V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
277
when low results are obtained the gravimetric and coulometric results are both low and the errors thus tend to cancel. Figure 3 represents such a combination. Because of this rather general occurrence of compensating errors the accuracy of the method is in practice somen hat better than might he predicted from an estimation of the probable error in each part of the procedure. Typical results for synthetic samples of sodium chloride and potassium bromide are shown in Table I. The average absolute error of the chloride result is 0.3 mg. and of the bromide result, 0.5 mg. The curves in Figure 4 indicate the manner in n hich the per cent relative error resulting from absolute errors of this magnitude changes as the weight ratio is varied, assuming a constant 4.5 meq. of halogen present. Figure 4 may be used to predict the average relative erior which may be expected a t various weight ratios. For example, from these curves it may be seen that chlorine to bromine iatios from 1 to 6 to 2 to 1 may be analyzed with an average relative c’rror of
no more than 0.7% for either halogen. With more extreme ratios the halogen present in greater amount may be determined with high accuracy, but the determination of the small amount of the other halogen is, of course. subject to large relative error. ACKNOWLEDGMENT
This research was supported in part from funds granted by The Ohio State University Research Foundation to the university for aid in fundamental research. LITERATURE CITED
Lingane, J. J., IND.ENG.CHEM.,AFAL. ED.,17, 332 (1945). Lingane, J. J., J . Am. Chem. Soc., 67, 1916 (1945). Lingane, J. J., and Small, L. A., ANAL.CHEX.,21, 1119 (1949). RECEIVEDfor review September 1.5, 1952. Accepted November 7, 1952. Presented before the Division of Analytical Chemistry a t the 122nd Meeting of the . ~ V E R I C A K C H E s r I C a L SOCIETY, Atlantic City, N. J.
Analysis of Sulfuric Acid and Acid Sludges from Petroleum Processes F. T. WEISS, J. L. JUNGNICKEL, AND E. D. PETERS Shell Development Co., Emeryuille, Calif., AND
F. W. HEATH Shell Chemical Corp., Pittsburg, Calif. The sulfuric acid consumed in a number of refinery processes can be more economically utilized when analytical methods are available for the determination of the components of the acid and sludge streams. .4nalytical methods have been evaluated for determination of the principal components of the “spent acid” streams from treatment of lubricating oils with sulfuric acid and from preparation of gasoline alkylate and alkyl benzene. The determinations which can be made include: total acidity, titratable acidity, ester (by difference), free sulfuric acid, sulfur dioxide, water, neutral oil and sulfonic acid content, and equivalent weight of the sulfonic acids.
T
HE present shortage of sulfur and sulfuric acid makes it
imperative for petroleum refiners t o consider means for the careful husbanding of sulfuric acid supply in refinery operations. One of the principal refinery uses of sulfuric acid is in the treatment of a number of straight-run and cracked products for improvement of stability and color and for increasing the viscosity index of lubricating oils. Considerable amounts of sulfuric arid are consumed by some refiners in the preparation of petroleum sulfonates, and in the production of alkylates in the manufacture of the alkylaryl sulfonate detergents, although in some cases the alkylation of aromatics is carried out with another catalyst. The large-scale processes for the manufacture of gasoline alkylates and polymers and for the synthesis of alkylated aromatics require the consumption of large tonnages of sulfuric acid. Important to the economical operation of these processes are analytical methods for the determination of the major and minor components of the spent arid recycle acids and of the acid sludges produced. Determinations of sulfuric acid esters, sulfonic acids, sulfur dioxide, water, and equivalent weight of the sulfonic acids are frequently required in order to evaluate process efficiency and to ascertain the fate of sulfuric acid consumed in the process. Over the course of a number of years’ study, reliable methods have been developed that are valuable for the determination of the components in various spent acid and sludge streams. The
applications of these methods to several important refinery prorwries are described and evaluated. SPENT ACID STREAMS
Spent acids from three processes have been studied: (1) treatment of lubricating oil with sulfuric acid, ( 2 ) manufacture of gasoline alkylate (C8), and ( 3 ) experimental preparation of alkyl benzene (detergent alkylate). In the first of these three processes, lubricating oil may be treated with sulfuric acids ranging in strength from 94yo to acid? heavily fortified with sulfur trioxide, depending on whether the process is being used for improvement of color and stability or the manufacture of sulfonates. The lower, spent acid phase, which is allon-ed to settle after treatment, is ralled the “acid sludge.” The commercially valuable mahogany sulfonates remaiii in the oil phase and are removed by a Rubsequent operation. The sulfonates of lower equivalent weight found in the acid sludges apparently have less commercial use. In the mariufacture of gasolinr alkylate, isobutane and butylenes are brought into contact with strong sulfuric acid catalyst t o produce a Cs alkylate. The acid stream is recycled, but because there is a gradual accumulation of organic material, a portion of the recycle stream is contiiiuously removed as spent acid and is replaced by fresh sulfuric acid. Alkyl benzene is produced as the first step in one method for the preparation of sodium alkylbenzenesulfonate, an alkylaryl sulfonate detergent. The alkyla-
278
ANALYTICAL CHEMISTRY
tion of benzene, if catalyzed by strong sulfuric acid, is conducted in a manner similar to the manufacture of gasoline alkylate and requires the same facilities as this process. Some work has been done also on spent acid from the preparation of alkylated toluene, which also can be converted into a sulfonate detergent. The approximate compositions of typical spent acids and acid sludges from the processes examined are given in Table I. All contain appreciable concentrations of free sulfuric acid, which constitutes the major fraction of the acidity for those streams commonly denoted as “spent acids.” However, there are several points of dissimilarity in the composition of these three types of acid discard streams. The acid sludges generally contain higher concentrations of sulfonic acids and of water-insoluble materials than the spent acids. The sulfonic acids found in the acid sludges are of high combining weight and appear to be sulfonation products of the aromatics in the oil. The sulfonic acids produced in the preparation of alkyl benzene appear t o consist chiefly of benzenesulfonic acid with small amounts of alkylbenzenesulfonic acid. The small concentrations of sulfonic acid found in spent acid from preparation of gasoline alkylate are assumed to be butanesulfonic acid. Appreciable quantities of sulfuric acid esters were found only in the case of spent acids from gasoline alkylate. PROCEDURES FOR ANALYSIS OF SPENT ACIDS
Determination of Free Sulfuric Acid. Because of the presence of sulfonic acids, sulfuric acid esters, and sulfur dioxide in spent acids, determination only of acidity, by titration, does not provide a measure of the concentration of free sulfuric acid. Fortunat,ely a satisfactory, rapid, and specific method is available for this determination-viz., the precipitation of sulfuric acid with aniline in chloroform. A detailed description of this method is given in the Procedure. It has been possible to check the validity of the aniline method d t h actual acid samples by means of a modification of the American Society for Testing Materials Method D 855-46T for the comprehensive analysis of commercial sodium petroleum sulfonates ( I ) . ANILINE PRECIPITATIOX METHOD. The method provides a direct determination of free sulfuric acid by precipitation from chloroform solution as aniline sulfate and titration with standard alkali. The aniline precipitation method was originated by Bacon (2) for the determination of free sulfuric acid in acid sludges from treatment of light oils. Bacon determined the amount of aniline sulfate precipitated by dissolving it in water, followed either by precipitation as barium sulfate or by quantitative titration of aniline with bromide-bromate solution. The method has been simplified by titration of the aniline sulfate with standard base to the phenolphthalein end point as in the method of Parke and Davis (IS),who employed dry liquid aniline medium for precipitating the sulfuric acid. Experiments have shown that the aniline salts of the alkyl sulfuric monoesters ( 2 ) , sulfur dioxide (or sulfurous acid), and ethane-, propane-, and butanesulfonic acids are sufficiently soluble in chloroform to offer no interference in the method. Methanesulfonic acid interferes severely, but is generally absent or negligible in spent acid samples. Benzenesulfonic acid forms an aniline salt of intermediate solubility in chloroform, such that no precipitation occurs if the amount of this sulfonic acid is less than 350 mg. in the sample taken for analysis. This condition is satisfied in the recommended procedure because the amount of benzenesulfonic acid is generally far less than this critical amount in the 1-gram sample taken for analysis. Toluenesulfonic acid shows even less interference. The presence of interfering amounts of sulfonic acids can be detected and avoided by determining the amount of sulfate in the aniline precipitate according to the barium precipitation procedure employed by Bacon (2). Work done by the authors has shown that erratic results are obtained by the aniline method unless at least one mole of water is present for each mole of sulfuric acid. To make certain that this condi-
Table I.
Approximate Composition of Typical Spent Acids and Acid Sludges
Component or Determination
Approximate Concentration, Weight % Spent acids from Spent acids Acid sludges manufacture of from preparafrom lubricat- gasoline (CS) tion of detering oil treatment alkylate gent alkylate
Titratable acidity, as Hgs0.1 Free sulfuric acid Sulfuric acid esters
20 t o 60 10 t o 55 Generally negligible 5 to 50 0.3 t o 1.5 1 to 8 10 t o 40
Sulfonic acids Sulfur dioxide Water Water-insoluble material E uivalent weight of sul250 t o 550 qonic acids, g. per eq. Calculated, assuming CaHsSO3H.
85 t o 90 80 to 87 1 to 5 2 to 0.1 t o 2 to 5 to
8
0.2 4
13ga
10
7 0 t o 85 60 t o 80 Generally negligible 10 t o 20 0.4 t o 1 5 t o 10 5 t o 15
170 t o 205
I
tion is eatisfied, the method described belox specifies that a small amount of water be added to the weighed sample in all cases. MODIFICBTIOK OF ASTJI PETROLEUM SULFOKATE ~ I E T H O D . The ASTM D 855 method ( 1 ) for the analysis of sodium petroleum sulfonates includes several procedures useful in the analysis of spent acids. Because of its complexity, the following brief description of the principles is given to assist in the understanding of its application: The method is based upon separation of components by a series of 1iquid;liquid partitions. The sample is dissolved initially in aqueous isopropyl alcohol and neutralized, and the solution is extracted with petroleum ether to remove neutral oil. Saturation of the oil-free solution with sodium carbonate gives two phasesan alcohol phase which contains the sodium sulfonates and an aqueous phase which contains the inorganic salts. Evaporation of the alcohol leaves the sodium sulfonates. The inorganic salt content is determined as the difference between the value for sodium sulfonates and that for the total, oil-free sodium salts, the latter value being obtained by evaporation of a portion of the oilfree solution before saturation with sodium carbonate. The ASTN method has a wide scope and can be applied to mixtures containing aromatic sulfonic acids, including benzenesulfonic acid, as well as acids of higher equivalent weight. The inorganic salt determination of the XSTM method can be employed to determine free sulfuric acid in spent acids by introducing into the procedure preliminary neutralization of the sample to convert all the acids t o sodium salts, and inserting a factor in the calculation of sulfuric acid t o correct for the sulfur dioxide content of the sample. Should sodium alkyl sulfate be present, it is included both in the determined value for sodium sulfonates and in the value for total, oil-free sodium salts and would, therefore, not affect the result for inorganic salts by difference. S o correction for free alkali, as required in ASTRI D 855, is needed, provided the neutralization is exact. The correction for sodium carboxylate usually is negligible. The sulfuric acid content is then calculated by means of the expression: Sulfuric acid, % w. = 0.6904 [(inorganic salt, % w.)- 1.97
(S02, % w.)l where 98 08 0.6904 = A = gravimetric factor for sulfuric acid, u-eighed as 142.06 sodium sulfate
1.97 =
126 06 = gravimetric
64 O7
factor for sulfur dioxide, weighed as sodium sulfite
and inorganic salt, % w. is calculated according to ASTM D 855that is, by difference between the determined total oil-free solids and the determined organic salts (sodium sulfonates plus sodium alkyl sulfates, if any) formed by neutralization, but based on the weight of acid sample taken for analysis, before neutralization. If desired, this method for determination of sulfuric acid can be incorporated into the recommended procedure for determination of sulfonic acids (see Procedure below).
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
279
This method is very accurate, but it is tedious and time-consumfonate produced in a sample of lubricating oil acid sludge or ing and has been used principally t o assist in the evaluation of the spent acid from alkyl benzene preparation upon neutralization aniline precipitation method. The ASTM method has not been with sodium hydroxide. From this result the sulfonic acid tested with nonaromatic sulfonates of low molecular weight, and content can be calculated, The details of this method are given consequently has not been applied to spent acids from gasoline in the Procedure. If sulfuric acid esters (RHSOa) are present alkylate production, in which butanesulfonic acid is suspected in the sample, these are included in the calculated value for to be present. sulfonic acids because the sodium alkyl sulfates formed by neuOTHERMETHODSFOR FREESULFURICID. In some cases tralization are not removed. The modified ASTM D 855 method it is possible to calculate the free sulfuric acid content of spent gives useful data, but is very time-consuming. A more rapid acids by difference between the titratable acidity and the sum method for routine plant control is highly desirable. of acidic components other than sulfuric acid. This difference BY DIFFERENCEFROM TITRABLE ACIDITY. The sulfonic acid method has found some use in plant control analysis of spent acid content of certain acids can be calculated by the difference streams containing relatively small amounts of sulfuric acid between the titratable acidity and the sum of other acidic comesters, sulfur dioxide, water, and water-insoluble material. For ponents, using the value for free sulfuric acid content determined the purpose of this calculation, two simultaneous equations are hy aniline precipitation, as follows: required, because the sulfonic acid content is usually also unknown. One equation is obtained by setting the sum of acidities Sulfonic acids, yo w. = __ (Ma) [ ( A , ) - (HzSO,,% w.) (49.0) due to free sulfuric acid, sulfonic acids, sulfuric acid esters, and (1.53) (SOs, % w.) - (Az - Ai11 sulfur dioxide equal to the titratable acidity. The other e q u e where tion is obtained by setting the sum of all components-Le., the M a = average equivalent weight of sulfonic acids, grams per foregoing plus water and water-insoluble materialkequal to 100. equivalent The amount of esters, calculated as sulfuric acid, can be obtained A I = titratable acidity, as H$3Oa, % y. A2 = acidity after hydrolysis (total acidity), as H2S04, % w. by the increase in acidity upon hydrolysis; the titratable acidity, 49.0 = equivalent weight of suWuric acid sulfur dioxide, water, and water-insoluble materials can be determined by appropriate methods. Thus, the two equations can be = factor for acidity of sulfur dioxide as HZSOI 1.53 = 64 07 solved for free sulfuric acid a i d for sulfonic acid content if a reasonably accurate assumption for the average equivalent weight HzS04,% w., a?d SOz, % w., are determined separately by of the sulfonic acids can be made. This differencemethod is actuanalysis of the original sample. The term (A2 - A,) is a correction for sulfate acid esters (RHSOa),calculated as sulfuric acid. ally time-consuming, inasmuch as it requires the results from the No correction for neutral sulfates (R2S04) is necessary because determination of four or five other components. SIoreover, these are usually negligible when a large excess of sulfuric acid is when substantial amounts of minor components are present, the present (14, 15). results of such calculations tend to be inaccurate because the The two methods for determination of sulfonic acid content accumulation of analytical errors falls on the results for free gave fair agreement when applied to spent acids from production sulfuric acid. of detergent alkylates (Table 11). The equivalent weight values Amperometric titration of free sulfuric acid with lead nitrate used in the calculation based on titratable acidity are those acusing the procedure of Iiolthoff and Pan (9) has been attempted tually determined with each sample; it is often possible to use with a number of spent acid samples but has not proved satisassumed values based on earlier determinations made on typical factory. Consistently low results have been obtained; the cause samples of the various streams. It is clear that, for this type of of the low results was not studied because the aniline precip taspent acid, the simpler method can be employed for determination method was found to be convenient and reliable. tion of sulfonic acid content, provided a reasonably correct value Alethods based on extraction of hydrocarbon solutions with can be assumed for the equivalent weight. With some acid water are applicable for determination of small amounts of sulfuric sludge samples the results by the two methods were very diveracid in crude oil-soluble sulfonic acids ( 5 ) and in sulfated oils gent (see Table 111). The poor agreement was probably due, ( 4 ) , but these methods are not directly applicable to samples of spent acid (aqueous phase) containing principally water-soluble sulfonic acids. Determination of Sulfonic Acid ConTable 11. -4nalyses of Spent Acids from Production of Detergent Alkylates tent. Rapid titrimetric methods for deSpent Alkylation Acid from Preparation of termination of the sulfonate group, such Alkyl Benzene Alkyl Toluene as the cetylpyridinium methods of Barr, A B C D E F Determination Oliver, and Stubbings (3) and Epton Titratable acidity, calcu7 1 . 5 7 7 . 6 , 7 7 . 6 7 9 . 0 , 79.1 7 0 . 8 8 0 . 2 7 4 . 5 lated as H z S O I , % w. (6) and the toluidine method of llarron Equivalent weight of suland Schifferli ( I O ) , ususlly are not applifonic acids 193 178 176 171 186 185 Individual components cable to spent alkylation acids and many found, % w. Free sulfuric acid acid sludges because sulfonic acids of low By precipitation with equivalent might, such as hutanesulfonic aniline 66.1 76.9 6 8 . 1 , 6 8 . 2 6 3 . 0 , 6 3 . 2 7 5 . 1 , 75.1 7 7 . 6 , 77.4 Froin ASTM D 855 64.6 76.6 67.3 62.1 75,O' 77.v acid and benzenesulfonic acid, are not Sulfonic acids Titratable acidity less quantitatively determined by these methHtS04 by S0z;nd ods. (These methods, however, are useaniline precipitation 20.3 10 2 20.5 27.5 9.20 5.4a From ASTM D 855 18.9 11.5 20.2 30.2 9.4 7.0 ful for determination of many oil-soluSulfur dioxide 0.4 0.3 0.4 0.3 ... .... Water 4.5 4.7 4.1 5.0 7.0 10.6 ble sulfonates). Possible methods for the Neutral oil, or waterdetermination of sulfonic acids in spent insoluble material By centrifugea 14.8, 15.5 8.1 9.2 ... 9.1 6.7 acids are described below: By extractionC D 8 5 i ( 9 . 7 ) ( 4 . 5 ) ( 4 . 6 ) ( 1 . 9 ) ( 5 . 0 ) ( 2.7) A S T I 1 -4STAI ~ I E T H O DD 855. This method, Component totald, 7 0 w. 104 101 102 >94 1005 101" developed for the analysis of sodium a Not corrected for sulfur dioxide. petroleum sulfonates, can be employed b Assuming density of 0.80 f,or water-insoluble material. c Nonvolatile oil only: not included In component total. for the analysis of other aromatic suld Valuea in parentheses not included in total: average values for sulfuric acid a n d eulfonic acids were fonate mixtures. It can thus be used employed. to determine the amount of sodium sul'
280
ANALYTICAL CHEMISTRY
in part, t o the uncertainty and magnitude of the factor Ma/49.0 gested that the esters in the aniline filtrate can be hydrolyzed by in the difference method, which may be of the order of 5 t o 10, heating with hydrochloric acid and the liberated sulfate ion then magnifying the experimental errors in the small difference bedetermined by precipitation as barium sulfate. Equivalent Weight of Sulfonic Acids. The ASTM D 855 tween titratable acidity and sum of sulfur dioxide and sulfuric method includes a procedure for the determination of the acid and ester contents. With acid sludges, it is considered that the sulfonic acid values obtained by the difference method are equivalent weight of aromatic sodium sulfonates by determining thesulfate ashof the alcohol-extracted sodium sulfonates. Thereunreliable because of this uncertainty in the equivalent m i g h t factor and in the other determinations. Further argument fore, the average equivalent weight of the corresponding sulfonic against this method of calculation is that the use of these values acids can be readily calculated. The equivalent weight of the sulfonic acids present in the acid samples from the preparation for the component balance sometimes leads t o anomalous totals. of alkyl benzene indicates the presence of benzenesulfonic acid It is not possible t o obtain a comparison of the two methods with (equivalent n-eight 158) with smaller concentrations of higher spent acids from gasoline alkylates, as the ASTM method has not been applied to nonaromatic sulfonates of low molecular alkylbenzenesulfonic acids. A considerable variation was observed in the equivalent weights of the sulfonic acids present weight. The difference procedure, from titratable acidity, provides values which, calculated RS CdHgSOaH, appear reasonable in the acid sludges obtained from sulfonation of lubricating oils. in that the component totals are in the region of 100%. “Pretreat” sludges had equivalent weights considerably higher than those of the sludges prepared by subsequent sulfonation of Attempts t o determine the sulfonic acids in the chloroform filtrate from the determinations of free sulfuric acid by aniline the treated oil probably because the heavier, PubPequent sulfonaprecipitation have not given satisfactory results. tion produces some disulfonation products. Determination of Sulfuric Acid Esters. The content of sulfuric Sulfur Dioxide. Sulfur dioxide can be determined by titration of the neutralized sample with standard iodine solution !Then the acid monoesters (alkyl- or alkylaryl hydrogen sulfate) can be mixture is not too dark in color, as in the case with most alkylate obtained by the difference between titratable acidity and the acidity after hydrolysis (commonly denoted “total acidity”), acid samples. The lubricating oil acid sludges are frequently t,oo dark t o allo\y this procedure t o be employed, but the amount assuming no neutral esters (R,SO,) are present. The absence of dialkyl epters in sulfuric acid mixtures with alcohols and in of colored material can be reduced by extraction with carbon alkylation acid has been noted by other investigators (14, 15). tetrachloride before titration, or a more lengthy evolution prcThe titratable acidity determination is made by titrating a cedure ( 1 6 ) can be used. Neutral Oil or Water-Insoluble Material. ASTN D 855 solution of the sample in ice water t o the phenolphthalein end provides a petroleum ether extraction procedure for the deterpoint. Inasmuch as the hydrolysis of most sulfate nionoesters in neutral or acid solutions is slow at room temperature and below mination of neutral oil. The value obtained, however, does not ( 1 5 ) , only one equivalent of acid per mole of monoester is obinclude volatile material found in alkylation acid samples, and consequently is loner than the value for water-insoluble material tained by this titration. I n the total acidity determination, obtained by centrifugation. I n the latter method, the sample is these esters are hydrolyzed by refluxing an aqueous solution of mixed with water, the mixture is centrifuged, and the volume of the sample for an extended period ( 7 , 8 ) , the initial acidity acting the water-insoluble (upper) phase is read. Some samples of as a catalyst. I n order t o recover any sulfur dioxide that may acid from the preparation of alkyl benzene have demonstrated be evolved during the reflux period, t,he condenser is connected such effects as formation of quasi-stable emulsions and producto a scrubber containing dilute hydrogen peroxide solution j the tion of two water-insoluble phases, and have given anomalous sulfuric acid formed can then be titrated and added t o the acidit,? values under apparently parallel conditions. For many samples, of the hydrolyzed sample. With acid sludges and spent acids the addition of 1 ml. of carbon tetrachloride, before centrifuging, from preparation of alkyl benzene, the titratable and total acidity determinations are generally identical, indicating the absence of improved the separation of phases, the water-insoluble material sulfate esters. With spent acids from gasoline alkylate manubeing extracted into a small lower phase, the volume of which could be measured and corrected for carbon tetrachloride added. facture, the difference between these two determinations is generally appreciable and coiresponds to as much as 4 or 5%, assuming the monobutyl ester for purpose of calcuTable 111. .linalyses of Acid Sludges from Treatment of Lubricating Oils lat,ion. with Sulfuric Acid With niany sample8, particularly acid Acid Sludge from Treatment of sludges, the total and titratable acidity 30 VI Industrial Oil 55 V I lube determinations are subject to small but Determination A’” B Ca D oil, E significant errors, resulting from diffiTitratable acidity, calculated as culties in obtaining representative homo43 0 , 4 2 . 8 2 3 . 5 , 2 3 . 6 HzSOa, % W. 5 1 . 6 , 5 0 . 6 5 8 . 2 , 5 7 . 5 42 2 , 4 2 , s Equivalent weight of sulfonic geneous samples and also from i n d i h n c t 302, 305 b 368, 380 261, 265 371 acids Individual ooinnonents found end pointf. When the difference be7% w. tween the two acidities is small, therefore, Free sulfuric acid By precipitation with anithe absolute accuracy in the determi35.4, 36.0 23.4, 23.4 -18.2 62.1, 53.0 37.6, 38.4 line 4 8 . 1 , 4 8 . 4 5 0 . 6 , 51.0 42.1C, 3 7 . 8 C 3 3 . 9 , 3 6 . 1 2 3 . 4 , 2 3 . 5 From ASTAI D 855 nation of esters by this method is poor. Sulfonic acids However, the accuracy obtained usually Titratable acidity less SO1 and HzSOd by aniline preis sufficient for reliable material balance ( 3 3 . 6 )C (35.8) (15.5) cipitation (0.1) (25 0 ) 45.0 0.4 0.4 2 1 . 8 , 21.6 1 7 . 2 , 1 7 . 5 32 5 , 32 6 From ASTM D 8.55 calculations. Carboxylic acids do not l . l d , I.4d O.ld: P . l d .... Sulfur dioxide 0 5, 0.5 0.3, 0 6 interfere because these are included in 4 9 5.3 5.5 4.1 4 5 Water Neutral oil, or water-insoluble both acidity titrations, but large amounts material, by extraction, K . 5 , 10.5 6 8 . 4 . 70.7 3 2 . 8 , 34.7 25.5, 3 0 . 0 9.0, 9.1 SSTRI D 855 of phenols, etc., decrease the accuracy Component t o t a l l , % w. 100 98 100 57 99 by causing indistinct end points. It is probably possible t o determine ester content more accurately by analysis of the filtrate remaining after precipitation of free sulfuric acid by aniline in the manployed ner of Parke and Davis (IS). It is sug-
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281
Water. FISCHER REAGENT METHOD.The water content of acids and sludges can be determined by titration with Karl Fischer reagent. Before titration the sample should be added to partially frozen C . P . pyridine or pyridine-dioxane (1 t o 1) and the pyridinium salt formed then dissolved in methanol ( 1 1 ) . Pretreatmelit with anhydrous ammonium chloride, instead of pyridine, has been employed by other investigators (7). After treatment with pyridine or ammonium chloride, there is no interfrrpncr. from sulfuric acid, sulfonic acids, or sulfur dioxide: sul-
the water-insoluble phase usuallywas found to possessa~niallacidity, hut thir source of error appeared t o be of minor importance. The centrifuge inethod for determination of water-irisoluble material has not heeii applied succedully to acid sludges because of the difficulty of sPp:tratiiig thew dark, viscous materials xcurntely iiito two recognizable phases. The addition of aolveiits has not assisted this ,srpar:itioir. For there materials, the estr:wtion procedure for detrrminntion of neutral oil ticcording t o .\ST111) 855 can hr rniployed. Table IV.
Dissolve in ice water, titrate to phenolphthalein
Reflux in water, titrate to phenolphthalein
I
acirty
I
Titratable -acidity
Total
By difference I
Acidify,’ titrate with iodine
I I
Sulfur
Outline of Methods for Analysis of Spent Sulfuric Acid Sample
I
*kid hot 10% aniline
I
I7
I
\Voter
I
I
Discard
I
I
Add to chilled pyridine, dissolve in methanol, titrate with Karl Fischer reagent
Read volume of insoluble phase, subtract 1 ml.
Cool and filter Residue Filtrate
I
Dissolve in hot water. titrate to phenolphthalein
dioxide
Add 100 nil. of water and 1 nil. of CC14, centrifuge
in
I
Esier
Dissolw in aqueous isopropyl alcohol, neutralize
Water-insoluble material
I
Extract with petroleum ether Petloleuin ether phase
I
I
ilqueous alcohol phase
1
Sulfuric acid Seuiral oil Trace of sodium sulfonates
I
Wash w’ith 50% v. isopropyl alcohol
I
Petroleum ether phase
Sodium .sulfate Sodium sulfonates
4
I
Combine
1 Alcohol phase
1
I
.idd isopropyl alcohol
Aliquot
to make equal amounts of alcohol and water
I
-1
I
Filter, evaporate,
and weigh
Alternative Drocedure for H ~ S(see O~ text)
Saturate with KazCOa
Nonvolatile neutral oil
Measure volume (-Y)
Dis&d
I
Sodiuiii sulfonates ‘Trace of SazCOa
I is,
Aliquot
Evadorate to dryne weigh
I
-’
d d d iyater, boil, titrate to
phenolphthalein
I
Weighed portion (‘2)
Resihue (R)
IC
Equivalent rveight of sulfonic acids
Correction
Sodium sulfonates
Correction for
I
ANALYTICAL CHEMISTRY
282 furous acid, if present by hydration equilibrium from sulfur dioxide and water, is determined as the stoichiometric amount of water. METHOD OUTLINE
.4 diagram of the recommended procedures for analysis of spent acids is shown in Table IV. The alternative method for calculation of sulfonic acids by difference from titratable acidity, also recommended and described above, is not shown in the diagram. APPARATUS AND MATERIALS
Atmospheric oven, 120" C. Beaker, glass, 1%-ithinverted rim turned in 10 mm. from the wall of the beaker and pointed dowm-ard to a point 5 mm. below shoulder; 65 to 75 mm. in diameter, 7 5 to 85 mm. in height, and wall thickness approximately 1 mm. Centrifuge, oil testing type, carrying 100-ml. tubes. Funnel, Buchner type, with 8-cm. fine porosity fritted-glass disk. Lamp sulfur absorber, ilST1I D 90. RIuffle furnace, 1000" C. Oil tubes, 100-ml. graduated pear-shape, ASTLI D 96. Lunge weighing bottle. Aniline solution, 10% v. in chloroform. Hydrogen peroxide solution, 1%, neutral to phenolphthalein. Iodine solution, standard 0.1 N . Petroleum ether (boiling range 30" to 75' C.; maximum nonvolatile matter 0.002%). Sodium carbonate, C.P. anhydrous. Sodium hydroxide solutions, standard 0.5 and 10 A'. Sulfuric acid, C.P. 36 N , and standard 0.1 A '. S4MPLING
RIost samples sloivly undergo changes in composition, even a t 0" C. For this reason, it is recommended that all portions for individual determinations be weighed out within a reasonably short time and processed immediately. If this is not practicable, store the sample in refrigerator a t -15' C. or below. Do not alloir the temperature t o rise except during weighing. Most samples can be made essentially homogeneous by rapid mechanical stirring just prior to sampling. If the sample is not too viscous, a Lunge weighing bottle is recommended for sampling operations because it is convenient to use and minimizes loss of sulfur dioxide. Viscous samples can be weighed in stoppered flasks. If the sample cannot readily be made homogeneous, it is sometimes possible to obtain homogeneous phases by centrifuging. PROCEDURE
Titratable Acidity and Sulfur Dioxide. Weigh approximately 1 gram of sample, A , into a 250-ml. Erlenmeyer flask containing about 100 grams of clean crushed ice. Titrate immediately to the phenolphthalein end point with standard 0.5 N sodium hydroxide solution, recording the volume of caustic required, B, and the normality, 6. Add 15 ml. of 0.1 N sulfuric acid to the neutralized solution and titrate the sulfur dioxide present t o the starch end point with standard 0.1 N iodine solution, noting the volume of iodine solution consumed, D, and the normality, E. Acidity plus Ester Content (Total Acidity). Place 25 ml. of neutral 1% hvdrogen peroxide solution in a lamp sulfur absorber and connect the Gbsorber t o the upper end of-a vertical reflux condenser by means of a flexible connection. Place 50 ml. of water in a 250-ml. Erlenmeyer flask equipped with a standardtaper joint and weigh into this flask approximately 1 gram of sample, F. Connect the flask to the reflux condenser and heat a t the boiling point for 20 to 24 hours to hydrolyze sulfate esters. Then slightly loosen the flask connection and at the same time apply suction a t the absorber, thus removing and collecting any sulfur dioxide that may be present in the condenser. Rinse the condenser with a little water, remove the flask, and titrate both the contents of the flask and the absorber with standard 0.5 N sodium hydroxide to the phenolphthalein end point, recording the total volume of caustic required, G, and the normality, C. Free Sulfuric Acid. Weigh approximately 1 gram of sample, H , into a 125-mI. Erlenmeyer flask. Chill the flask in an ice bath and add 0.20 ml. of ice water from a pipet. Remove the flask from the ice bath and, with constant swirling, slowly add from a graduated cylinder 50 ml. of a boiling-hot 1Oyo solution of aniline in chloroform. Break up any large lumps of aniline sul-
fate with a glass rod which has been flattened a t the end, and allow the mixture to cool t o room temperature. At the end of this time, attach a glass funnel equipped with a fine fritted disk to a filter flask, transfer the reaction mixture t o this funnel, and filter off the liquid by application of vacuum. Wash the reaction flask with a 10- to 15-ml. portion of chloroform and add the washings to the funnel. ( I t is not necessary to remove all the precipitate from the flask as the flask is to be washed again later in the procedure.) Triturate the precipitate in the funnel with the flattened glass rod, then draw the washings through the filter by application of vacuum. Continue washing the precipitate with 10- t o 15-ml. portions of chloroform, until the last washings are colorless, using a minimum of 40 ml. of chloroform. Then draw air through the filter for several minutes to remove chloroform. Any turbidity in the washings may indicate that aniline sulfate has been drawn through the filter. Discard the filtrate and attach the filter to a clean 500-mI. filter flask. Moisten and triturate the precipitate with 2 or 3 ml. of acetone and then apply vacuum to the flask until the precipitate is quite dry. (The purpose of the acetone is to remove the last traces of chloroform which would cause spattering upon addition of hot water. The acetone is retained with the subsequent aqueous filtrate, as it may contain a small amount of dissolved aniline sulfate.) Add 10 to 15 ml. of hot water t o the reaction flask to dissolve any precipitate which may not have been washed out. Use this washing, plus an additional 100 to 150 ml. of hot water, to dissolve the precipitate on the filter, drawing the washings into the flask by application of vacuum. Titrate the contents of the filter flask with standard 0.5 N sodium hydroxide solution to the phenolphthalein end point, recording the volume of caustic consumed, I , and the normality, C. Water-Insoluble Material. Pipet 1 ml. of cai bon tetrachloride into a 100-mI. 011 tube, add approximately 100 ml. of distilled water, and chill to about 0" C. Weigh 5 to 10 grams of sample, J , into the oil tube. The sample should preferably contain at least 1 ml. of water-insoluble material. Stopper the tube firmly with a clean cork and shake it vigorously. Centrifuge the miuture until a clear, aqueous phase is obtained. Record the volume, K , of the lower phase. hONVOLATILE NEUTRAL OIL, SULFONIC ACID CONTENT, AND AVERAGE EQUIVALENT WEIGHT, BY 3ZODIFIED ASTM METHOD
These procedures are applicable only t o aromatic sulfonic acid miutures, such as in lubricating oil acid sludges and spent acids from manufacture of detergent alkylates. Nonvolatile Neutral Oil. Weigh into a 250-ml. beaker an amount of sample, L,sufficient t o give, if possible, 3 t o 5 grams of sulfonic acid, but in general limit the sample to 25 grams. Chill the sample beaker in an ice slurry and add slowly 125 ml. of distilled water and 50 ml. of isopropyl alcohol while stirring. Neutralize vc-ith 10 .V sodium hydroxide solution to the phenolphthalein end point. If the mixture is so dark that the phenolphthalein color change cannot be observed, calculate from the titratable acidity the amount of standard 10 iV sodium hydroxide solution necessary to neutralize the mixture. Quantitatively transfer the neutralized solution to a 500-ml. separatory funnel, rinsing the beaker and stirring rod TT-ith 10 ml. of distilled water followed by 10 ml. of isopropyl alcohol. [Salts precipitated upon neutralization must be dissolved before proceeding further. This is accomplished by alternately adding water and shaking the mixture until solution is effected. Some samples may require the use of larger separatory funnels than are specified in the method. Should stable emulsions occur during the subsequent extraction with petroleum ether, additional isopropyl alcohol must also be added to the mixture. I n cases in Jvhich increased volumes of water and/or isopropyl alcohol are necessary, the volume of aliquot taken for determination of sulfonic acid must be increased so that a reasonable amount of residue is obtained, and the addition of the 65 ml. of isopropyl alcohol specified in this determination (below) should be ignored and instead the contents of the cylinder should be adjusted to contain equal amounts of water and isopropyl alcohol.] Rinse the sampling beaker with 30 ml. of petroleum ether and use this portion of ether t o extract the combined alcoholic solution; retain the interface "cuff" with the ether phase. Extract the alcoholic phase five more times with 30-ml. portions of petroleum ether, using two 250-ml. separatory funnels and collecting the petroleum ether extracts in the 500-ml. separatory funnel. Drain the oil-free alcoholic sulfonate solution into a 500ml. mixing cylinder. Rinse each 250-ml. separatory funnel with two 10-ml. portions of isopropyl alcohol-water (1 to 1 by volume) and add to the mixing cylinder. Wash the combined ether extracts with 50 ml. of isopropyl alcohol-water (1 t o 1); add the
V O L U M E 2 5 , NO. 2, F E B R U A R Y 1 9 5 3 wash to the miving cylinder, and retain the contents of the cylinder for determination of sulfonic acid content (see below). Filter the combined ether extracts through a small plug of cotton, placed in the vortex of a filter funnel, into a tared, inverted-rim beaker. N-ash the separatory funnel and filter with 20 to 30 ml. of fresh petroleum ether, adding the washings t o the inverted-rim beaker. Evaporate the ether solution to dryness on a steam bath, adding small portions of isopropyl alcohol (or acetone) to aid in removing miter. Heat for 15 minutes after the disappearance of the odor of solvent. Cool to room temperature and bring to constant weight in a vacuum desiccator a t room temperature and under 3 mm. or less of mercury pressure, applying vacuum gradually and venting frequently to avoid spattering at the start. Denote the residue as 31. Sulfonic Acid Content. Add 65 ml. of isopropyl alcohol t o the mixing cylinder (see foregoing section) and warm in a water bath a t 40" to 50" C. Weigh out 18 grams of C.P. anhydrous sodium carbonate per 100 nil. of alcohol solution remaining in the mixing cylinder. Carefully admit a few grains to the mixing cylinder, allowing the dissolved petroleum ether t o evaporate s l o ~ l y . JYhen danger of boiling is past, add the remainder of the carbonate and shake the mixture vigorously. . 4 ~ ~ ot ox stand for a few minutes and repeat the shaking. If there is no excess of the solid carbonate, add 1 to 2 grams and repeat the vigorous shaking. Allow the phases t o separate, swirling the solution to dislodge any solid particles clinging to the upper part of the miving cylinder. Replace the mixing cylinder in the Iyater bath a t 40" to 50" C. until separation into t x o layers is complete. Cool the miving cylinder to room temperature (3 to 4 hours). Kote the readings on the neck of the cylinder a t the top of the alcohol phase and a t the alcohol-aqueous phase interface. Designate the difference between these readings as -V. Pipet a 75-ml. aliquot of the dehydrated alcohol phase into a tared 250-1111. beaker and evaporate it to dryness on a steam bath. D r y for 8 hours in an atmospheric oven a t 120" C. Cool the residue in a desiccator after removing from the oven and prior to iyeighing. Denote the Jyeight of residue as P; save for equivalent weight determinations. Retain the balance of the dehydrated alcohol phase for determination of the carbonate correction (see below). Average Equivalent Weight of Sulfonic Acids. Weigh a portion of the dried residue into a tared, ignited platinum dish. Denote this portion as Q. Carefully heat the dish over a small flame until the contents ignite and burn. Place the dish on a hot plate and allow the contents to burn gently. After the dish has cooled, add a few drops of 30 'Ir sulfuric acid and heat gently over a small flame, rotating the dish in such a manner that all the contents come in contact with the acid, until fuming ceases. Do not heat strongly enough to cause spattering. Then ignite over a burner, never allowing the dish t o get hotter than a dull red, until all carbon has disappeared. Cool, add 3 or 4 drops of 36 S sulfuric acid, and fume off the acid as before, taking care to avoid spattering. When fuming ceases, heat in a muffle furnace a t 800" to 1000" C. to constant weight, R. Cool in a desiccator prior to neighing. Obtain a correction for the effect of the small amount of sodium carbonate remaining in the dehydrated alcohol phase by the following procedure: Pipet 25 ml. of the alcohol phase into a 150ml. beaker containing 50 ml. of distilled n-ater and a few drops of phenolphthalein indicator solution. [If more than the usual 75ml.aliquot of the alcohol phase x-as used in the sulfonic acid determination, take a proportionately larger aliquot for determination of the carbonate correction (one third of the volume of the aliquot taken for the wlfonic acid determination).] Heat to a gentle boil and titrate with standard 0.1 iV sulfuric acid to the disappearance of the pink color. Repeat the boiling and titrating until the pink color fails to return on additional boiling. Record the total volume of sulfuric acid required, S, and the normality, 7'. Calculations Titratable acidity as &SO4
%
=
(B)(C)(49.04)
( K - 1)(100) JYater-insoluble material, m1./100 grams = (J) \VEIGHT OF SODICM SELFONATE I X RESIDGES.Calculate the \wight of sodium sulfonate in the residues by subtracting the weight of sodium carbonate from the dry residue weights as follo\vs:
283 Sodium carbonate correction, V = (O.O53)(S)( T ) = ( P )- (3)(V)
P (corrected)
(Q) [ P (corrected)] Q (corrected) = --
(PI
SULFATE Ak~ Calculate . a corrected value for ash, CORRECTED R, corresponding to the ash due to sodium sulfonates alone, by subtracting the weight of ash contributed by the sodium carbonate contaminant present in the portion of the residue, P , taken for ashing, Q.
R (corrected)
=
R
- -("(Q)
(o.oi1)(s)(2')
(PI riverage equivalent might of sodium s~dfonates,TV = (71)[& (corrected)] [ R (corrected)] Average equivalent weight of sulfonic aciJs X = W Sonvolatile neutral oil content, % Sulfonic acid content, % =
=
-
22
( 100 )( M ) ~
(L)
(loo)[P (correcteJ)](S)(X) __( i 5 ) ( L )IV) (
where A = xeight of sample taken for titratable acidity and sulfur dioxide determinations, grams B = volume of sodium hydroxide solution, millilitcrs, consumed in titratable acidity determination C = normality of sodium hydroxide solution D = volume of iodine solution consumed, milliliters E = normality of iodine solution F = n-eight of sample taken for total acidity determination, grams G = volume of sodium hydroxide solution, milliliters, consumed in total acidity determination H = n-eight of sample taken for sulfuric acid determination, grams Z = volume of sodium hydroxide solution, milliliters, consumed in free sulfuric acid determination J = neight of sample taken for water-insoluble materials determination, grams K = volume of lower phase in water-insoluble materials determination, milliiiters L = weight of sample taken for sulfonic acid determination, grams Jf = weight of oil residue, grams X = volume of dehydrated alcohol layer, milliliters P = weight of oil-free, desalted sulfonate, grams Q = weight of oil-free, desalted sulfonate taken for ashing, grams R = weight of sodium sulfate ash from portion Q of oil-free, desalted sulfonate residue P , grams S = volume of sulfuric acid solution consumed, milliliters T = normality of sulfuric acid solution Ti = sodium carbonate correction for residue from aliquot -portion of N,grams TV = average equivalent weight of sodium sulfonates X = average equivalent weight of d f o n i c acids GEYERA L DISCUS SIOh-
Because of the complex and unstable nature of spent alkylation acids and acid sludges, the validity of analytical results can be judged only by indirect evidence, such as by comparison of values obtained using methods based on different principles and by calculation of material balances for the sum of determined components. The aniline precipitation and the ASTAI D 855 methods for determination of sulfuric acid have given good mutual agreement, both with spent acids from detergent alkylate preparation (Table 11),and with acid sludges (Table 111), indicating the validity of these methods for such materials. The spent acids from gasoline alkylate production were not analyzed bv the ASTM D 855 pro:edure, but other considerations show that the aniline precipitation method is applicable t o these acids also. As shown in Tables 11, 111, and I-,reasonably good component totals were obtained for all three types of spent acid streams investigated, provided that volatile coniponents were not lost in the
ANALYTICAL CHEMISTRY Table V.
Analyses of Spent Acids from Production of Gasoline Alkylates
Determination
Spent Acid from Alkylation of C I Hydrocarbons A B C D E
Total acidity, calculated as HzSO4, % w. 87.0 9 2 . 2 90.2 92.6, 9 2 . 7 91.8 Titratable acidity, calculated a s HzS04, % w. 8 5 . 6 90.9 89.4 9 2 . 2 , 92.3 91.6 Individual comuonents found. % w. Free sulfuric acid, by precipi8 1 . 4 87.4 8 5 . 7 9 0 . 0 , 9 0 . 2 9 0 . 4 tation with aniline Sulfonic acid, 8.1 4.8 3.7 7.d 1.9 calculated as C4HsS03Ha Sulfate acid esters, 4 4 3 1 3 0.6 calculated as C I H Q S O ~ H ~ 0.01 0 2 .. 0 02 Sulfur dioxide 0.1 3 1 2 8 . 4.3 7 7 Water Water-insoluble material, b y centrifuge C 4.9 2.7 .. 1.3 Xi1 Component total, 70w. 101 99 >97 102 101 a Calculated from titratable acidity less the sum of ester, Sot, and HrSO, by aniline precipitation. b Calculated by difference between total and titratable acidities. e .lssnming density of 0.80 for water-insoluble material.
considerable use in research and control laboratories for more than 15 years. During this period, numerous improvements in analytical procedures and techniques have been developed and, in the opinion of the authors, the uaefulness of these procedures h:ts been sufficiently established to warrant their detailed dewription as a comprehensive schcmcs of analysis for many types of industrial sulfuric acid strcarnr. ACKNOWLEDGMENT
It is :Lpleasuie to acknowledge the capable amstance ot It. J. Shreve in conducting many of t h r analyses in this investigation, tiis helpful suggcstions LITERATURE C I T E D
Ani. Soc. Testing Materials, “ASTM Standards on Petroleum Products and Lubricants.” D-3, p. 320, 1951. Racoii, F. S., IXD. ENG.CHEM.,.\SAL. ED.. 1, 89 (1929). Barr, T., Oliver, J., and Stubhings, W ,V...I. S O C . Chem. I n d . , 67, 45 (1948).
Hnt,ton, D., and Robertshaw, G . F.,“Sulfated Oils and Allied I’roducts,” 1st ed., pp. 116, 148. Xew Tiork, Chemical Publishdetermination of water-insoluble material. Because the equivalent weights of the sulfate esters and sulfonic acids are much greater than that of sulfuric acid, appreciable anomalies in component balance should be evident if substantial errors in analytical results were present. Det,ermination or correction for carboxylic acid content u~u:tlly has not heen found necessary in actual practice, inasniurh as the concentration of these acids has been very low or negligible in the samples investigated. Because of t h e large esvess of strong acids, it has not h e n found possible t o determine rart~osylic acids reliably by p3tentiometric titration directly on the sample. A solvent, extraction step with an organic solvent t o con(~etitrate organic acids mav be possible. For example, it has hectn reported that n-tributyl phosphate ( 1 8 ) estrarts more thau 90% of C, and higher monobasic carbosylic acids from aqueous solutions, but estracts only 1% of the sulfuric acid. The bchavior of sulfonic acids has not been determined for this solvent. Rlost of the analytical methods described ahove h a w found
ing Co., 1940.
Dawson, 0. H., ASAL. CHEX, 20, 383 (1948). Epton, 8.R., Trans. Faraday SOC., 44, 226 (1948). Goff. \I* H.,,Palmer, W. S..and Huhndorff, R. F..ANAL. (.‘HEM..
20, 344 (1948).
Hart. R., J . I n d . Eng. Chem.. 9, 860 (1917). Kolthoff, I. JI., and Pan, T.I).. .I. A m . Chem.
Soe., 62, 3332
(1940).
SIarron, T. V.,
arid Srhifferli, J . , ISD. E s o . CHEM.,A x . ~ LED., . 18, 49 (1946). Mitchell, J., Jr., and Smith, D. lI., “.lquanietry,” 1st ed., p. 245, New Tork, Interscience Publishers, 1948. Pagel, H. A, and hIcLafferty, F. IF-,,A N a L . CHEM.,20, 272 (1948). Parke, T. V., and Davis, W.JV.,Ibid.. 21, 1570 (1949). Rohey, R. F., I n d . E’/?Q.Chem., 33, 1076 (1941). Suter, C. 11..and Oberg, E., J . A m . Chem. SOC.,56, 677 (1934). Thompson, ,J. E., and Toy, E., ISD. E s c . CHEM..AN.AL.Eo., 17, 612 (1945). 17, 1952. Accepted October 20, 1952. Presented hefore the Division of Refining, American Petroleum Institute, a t San Francisco. Calif., May 1952.
RECEIVED for review Jnne
Spectrophotometric Determination of Iron as Ferric Sulfate Complex A n Ultraviolet Study ROBERT BASTIAN, RICHARD WEBERLING, AND FRANK PALILLA Sylrnnin Electric Products Znc., Kew Gardens, ?I.’ Y .
T
Beckman Model D U spectrophotometer with ultraviolet accessory set and I-em. silica cells. .$I1 unspecified reagents were of C.P. grade.
peak shoivn i)y (1 (encircled) Lvhich contains about 0.02 nil. of free i o % prrchloric acid per 100 ml. is destroyed so rapidly hy escess acidity that it would h r difficult to employ for analytical purpoucs. T h e peak shon-n a t 240 m p by solution b containing 7 nil. of acid could he used t,o determine iron because t,he ahsorb:tiicy is independent of acidity over a wide range, and adherence to the norinal ahsorption l a w is obtained. However. at this wave length many other elenicnts show appreciable ahsorption. Figure 2 shows the effect of sulfuric acid on the ahsorption spectra of ferric sulfate solutions. Solution a contains ahout 0.02 nil. ?mess sulfuric acid, while h contairi.; 10 ml. ( w e s s . Thr shift in t h e ahsorption peak over thii: wide range of acidity is not more than n few niilliinicrons.
ABSORPTION SPECTRA
E F F E C T O F ACIDITY ON F E R R I C S U L F 4 T E
Figure 1 shows the dependence of t,he absorption spectrum of ferric perchlorate solutions on acidity. The near ultraviolet
Figure 3 shows the effect of sulfuric acid upon the ahsorbaticy of a ferric sulfate solution a t threr wave lengths. (The longer
HE ahsorption spectra of ferric sulfate (1, 2 ) anti ferric perchlorate solutions (1, 3-7) have been studied from a
theoretical point of view. The former has been attributed to ferric sulfate complex formation ( 2 ) . and the latter to F e + + +and Fe(0H) + + ions, their proportions depending on t’he acidity of the solution (7‘). No analytical applications are reported in these papers, and much of the work has been done in a concrntration rang? higher than that of analytical interest. APPARATUS AND M A T E R I A L S
