March, 1928
INDUSTRIAL A N D ENGINEERING CHEMISTRY
315
Effects of Certain Impurities upon the Determination of Causticity of Limes by Modifications of the Sugar and Scaife Methods’ W. M. Shaw and W. H. MacIntire THEUNIVERSITY
OF
TENNESSEE AGRICULTURAL EXPERIMENT STATION, K K O X V I L L B , TEIN.
H E modified sugar and modified Scaife methods are dioxide, immediately prior to final titrations. Two efficient probably the most widely used for the determination and rapid filtration devices were perfected7 in certain of causticity of burnt and hydrated limes. Both collaborative work. The homemade filtration device7 remethods have been adopted by the Association of Official ported by the present authors may be adapted to the modified Agricultural Chemists2 as tentative for the determination Scaife procedure as well as to the sugar method. It was of “calcium oxide in burnt and hydrated lime.” The Scaife therefore used t o obtain clear filtrates in the comparison method is prescribed by the American Society for Testing of the systems produced by the two procedures. Unless hfateriais3 as tentative for the determination of “available otherwise stated, a constant charge of 1.4 grams was used in lime.” all the experiments. The term “available lime” has come into rather general Effect of Magnesia upon Causticity Determinations usage in the industries. It is, however, somewhat indefinite and unscientific. It has one meaning when used with Known amounts of “chemically pure” MgO were added to definite quantities of CaO. reference to soils and anThe two oxides were obother in industrial practices. tained by separate calcinaCalcium oxide, hydroxide, A study was made of the influence of the usual imtions of “c. P.,’ calcium and carbonate, and silicate are purity components of caustic lime upon the accuracy of magnesium carbonates in an all of value when incorpodeterminations of causticity values by the authors’ electric furnace for 5 hours rated in soil, whereas only modification of the sugar method and the modified at 950” C. No further loss that amount of CaO which Scaife procedure. Especial objectives were (a)to show in weight occurred when the reacts under imposed conthe adaptation of a previously devised apparatus to the oxides were again heated ditions is considered a s analysis of high-magnesic and slow-settling limes; ( b ) for 2 hours. The two oxides “available” in i n d u s t r i a l to determine the extent to which the different impuwere then passed through a usages. Haslam, Adams, rities may be expected to vitiate analytical results; and and Kean4 conclude that 120-mesh sieve, mixed, and ( c ) to compare the two analytical procedures designated “for an availability test for bottled. Charges of lime as tentative by the A. 0. A. C. and A. S. T. M., respecand magnesia were weighed lime to be of any great value tively. The factors of magnesia, alone and in silicate out separately to give the the test must be based on combinations, interference of silica, neutral salt conthe particular process in respective proportions of 10 centration of the titration system, and the formation which the lime is to be and 40 per cent of MgO in of ternary compounds were considered. used.” RecentlyBlair5has Ca0-Mg0 charges of 1.4 stated, “ a v a i l a b l e lime grams. The causticity of means nothing until it is each charge of both -maknown for what purpose it is available.” It would seem that terials, separately and in combination, was then determined. the uncombined CaO, or Ca(OH)2, content of a commercial The results are given in Table I. lime should be the basis on which the producer is entitled The value obtained for each mixture of the two oxides to compensation, irrespective of any lack of complete reaction was greater than that for the corresponding charge of CaO arising from imperfect commercial utilization. The present alone, but in every case the actual determination for each authors, therefore, use the term “causticity,” or “caustic CaO-MgO mixture was less than the summation of the two value, ” in preference to “available lime.” separately determined oxide values. The smaller amount of Mg(OH)2 suspended alone in each of the two solvents was Adaptability of Two Methods sufficient to produce a saturated solution, but the titration It has been shown6 that the sugar and modified Scaife for the 40 per cent charge was about three times as great methods give concordant determinations of the CaO-Ca(0H)t as that for the 10 per cent charge, in both sugar and aqueous content of high-purity limes. Even the highest grade calcium chloride solutions. These data indicated that the commercial lime, however, contains some extraneous ma- original magnesium carbonate contained some soluble alkali terials. This is especially true of high-magnesic limes. or alkali earth. The supposedly “c. P,’, MgC03 had been The persistent suspension of finely divided magnesium subjected to the usual oxalate procedure, and only a trace hydroxide was a serious handicap to the accurate determina- of CaO was obtained. The oxide was then extracted with tion of causticity values by either method, in its original form. boiling water and the alkalinity yielded by 10 grams in 500 cc. A study was undertaken to determine the extent to which of water determined. The first water extraction gave a this and any other factors influence the values given by the 47.5-cc. titration against 0.05 N acid. Three additional two methods. It became necessary, therefore, to eliminate extractions of the moist oxide gave titrations of 5.5, 3.81, all suspended material, with protection against carbon and 3.81 cc. The residue was again ignited and extracted three times. Considerable titratable alkalinity was de1 Received October 24, 1927. * Assocn. Official Agr. Chem., Methods, p. 35 (1925). veloped by the second ignition, as shown by 0.05 N titrations * Pro6 A m . SOC.Tesfing Maferials,26, 779 (1926). of 29.0, 9.0, and 4.0 cc. A third ignition and two extractions 4 I n d . Eng. Chcm., 18, 19 (1926). gave titrations of 14.0 and 3.8 cc. Equilibrium was apparently 6 I b i d . , 18, 389 (1926).
T
Shaw, J . Assocn. Oficial Agr. Chcm., 9, 141 (1926).
7
Shaw, MacIntire, and Underwood, Ind. Eng. Chcm., 20,312 (1928).
316
INDUSTRIAL AND ENGINEERING CHEMISTRY
established after the fourth ignition and extraction. The extracts from each of seven successive ignitions were combined and analyzed for calcium, by the oxalate method. This analysis gave a CaO content of 0.70 per cent, as against the 0.001 per cent given on the label. An occurrence of 0.70 per cent was also obtained by the use of the modified Stolberg method,* through the courtesy of G. E. F. Lundell, of the Bureau of Standards. Table I-Effect
Vol. 20, No. 3
impurities being responsible for this discrepancy will be considered in succeeding paragraphs. Effect of Silica upon Causticity Determinations
Silica is found as an impurity combined, or uncombined, in calcined products. The influence of this impurity was therefore studied by adding silica to give 10 and 30 per cent occurrences in the lime-silica mixture. The silica gel was
of Added M a i n e s i a uDon Causticity Values“ MODIFIED SUGAR METHOD
COMPOSITION OF CHARGE
40 PER CENT MAQNESIA E-Lime alone 60% charge F-Magnesia done 40% charge G - S u m m a t i w of apd F H- Actual determination Variation of determined value H from calculated value G
MODIFIBD SCAIFEMETHOD
4v.
59.10 1.10 60.20 59.00 -1.20
59.10 1.10 60.20 59.00 -1.20
59.10 1.10 60.20 59.00 -1.20
2
2
58.95 0.85 59.80 59.25 -0.55
Av.
58.95 0.85 59.80 59.50 -0.30
58.95 0.85 59.80 59.38 -0.42
4 (Expressed as per cent CaO equivalent.) To afford uniform comparisons under experimental conditions the CaO equivalent was calculated from the total titration equivalent of the 500 ml. of solution in each instance and referred to the standard charge of 1.4 grams, regardless of the actual quantitiu used in the additions. Thus the 0.35 per cent CaO equivalent for a water extraction was derived from a titration of 0.70 ml. of 0.05 N acid per 100 ml. 0.0014 X 0.7 X 5 X 100 o.35. of the extract: 1.4
made by the procedure of Holmes and Anderson.9 The lime and silica were intimately mixed by repeated fractional grindings in a mortar and then passed through a l2O-mesh sieve. These mixtures were then calcined in an electric furnace for 5 hours a t 950’ C. The calcined mixtures were again passed through a 12Gmesh sieve and intimately mixed. The causticity values of filtrates obtained by both the sugar and the Scaife method are given in Table 111. These data show that a decided reduction in causticity resulted from the calcination of the limesilica mixture. The reduction in alkalinity caused by the 30 per cent addition was 2.13 times that caused by the 10 per cent addition when measured by the modified sugar method. When measured by the modified Scaife method a corresponding ratio of 2.29 to 1 was obtained. Thie, of course, must be ascribed to silicates formed by reaction of a part of the added silica with CaO during calcination and with Ca(OH)* during the subse-
The experiment was then repeated with the pure MgO thus prepared from the “c. P.” hydrated carbonate. Table I1 shows the same,titration value for the 10 and the 40 per cent charges of purified MgO. These data also show the same value for-the lime alone, lime plus 10 per cent of MgO, and lime plus 40 per,cent of MgO by the filtered sugar solution, even though the causticity value found for MgO alone by the sugar method was three times as great as that found by the modified Scaife method. Nevertheless, the causticity value registered by the Scaife procedure for the CaO-MgO charge which contained 10 per cent MgO was considerably less than that obtained for CaO alone. The disparity registered by the 40 per cent MgO inclusion was still greater. It therefore seems justifiable to conclude that the causticity ‘value of CaO in high-magnesic limes is more accurately determined by the modified sugar method than by the Scaife procedure. The possibility of silica 8
Hillebrand,
U.S. Gcol. Survey, Ball. 700, p. 143 (1919). Table 11-Effect
8
f n d . Eng. Chem., 17, 280 (1925).
1
of F u r t h e r Purified C. P. Magnesla upon Causticity Values
I%
MODIFIEDSUGAR METHOD
COMPOSITION OF CHARGE
A-Lime alone,rl.40 grams B-MgO alone,rO.l4 gram C-MgO alone 10.58 gram 1 4 ;rams*+ MgO, 0.14 gram D-Lime Mg, 0.66 €Tam %Lime,’ 1:4 grams Variation of determmed,value D from summation of A and B Vnriatinn of determined value E from summabon of A and C
+
Table 111-Effect
M O D I F ~SCAIFE D METHOD
1
2
Av.
% 90.-25 1.10 1.10 99.25 99.25 0.00 0.00
%
%
%
99.i5 1.10 1.10 99.25 99.25 0.00 0.00
99.25 1.10 1.10 99.25 99.25 0.00 0.00
90:00 0.35 0.35 98.60 97.90 -0.40 -1.10
99.00 0.35 0.35 98.60 97.90 -0.40 -1.10
1
1
2
Av.
%
99 m : 0.36 0.36 98.60 97.90 -0.40 -1.10
of Silica Additions Followed by Calcination upon Causticity Values MODIFIEDSCAIFE METHOD MODIPIBDSUGAR METHOD
COMPOSTION OF CRARGE
A-Lime alone 9 0 7 of charge B-Silica alonh, 10 0 of charge C 4 u m m a t i o n of d a n d B D-Actual determination Variation of determined va1ue:D from caIculated value C +Lime alone 70 of charge F-Silica alon;, 80 0 of charge M u m m a t i o n of and F €I-Actu.al determination Variatron of determined value H from calculated value G
I :
ss:es 0.00
88.65 82.05 -6.60 68.96 0.00 68.96 54.90 -14.05
2
Av.
% ss;6s 0.00 88.65 82.05 -6.60 68.95 0.00 68.95 54.90 -14.05
88165 0.00 88.65 82.05 -6.60 68.95 0.00 68.95 54.90 -14.05
%
1
% 88.43 0.00 88.43 83.75 -4.68 68.78 0.00 68.78 57.50 -11.28
2
% 88.43 0.00 88.43 83.25 -5.18 68.78 0.00 68.78 57.50 -11.28
Av.
% 88.43 0.00 SS .43 83.50 -4.93 68.78 0.00 68.78 57.60 -11.28
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1928
quent suspensions. It is evident that the full CaO-SiOz reaction registered by the differential titrations did not take place during calcination; but the extent of any reaction between Ca(OH), and that part of the silica which did not react with CaO during calcination was not definitely established. Using the sugar method as an index in the larger proportion of 9 of CaO to 1 of SiOz, the full CaOSiOzreaction during calcination and suspension had accounted for 71 per cent of the added silica, as against 53 per cent by the modified Scaife method. The outstanding feature of this experiment is the consistently diverging difference between the sugar and the Scaife results with increase in quantities of silica in the lime-silica mixtures. I n each case the sugar method shows a greater decrease in the causticity found as a result of the silica addition. The difference between the two methods was about 1.6 per cent CaO equivalence for the 10 per cent addition, whereas the corresponding difference for the 30 per cent addition is about 2.8 per cent. These differences may not be accounted for by assuming that the calcium silicate calcination product is less soluble in the lime-sugar solution than in the CaCl2-Ca(OH)z-H20 system. The opportunity for further combination between Ca(OH)2 and the uncombined silica residuals mentioned above must be taken into consideration. The results show that the calcium silicate impurities register as Ca(OH)z to a greater extent by the modified Scaife than by the modified sugar procedure.
complete neutralization of Ca(0H)z-as determined by the preliminary determination. If these results are taken as criteria, it may be concluded that where the Scaife procedure is followed strictly as to incomplete neutralization of the charge there is to be expected no great difference between the results by that method and those by the sugar method, in so far as the interference of ignited silica is concerned. Should the residual alkalinity by the Scaife method be reduced beyond the prescribed normality of 0.01, silica impurities may be expected to cause appreciable differences in the results obtained by the two methods. These data emphasize the importance of making the titrations as soon as possible after solution is effected and the importance of filtration to admit of so doing, as contrasted to the unpermissible wait required to secure a clear solution by settling. Table IV-Reactivit of Calcined Silica Gel 0.7 g r a m ) i n 400 cc. of CaC12-Ca(OK)I S o l u t i o n s of V a r d n a Loncentrations
E~.,..
CHARGE
A-Lime alone, 50% of charge B-Magnesia alone, 40 of charge C-silica alone, 10% oycharge >Summation 0f.A and B and C E-Actual determination Variation of determined value E from calculated value D
1
2.5
2 3 4 5
20.0 97.6) a
b
Several charges of a high-grade lime were completely dissolved in 5 per cent sugar solution and several others were partially dissolved in varying quantities of HCI, to obtain CaClz-Ca(OH)z-HzO systems of varying alkalinity. The solutions were all filtered free of suspended matter and the alkalinities of 50-cc. aliquots determined by 0.05 N acid. A 0.7-gram charge of calcined silica gel was then introduced into each filtrate. Each suspension was then shaken for 5 minutes, permitted to stand for 1 hour and for 18 hours, refiltered, and alkalinities again determined. The data of Table IV indicate appreciable reaction between the Ca(OH), solution and the suspended ignited silica. It is interesting to note so extensive a reaction between the ignited silica gel and the approximately 0.0025 N Ca(OH), solution obtained in simulating the conditions of the modified Scaife procedure. The speed of the lime-silica reaction increased with the concentration of Ca(OH)z, and for dilute solutions of Ca(OH), it had apparently reached equilibrium during the first hour or two. With Ca(OH)2concentrations of 0.01 normality, or greater, the reaction was more extended. With a Ca(OH), concentration of approximately 0.01 nor-
OF
1.4-GRAM LIME CHARGE UNNEUTRALIZED BY HC1 BEFORE SILICAGEL ADDITION
TITRATION OF 50
BY
ACID
Before addition
1 hour
After
cc.
cc.
2.70
0.95
18.60 97.00
12.200 93.70
After
1 hour
I
OF
Sios GEL After 18 hours
%
%
1.75 2.40 4.10
1.85 2.50 5.70 6.40 6.00
3:QO
After 18 hours’ standing. Calcium sucrate control.
Effect of Calcination of Silica-Magnesia-Lime Mixtures, as Indicated by Causticity Determinations
Calcination of high-magnesic limes aiTords opportunity for Ca0-Mg0-SiO2 reactions. The conditions of such a calcination were simulated to show their effects upon causticity values recorded by the two methods. Silica gel, magnesia, and lime were mixed to give SiOrMg0-Ca0 proportions of 10, 40, and 50 per cent, respectively. The mixture was calcined, ground to fine powder, and sifted through a 120-mesh sieve, as in the preceding experiment. The causticity of the mixture and that of each component was then determined separately by both methods. The results are incorporated in Table V. In spite of the large excess of magnesia over silica during calcination, the reaction between these two oxides was not complete. A part of the silica evidently combined with lime in both systems, as indicated by the reductions in causticity. The variable influence of impurities during the manipulations of the sugar and Scaife methods is again apparent. B decrease of 3.40 per cent in causticity was MODIFIED SCAIFEMETHOD
MODIEIED SUGARMETHOD 1
2
Av.
%
%
%
49.25 1.10 0.00 50.35 46.90 -3.45
49.25 1.10 0.00 50.35 47.00 -3.35
49.25 1.10
mality the speed of reaction with the silica was practically the same as that determined for the calcium sucrate solution of tenfold alkalinity. This CaClrCa (OH), concentration corresponds to a solution of high-grade lime by the Scaife method where the charge was almost neutralized by an addition of normal acid-5 cc. less than that required for
ABsoRpnoN
cc. USING0.05 N
I
%
Effect of Calcined Silica Gel upon Causticity of AqueousSugar and CaClc-Ca(OH)a-H20Systems
COMPOSITION
317
0.00
50.35 46.95 -3.40
1
% 49.13 0.85 0.00 49.08 47.75 -2.23
2
% 49.13 0.85 0.00 49.98 48.00 -1.98
Av.
% 49.13 0.85 0.00 49.98 47.88 -2.10
registered by the sugar method, as against 2.1 per cent by the Scaife method. The magnesium silicate fusion product runs parallel to the previously mentioned calcium silicate in showing a smaller solubility and causticity value by the sucrose solution than that recorded by the system created when carrying out the Scaife procedure.
INDUSTRIAL AND ENGINEERING CHEMISTRY
3 18
Effect of Calcination of a Lime-Serpentine Mixture, as Indicated by Causticity Determinations
It is apparent that the solubility of CaSi03 in water alone is almost identical with that found for water containing 5 per cent of sugar. The addition of both 1.0 and 0.5 per cent CaClz considerably depressed the solubility of CaSiO3. The Ca(OH)2 in the CaClz solution resulted in practically complete insolubility of the CaSiO3 addition. Conversely, the CaSiO3 addition caused a diminution in the original Ca(OH)2 concentration. The chemistry of this phenomenon is of
I n balancing the Ca0-Mg0-C02 content of certain dolomites, it has been found necessary to assume the presence of magnesium silicate. It seemed desirable to determine what effect would follow from the ignition of such dolomites. Serpentine (H4Mg3Si2Oe)was powdered and washed. A mixture of 10 per cent of this silicate and 90 per cent of Table VI-Effect
Vo1. 20, No. 3
of Additions of M a g n e s i u m Silicate Followed b y Calcination upon Causticity Values MODIFIED SUGAR METHOD
COMPOSITION OF CHARGE I A- Lime alone 90% B-Serpentine 'alone, 10% C-Actual determination D-Summation of A and B Variation of actual determination D from calculated value C
1
MODIFIED SCAIPE METHOD Av .
2
I
1
%
%
%
%
88.65 0.00 88.65 86.50 -2.05
88.65
88.65
88.43
88.65 86.50 -2.05
88.43 87.75 -0.68
lime was calcined, ground, and sifted through a 120-mesh sieve, as in the preceding experiments. The washed and ignited serpentine gave no determinable alkalinity when suspended alone in a 5 per cent sugar solution, nor by a Scaife-method control. Table VI shows the causticity value of the calcined CaO-serpentine mixture by both methods, as compared with that found for a corresponding charge of lime without serpentine. The causticity decreases of 2.05 per cent and 0.68 per cent by the modified sugar method and the modified Scaife method, respectively, may be attributed ((0the interchange between CaO and the magnesia of magnesium silicate during the period of ignition. These two experiments establish the fact that the presence of a large excess of magnesia over Si02 does not insure a complete tie-up between silica and magnesia during the ignition of Ca0-Mg0-SiO2 mixtures, and the further fact that both calcium and magnesium silicates are formed when a dolomitic lime containing silica is calcined.
0.00
0 00
88.65 86.50 -2.05
2
Av.
% 88.43 0.00 88.43 87.75 -0.68
0.00
% 88.43 0.00 88.43 87.75
-0.68
sufficient importance to warrant further investigation. It is apparent that not only the concentration of Ca(OH),, but also the total concentration of calcium ions, is a factor in the repression of the alkalinity induced by the CaSiO3. The experiment with CaSiOa was repeated with the lime of Table I, using both the modified sugar and modified Scaife methods. The usual time of contact, 20 to 30 minutes, was allowed. The results are given in Table VIII. Table VIII-Effect
of Additions of Precipitated a n d Ignited Cas108 upon Causticity D e t e r m i n a t i o n s
COMPOSITION OF CHARGE
Lime only Lime 5% CaSiOs Lime 1 0 7 ~CaSiOa
+ +
1 I
MODIFIED SUGAR METHOD 1
2
Av.
9i:s 91.8 91.3
.~ 91.5 92.3 91.5
9i:7 92.0 91.4
MODIFIEDSCAIPE METHOD Av.
1
9i:e 91.8 91.5
9i:s 91.5 91.5
9i17 91.7 91.5
It is evident that neither the 5 nor the 10 per cent addition of CaSiO3 had any effect upon the CaO value obtained by either of the two methods. These results are in apparent Calcium silicate may result from reactions between silica contradiction to those of Table VII, which were obtained by impurities and CaO during the process of calcination and the addition of the CaSiO3 to solutions of CaClz-Ca(OH)z. may also be formed readily when finely divided silica is It was assumed that the difference in time of contact was suspended in aqueous solutions of Ca(OH)Z, hence it was responsible for this apparent discrepancy, since in the exessential to determine the effect of calcium silicate upon the periment with the pure Ca(OH)z solutions the contact procedures followed in the two methods. Calcium silicate period was 24 hours, whereas with the lime charges it was was prepared by the addition of a solution of calcium chlo- only 30 minutes. The residual lime solutions were therefore ride to one of sodium silicate. The silicate was washed, again titrated after 24 hours, and it was found that the soliddried, and ignited. Two charges, 0.070 and 0.140 gram, phase CaSiO3 had brought about a decreased concentration were shaken with 500 cc. of each of the following media: of Ca(0H)z during the longer period. This proved true for water, 5 per cent aqueous sugar solution, 0.5 per cent and both the modified sugar and modified Scaife methods. The 1.0 per cent CaClz aqueous solutions, and the same CaClz resultant decreases in values were from 1.0 to 1.5 per cent of solutions plus sufficient calcium hydroxide to give an alkalin- the charge. It may therefore be concluded that the presence ity of 0.095 N . The systems were shaken 1 hour and then of CaSi03 in the lime sample has a tendency to produce a allowed to stand for 24 hours. Aliquots of 100 cc. of the minus error by both methods. When contact extends for clear solutions were then titrated with 0.05 N HC1 and the only 20 to 30 minutes the error attributable to calcium alkalinities expressed as per cent of the lime charge, as in silicate is negligible. Since the magnitude of this error depends upon the period of contact, further emphasis as to Table VII. the desirability of insuring rapidity through filtration is thus Table VII-Solubility of Pre ared a n d Ignited CaSiOs in Aqueous, obtained. CaCh. a n d 8aClr-Ca(OH)r S y s t e m s Because of the decided reduction in the Ca(0H)z concen(Expressed as CaO in a hypothetical charge of 1 . 4 grams) tration caused by the presence of added prepared calcium I 0.0095 N Ca(OH)r silicate during an extended period, there arises the question CONTAINING:' 5% SUGARIN 1 R IN 0.5% CaSiOa IN KzO INSOLUTION of the actual composition of the calcium silicate. The CaClz CaClz 1.0% 0.5% phenomenon may be due to: (a) the absorptive properties CaC12 CaClz of the pulverulent CaSiOs; (b) reaction between Ca(OH)2 Gvam % % % % I % % and any slight excess of uncombined SiOz in the prepared None ... ... ... 9.4 9.5 product; or (c) the formation of dicalcium silicate upon 0.070 0.4 0.40 0.3 0:30 8.4 8.8 0.45 0.4 0.35 7.7 8.3 0.140 0.5 standing. Effect of Calcium Silicate
I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1928
319
Influence of Impurities of A1203,Fe203,and CaSO. upon Causticity Determinations
It may be that the CaO so combined is of value under some conditions, but the object of the methods proposed is the Alumina, ferric oxide, and calcium sulfate are present determination of uncombined CaO. The effect of calcium in varying quantities in all limes. It has been shown10 aluminate per se upon the two analytical procedures was that after Ca(OH)z has reacted with A1203 and Fe203to form therefore studied by means of 2, 5, and 10 per cent additions. The results of Table X show that some causticity is derived 3Ca0(AlZ03) and 3CaO(Fe203), respectively, further reaction with Cas04 ensues to form 3Ca0. A1203.3CaS04.- from the hydrolysis of the added calcium aluminate. It 33H20 and 3Ca0. FezO3. 3CaS04. nH20. The taking up further appears that the increase in causticity found by the of Cas04 to form the ternary systems does not affect the sugar method is in proportion to the amounts of calcium amount of Ca(OH)2fixed, but it produces a crystalate which aluminate added, whereas with the Scaife method the differhas properties different from those of calcium aluminate. ence between the increase from the 2 and the 10 per cent The conditions imposed were not the same, however, as additions was only 0.3 per cent. Again, however, the inthose encountered in carrying out the determination of creases in causticity found by the two methods are practically the same for the 2 per cent addition, which would caustic values by either of the methods. represent the maximum calcium aluminate occurrence for a Some reaction undoubtedly occurs between CaO and A1203 during calcination of limestone, and it is very probable that commercial product. most, if not all, of the alumina content of a commercial lime Table X-Effect of Additions of C a l c i u m A l u m i n a t e upon Causticity will have combined with the excess of CaO during that Determinations process. MODIFIED SCAIFE MODIFIED SUGAR METHOD METHOD Aluminates so formed are stable in concentrated solutions COMBINATION of Ca(OH)2, but they readily hydrolyze in concentrations InIn1 2 Av. crease 1 2 Av. crease of less than 0.01 normality. Consideration was therefore given to the effect produced by such compounds while % % % % % carrying out the two analytical procedures with both high- Lime alone 91.8 9 1 . 5 91.7 . . . 91.6 91.8 91.7 . . . Lime + 2~oCaAIz04 9 2 . 5 9 2 . 4 9 2 . 5 0 . 8 9 2 . 8 9 2 . 0 9 2 . 4 0 . 7 calcic and high-magnesic limes. . .. 9 3 . 8 9 3 . 8 2 . 1 9 2 . 7 9 2 . 5 9 2 . 6 0 . 9 Lime +5%CaAlzOd
I %
Lime
Table IX-Effect
SAMPLE
High-rnagnesic
+ 10% CaAlz01 ...
96.3 96.3 4.6
93.0 92.4
92.7
1.0
of Additions of A1203 upon Causticity
A1203 AnnF.n
Determinations
II
MODIFIED SUGAR METHOD Found
Sone
54.2
5.0 10.0
52.0 51.2
Decrease
11
MODIFIEDSCAIFE FILTRATES Found
2.2 3.0
Decrease
54.4
...
53.4 52.7
1.0 1.7
The vitiating influence exerted by 2.5, 5.0, and 10.0 per cent additions of A1203 upon causticity values is shown in Table IX. A definite decrease for each addition was registered for each type of lime by both methods. The decrease caused by the 2.5 per cent A1203addition to the high-calcic lime was 0.7 per cent and 0.6 per cent for the modified sugar and modified Scaife methods, respectively. Corresponding decreases of 1.7 and 1.1 per cent were obtained with the same lime by the two methods when the A 1 2 0 3 addition was 5 per cent of the charge, whereas still further respective decreases of 2.7 and 1.9 per cent were caused by the 10 per cent A l 2 0 3 additions. Similar diminutions in causticity were also obtained from the high-magnesic lime by both procedures. An A1203 occurrence of 2.5 per cent is, however, more than would be expected in commercial limes, and the results from the two large additions may therefore be considered mainly as of academic interest. It is, nevertheless, plain that the full potential causticity of a lime having a high content of A1203 cannot be determined accurately by either of the two methods. It is also evident that the kiUing off of the larger part of the Ca(OH)2before filtration, as in the Scaife method, results in a diminished reaction between calcium hydroxide and alumina. This was true for both types of lime. Effect of Calcium Aluminate Present before Analysis
It has been pointed out that calcium aluminate may be present in the original lime as a result of calcination reactions, lo
%I%
MacIntire and Shaw, Soil Scrcncr, 19, 128 (1925).
The foregoing results indicate that the added calcium aluminate underwent considerable hydrolysis in the two analytical procedures. The solubility of the added calcium aluminate was, therefore, determined for (a) aqueous, (b) 5 per cent sugar, (c) neutral CaC12 solutions, and (d) CaC12 solutions containing Ca(OH)2to the extent of 0.0095 normality-a value corresponding to that of the Scaife method after the preliminary addition of hydrochloric acid. The results in Table XI show that the solubility of the aluminate is in approximate proportion to the charge, for the pure-water solution, the 5 per cent sugar solution, and the neutral CaClz solution. There was only a slight hydrolysis in either the 0.5 or the 1.0 per cent CaClz solution in 0.0095 N Ca(OH)2 and no marked increase in the hydrolysis was induced by doubling the amount of calcium aluminate. This indicates a repression of hydrolysis of the aluminate and a diminished dissociation of the hydroxide as a result of the excess of calcium ions in the chloride and hydroxide combination. Table XI-Effect of Certain Concentrations of C a ( 0 H ) t a n d CaClt upon the Solubility of C a l c i u m A l u m i n a t e , in T e r m s of CaO
CaAlz04 ADDED
Gram None
0.0700 0.140b
IN WATER
$2 2__ ____-
%
%
20:5 . 3
205: . 5
AQUEOUS SOLUTION Per cent CaClz in solvent
I1 20:%; % I1 0.5
1.0
0.5
1.0
%
%
3.3
y:: 2.5
9.5 9.8 10.0
9.3 9.7 9.7
5 per cent of charge. b 10 per cent of charge. e Simulating usual alkalinity and CaCt content of solution at end of Scaife method titrations of high- and low-calcic limes. a
Correction In my article entitled “Bromocresol Green as an Indicator in the Manufacture of Grain-Curd Casein” [Id.Eng. Chem., 20,15, (1928)] the next to the last line in the second paragraph should read “(0.05 per cent solution)” instead of “(0.5 per cent solution).” ANNE G. BENTON
