
1
BLANK 0.1 0.15
0 28-0 30 0 33-0 34 0 35 0 38-0 40 0 39-0 40 0 50 0 64-0 66
1
4
7
?
1.2-1 2 5 0.30
10.8-11.0
56-57
1,59-1,64 1.56-1.61 1.67-1.69 1.61-1,64 1.78-1.82 1.70-1.78 2.17-2,27
11,1-11,4 11.2-11.5 11.5-11.9 11.1-11.6 11.1-11.6 11.6-12 0 12 ( t 1 2 2
55.5-56.2
0.30
0.25
53.6-55.5 54.3-55.0 55.0-L6.2
54.3-55.5 55.0-56.2 .55 5-66
2
6
119-122 0.25
143-147 0.50
294-308 0.73
112-116 113-115 110-111 111-112 107-109 11&113 111-116
135-139 131-133 126-128 122-125 114-117 105- 107 116-1 17
112-1 18 94.0-94.0 78.0-82.0 107-1 1 1 122-128 125-128 126- 128
B S A LY TI CAL EDI TIO-V
282 T a b l e IX-Removal
I
1 Untreated Reagent blank Treated: Me. iron added Der liter: 0 1 5 25 50 75 100 250 a
of I n t e r f e r e n c e of P h o s p h a t e s a n d I r o n w i t h C a l c i u m C h l o r i d e (Values in milligrams of Si02 per liter)
I
h-0 SILIC.4
Fe-+
FeT-+ 0.1
Fe--and P2O;a
Fe+-'and P2O.a
1.85(?)
1.96-2.04 0.23-0.27
0.6410,65 0.58-0.61 0.91-1.00 0.56-0.58
0.62-0.68 0 46-0.47 0.98-1.02 0.65-0.67
0.57-0.59 0.3610.31 0.58-0 59 Brown Ppt. 0.55 0.67-0.69
1.0b-1.06 0.34-0 39
...
0.89-0.94(?)0.23-0.26 0.43-0.45 0.59-0.62 0.33-0.35 0.4&0.41 0.29-0.30 0.17-0.19 0.18-0.20 0.15
Yol. 2, s o . 3
0.3
10 XG. Si02 PER LITER
Fe-+
F e - - and Fer-' a n d P.O,a PZOiO
Fe-++
10.6-10.7
10 2-10.3
10.6-10.7
...
...
..,
. .
10.2-10.3.
...
...
...
. . ... S o t detd. becauseevidentlylow 9.5-10.0
7.2-'7.4 ,..
6 7- 7.1 4.C4.7
5.2- 5 , 4
io. &io.
3
250 m g . P20, per liter
animal tissues b y the D i h e r t and Wandenbulcke niethod after removing the phosphates with magnesia mixture. When no iron is present King's method should give quite satisfactory results, for there is little choice between the use of magnesium and calcium chloride. King states that the iron is precipitated along with the phosphate; he does not cite data to show that there is no loss of silica froin his solutions a t the same time. The experiments cited above, however, make i t seem highly probable that part of the silica is precipitated as ferric silicate during the precipitation of the phosphate in King's procedure. The method finally developed, then, requires the use of the following reagents: (1) Acetic acid: 10 cc. glacial acetic acid diluted to 100 cc. Approximately 2 N . ( 2 ) Sodium phosphate solution: 5 mg. P2O5 per cc. (3) Calcium chloride solution: 10 grams (anhydrous) calcium chloride in 90 cc. of water; filtered. (4) Ammonium hydroxide: 35 cc. concentrated solution diluted to 200 cc. Approximately 2.6 AV.
analyzed without treatment. Three readings were made f o r each det'erniination, the mean of these three readings to be taken as the value. The variation in all eighteen readings \vas from 1.00 to 1.06 mg. of Si02 per liter. The variation in the six means was from 1.015 to 1.05 nig. of S O , per liter. The niean:value derived froin either set wai 1.03; this was also. the average for all of the readings. G a i n e d by Use of Modified M e t h o d f o r Silica Determination (Values in milligrams of Si02 per liter. Values of treated samples have been corrected for reagent blank error ) T a b l e X-Results
I
Distilled water: Reagent blank Untreated Treated: STg. Fe added per liter:
10
PROCEDURE-TO 100 cc. of the silica solution, containing not more than 10 mg. of iron (100 mg. per liter), add 2 cc. acetic acid and 3 cc. of the sodium phosphate solution. If more iron is present than the quantity named, correspondingly more phosphate should be used. Heat just to boiling to coagulate the precipitate, filter, and cool to room temperature. If the solution is made alkaline before cooling silica may be dissolved from the glass and the results will be erratic. If the silica solution which is being determined does not contain sufficient calcium or magnesium to effect the precipitation of all the excess phosphate-as. for instance, in the case of natural fresh waters-2 cc. of the calcium chloride solution should be added to it after cooling. S o calciurn chloride need be added to sea water, as the combined calcium and magnesium present in sea water amount' to 0.133 4, Idlereas the concentration of P206produced by adding 3 cc. of the sodiuni phosphate reagent to 100 cc. of water is only 0.006 S. The solution is then made alkaline by the addition of 2 cc. of ammonium hydroxide solution. shaken and allowed t'o stand 10 to 20 minut,es or longer before filtering off the calcium phosphate precipitate. To the filtrate add 2 cc. of the 10 per cent aninioniuiii molybdate reagent and enough acid (8 drops) to bring the p H to about 1.5, shake and after 10 minutes compare with the appropriate picric acid standard in Hehner cylinders. A reagent blank must be carried through a t the same time, for the correction made with the aid of it is by no means negligible. Results gained by the use of this method are given in Table X. To get some idea of the accuracy of the method, six solutions containing about 1.0 mg. of SiOz per liter were made up and
50 100 -ariation Sea water: Reagent blank Untreated Treated: Slg. F e added per liter: n 1; 50 100 Variation
1 1
I
i
i
SILIC.A ADDED 50 mg. per 110 mg. per liter dil. liter dil. 5 X before 1 0 X before determining determining
1 mg. per liter
10 mg. per liter
1.73-1.76 1.00-1.03
1 35-1 37 9 . 9 -10.1
1 40-1 43 46 8-47 3
1.6 110-112
1.19-1.24 0 98-1.01 0 98 0.95-0.98 28.2% 0 52-0.53 1.43-1 47 1.50-1 53
10 0-10.3 9 4- 9 . 7 9.5- 9 S 9.8-10 0 9.1%
45.5-46 46.9-48 47 0-48 46-0-47
112-114 110-112 111-113 111-115 4.47,
1 43-1 4 3 9 . 9 -10.0
1.68 50 5-51 0
9.7-9 8 9.4-9 5 9.4-9.6 9.2 8.4%
51.8-32.8 51 8-32.9 52 9-53 4
1.40-1.41
1 58 1.77-1 86 1.47-1 50 28 2%
5 0 0 0
a.5-'
z 0 . 0-50.5 6.65
Likewise, a set of six reagent blanks was treated according to the procedure given above. The total variation in the eighteen readings was from 1.63 t o 1.95 mg. ( = 0.39) of S O ? per liter. The variation in the six niean5 was froin 1.635 to 1.925 (= 0.27) nig. of SiO, per liter. On the basis of all six samples the mean value is 1.79, the average 1 . i 2 nig. of SiO? per liter. The accuracy of the method is limited by the accuracy of making readings, which ordinarily vary about 5 to 7 per cent in unt,reated samples for all concentrations of silica that can be determined. Weaker solutions can be read more accurately than strong ones, and for this reason it is ad\-isable to dilute stronger solutions to about 10 nig. of S O z per liter before determining. I n the case of treated samples, hon-ever, variations in the reagent blank introduce a further error, this time a constant amount of about 0.3 mg. of SiO? per liter. Therefore, the error will be large in proportion to the total silica content in solutions of low silica concentration, and will become smaller and finally disappear within the reading error as the concentration of silica increases. I n solutions concentrated enough to be diluted after removing phosphates and iron. before the silica,
ISDCSTRI.4 L AND LC’SGISEERISG CHEJIISTRY
July 15, 1930
can be deteriiiined the reagent blank error is negligible, as an inspectioii of Table ,Y will show. Literature Cited (1) Arnfeld, Dissertation, Berlin, pp. 1-60 (18991. (2) Asch, lv. and D . , “Silicates in Chemistry and Commerce,” translated by Searle, p. 16, London, 1913. (3) Atkins, J . .lIariiie B i d I s s o c n . , Cniied Kingdom, 14, 89 (1926). (4) Atkins, IhiLl , 15, 191 (1928).
(5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
283
Atkins and Wilson, Biockem. J., 20, 1223 (1926). Bertrand. Bull. soc. chim. b i d , 6, 656 (1934). Dienert and Wandenbulcke, Compt. rend., 176, 1 4 7 8 (1923). Isaacs, B d l . soc. chim. bioi., 6, 157 (1924). King, J. B i d . Chem., 80, 25 (1928). King and Lucas, J . .Am. Chem. Soc., 50, 2395 (1928). S i m e c , Biochem. Z . , 190, 42 (1927). Travers and Nalaprade, Compt. r e n d . , 183, 292 (1926). Truog and I f e y e r , 1x0. ESG. C H E x , Anal. E d . , 1, 136 (1929‘1 Wu, J . B i d . Chem., 43 189 (1920).
A Source of Error in Polariscopic Measurements’ H. K. Miller a n d J a m e s C. Andrews DEP.4RTMEST
OF l”YSIOLOCIC.4L
CHEMISTRY, S C H O O L
w
OF
hfEDIClKE, UNIVERSITY O F PEVSSYLY.4SI.4, PHILADELPHIA, PA.
H I L E it is a matter of coniinon knowledge that accurate polariscopic readings require t h e use of clear solutions, there is little information available as to the extent and nature of the errors involved when polariscopic examinations are made of solutions which are very faintly turbid but not sufficiently so to prevent readings. The turbidity in solutions of amino acids is often of such nature that filtration is quite ineffective. Under these circuiiistances, if readings in short tubes are a t all possible, one is inclined to resort to this procedure rather than to other methods of clarification. The equivocal results of such a procedure are evident from the data shown below. While making polariscopic measurements on arginine and other ainino acids with comparatively lon. rotations, the writers observed rather large discrepancies for TThich there appeared to be no adequate explanation. This was first called to their attention by the values for specific rotation of d-cysteine published by 1-ickery and Leavenworth ( Z ) , who obtained figures considerably larger than those previously published by A n d r e w ( 1 ) . Repetition of these prepsrations under identical conditions still resulted in values which, for the present authors, duplicated the previously published figures of $9.0 to +10.0, while those obtained by Kckery and Leavenworth usually ranged from about 12.0 to 18.0. The polariscopes used. when checked with pure sucrose solutio’ns, gave practically identical results. A check solution of 1 per cent glutamic acid also gave identical results. However, the writers noted that in several cases the amino acid solutions shoned a slight turbidity which was very difficult to remove by filtration, and this sometimes obscured the field to the point of necessitating the use of a 1-dm. tube rather than the 4-dm. tube usually used by them. The observations of Vickery and Leavenv-orth were made with 2-dni. tubes. A series of measurements was then made of the optical activity of several amino acids and also of pure sucrose in 1-din. and 4-din. tubes. Table I shows the results obtained. It will be noted that the differences range from practically zero to a very considerable figure and that the greatest differences occur consistently with the more turbid solutions. I n the solutions of arginine (not too acid) and glutamic acid enough putrefaction can result from standing overnight in the laboratory to give a definite turbidity and, while this may not be so great as to prevent the use of the longer tube, one is far more prone to resort to shorter tubes in such cases. The cysteine samples were all prepared by reduction of 1cystine with tin and hydrochloric acid. The usual procedure
+
1
Rereived .4pril 26, 1930.
+
is to precipitate the excess tin as stannous sulfide and this is invariably followed by slow precipitation of free sulfur. When the solution with the stannous sulfide is filtered directly into the polariscope tube (the procedure used by T’ickery and Leavenworth and also by the authors in Experiments 8, 9, and lo), one is always on the verge of too much turbidity due to the sulfur, even with rapid manipulation. I n Experiment 7 the solution was filtered into the tubes without contact with hydrogen sulfide or removal of the tin; in Experiment 11 the solution was carefully protected from air for a day after the hydrogen sulfide treatment and then filtered into the tubes. Presumably the formation of free sulfur had ceased. T a b l e I-Specific
EXPP. 1 2 3 f a
6
k9
10 11
R o t a t i o n of V a r i o u s S o l u t i o n s i n T u b e s of Different Lengths
SOLS.
COSCS. Grams per
Sucrose Sucrose Glutamic acid hrginine Areinine
100 c c . 1 0470 1 0795 0 7213 8 264 1 030
Cystine Cysteine Cysteine Cysteine Cysteine Cysteine
1 000 1.250 1,000 0 995 0 825 0.902
[el?
[al?
LDM. TUBE
4-n~.
-C66.90 f67.00 f13.9 +26.7 -18.0 -213.0 f 8.8 f12.0 fl5.Z
+15.s f10.2
TUBE
REM.ARKS
f67.10 f67.00
Clear Clear Very faintly turbid Clear Lower aciditv than in Expt. 4 : faintly turbid Apparently clear Apparently clear Turbid Turbid Turbid Apparently clear
+11.4 +26.5 +14.5 -210.0 8.2 8.2 9.6 9.4 9.8
++ + ++
I t appears reasonable to conclude that in faintly turbid solutions of weakly rotatory substances identical specific rotations may not be obtained with tubes of different lengths. This is probably due to the effect of the turbidity in changing the character of the spot of light, and no doubt the possibilities of such a change are modified by the peculiarities of the polariscope and of the eye of the observer. If the error is caused by the interposition of so inuch turbidity as to change the appearance of the field, one would expect to obtain more correct values from shorter tubes, with, however, the disadvantage of a correspondingly greater percentage error in the final values obtained. Making duplicate determinations in tubes of two different lengths would appear to be a practicable means of insuring the absence of the turbidity factor. Literature Cited (1) Andrews, J. B i d . Chem., 69, 209 (1926). (2) Vickery and Leavenworth, I b i d . , 86, 129 (1930).
