1844
ANALYTICAL CHEMISTRY
traction units can analyze two samples in 1.5 hours of over-all time. This figure includes the time required to clean up the apparatus for the next determination. The operator time required is about 40 minutes, which also includes cleaning time. Nevertheless, the method is not as rapid as the U-tube method, since the same nontechnical operator can do six determinations in an over-all time of 2.5 hours, of which only 50 minutes is operator time. However, results by the direct extraction method are much more accurate. Some samples, after standing for several days, required the use of compressed air to force the bulk of the carbon tetrachloride solution through the fritted glass. Duplicate samples analyzed within a few hours of receipt gave no difficulty a t all. It is therefore recommended that samples be analyzed as soon as possible after they are received. LITERATURE CITED
American Petroleum Institute, “Disposal of Refinery Wastes,” 4th ed., Sect. 1, Appendix 5, 1949.
(2) American Petroleum Institute, “Manual on Disposal of Refinery Wastes,” Vol. IV, 1st ed., pp. 81-8, 1952. (3) American Public Health Association, “Standard Methods for the Examination of Water and Sewage,” 9th ed., p. 42-3,1946. (4) Headington, C. E., ANAL.CHEM.,25, 1681 (1953). (5) Kirschman, H. D., and Pomeroy, R., Ibid., 21, 793 (1949). ( 6 ) Lipkin, M. R., Davison, J. A,, Harvey, W. T., and Kurte, S. S., Jr., IND.ENG.CHEM.,AKAL.ED., 16,55 (1944). (7) Melpolder, F. AI., Warfield, C. W.,and Headington, C. E , ASIL. CHEM.,25, 1453 (1953). (8) llusante, A. F. S., Ibzd., 23, 1374 (1951). (9) Noll. C. A. and Tomlinson. W. S.. IND.ENG.CHEM, L hED.. ~ ~ . 15,629 (1943). (10) Scott, W. W,, “Standard Methods of Chemical Analysis,” Vol. 1 1 , 5th ed., p. 2078,New York, D.Van Nostrand Co., 1939. (11) Simard, R. G., Hasegawa, I., Bandaruk, W., and Headington, C. E., ANAL.CHEW,23, 1384 (1951). (12) Snell, F. D., and Biffen, F. M., “Commercial Methods of dnalysis,” p. 253,New York, McGraw-Hill Book Co., 19+4. RECEIVED for review May 6, 1953. Accepted August 28, 1953. Presented before the 18th Midyear Meeting of the American Petroleum Institute, Group Session on Analytical Research, New Yorh, May 1953.
Uncombined Calcium Oxide or Hydroxide in Lime and Silicate Products Volumetric Determination GUNNAR 0. ASSARSSON AND JEAN M. BOKSTRORI Chemical Laboratory, Geological Survey of Sweden, Stockholm 50, Sweden The methods for the volumetric determination of uncombined calcium oxide and hydroxide extracted from lime and silicate products by glycerol, ethylene glycol, and acetoacetic ester have been studied. The best results are obtained by conductometric titration with a strong acid. Reasonable values can also be obtained using an indicator: methyl red for glycerol and ethylene glycol extracts and bromophenol blue for the acetoacetic extracts. When an accelerator such as strontium nitrate is used in the glycerol extraction, the only convenient method is titration with an indicator.
0
NE of the most important determinations in the lime and
cement industry concerns the percentage of lime occurring as calcium oxide or hydroxide in the products, the so-called free or uncombined lime. Much effort has been expended in attempts to find suitable methods for this determination. h summary is found in modern handbooks on the lime industry (3). In connection with some investigations on pozzuolanas the authors required a tolerably accurate method for the determination of uncombined lime, but after a scrutiny of the methods described in the literature and after some experiments it was found that some of them are unsuitable and most of them need improvement. Thus the first step in this investigation was to find the conditions under which the determinations were sufficiently accurate and the analytical work could be best adapted. There are two chief groups of methods. The first involves the measurement of ignition loss (determination of calcium hydroxide) or of the heat of hydration of an anhydrous or an ignited sample a t well-defined humidification conditions. The second group includes methods based on an extraction of the uncombined lime with a suitable solvent. The former methods cannot be used in the presence of kinds of pozzuolanas usually encountered and are not discussed here. The extraction methods are suitable only when the solvent is sufficiently selective, completely dissolving calcium oxide or hydroxide without decomposing hydrated silicates or aluminates. Several solvents have been proposed and investigated. The
most acceptable are glycerol, acetoacetic ester, and ethylene glycol. The extracted lime can be determined volumetrically or gravimetrically, but in view of the difficulties encountered during the precipitation of calcium from organic solvents, volumetric methods have been exclusively employed. The problems involved are of such embracing nature that it was necessary to limit the scope of this investigation. An examination of the volumetric determination of calcium oxide or hydroxide dissolved in the solvents has now been undertaken, while the actual extraction methods for pozzuolanas must be treated separately. REAGENTS AND APPARATUS
The titration solutions used in the experiments described were: Hydrochloric acid (aqueous solution), 1 ml. equivalent to 10 mg. of calcium oxide. Sulfuric acid (ethyl alcohol solution), 1 ml. equivalent to 10 mg. of calcium oxide. Benzoic acid (ethyl alcohol solution), 1 mg. equivalent to 1 mg. of calcium oxide. Ammonium acetate (ethyl alcohol solution), 1 ml. equivalent to 2 mg. of calcium oxide. These solutions were calibrated in aqueous solution according to standard methods-that is, they were not standardized by carrying out a control titration of a known amount of calcium oxide under the conditions of the determinations. Usually a microburet was used for the acids.
V O L U M E 2 5 , NO. 1 2 , D E C E M B E R 1 9 5 3 The concentrations of the indicators used were: Methyl red, 1%. Mixed indicator of equal parts of 1% phenolphthalein and 1.5y0 naphtholphthalein. Bromophenol blue, 1%. The titrations were followed bv two physicochemical methods: conductometric titration and titration under measurements of the potential difference between the solution and an antimony electrode. The electrical conductivity of the solution was measured with the aid of a commercial resistance instrument (Philoscop). The measuring instrument was used together with a frequency modulator for very high resistances. The resistance determination was accurate within about 1% of the measured value, which was sufficiently accurate for the purpose. No thermostat for regulation of the temperature of the reaction liquid was used. A s the conductivities of the different solutions varied over a considerable range, i t was found necessary to employ three resistance cells with constants 1.2, 2.5, and 3.5 reciprocal ohms. The resistance measured (reciprocal of the conductivity) is plotted in the lower half of Figures 1, 2, and 3. For the measurement of the potential difference between the antimony electrode and the solutions a saturated calomel electrode was used a~ reference electrode. The potential difference was converted into normal p H values and is referred to hereafter as pH. Glass electrodes as well as a hydrogen electrode [palladium (IO)] were also tried, but they became poisoned rapidly by the organic solvents used. The antimony electrode, on the other hand, retained its correct potential during a titration under the conditions involved. Before i t WRS used it -a-as cleaned by gentle polishing with fine emery paper. If absolute values are required, the surface must be kept clean-e.g., by rotating brushes-but for this investigation this was not necessary. As the proportion between the solvents in the reaction mixture cannot be kept constant, an accuracy greater than 3~0.02volt has no significance. The calomel electrode was connected with the solution to be titrated by an agar-agar saturated pqtdssium chloride siphon. The reaction mixture was thoroughly stirred by a mechanical stirrer. A magnetic stirrer cannot be used because of the influence of its electromagnetic field on the high-frequency current in the conductivity measurements and on the pH measurement instrument and its antimony-calomel electrodes. With certain indicators no sharp change of color was obtained, and when this was the case the color WRS compared with that of a reference solution. EMLEY METHOD, DENNIS MODIFICATIOh
The uncombined calcium oxide or hydroxide is extracted with a boiling mixture of glycerol and ethyl alcohol and the boiling solution is tit rated with ammonium acetate, using phenolphthalein as indicator. Since the first publication of the method (6) some modifications have been described (1, 2 , 8, 9). One of the inconveniences of the method is that the rate of extraction of the calcium oxide is very slow and that of the calcium hydroxide is still slower. To avoid this, Brandenburg ( 4 ) recommended the addition of an acid-base neutral salt (barium chloride), which by ion exchange could form a more suitable glycerate. Dennis (5) investigated a large number of such possible accelerators and considered strontium nitrate to give the best results. Accurate values can be obtained by this method only if the experimental conditions are ideal, which is often not the case. The procedure vias carried out in accordance with the directions of Lerch and Bogue (8) and Dennis (5), except that the ammonium acetate solution was standardized by analysis, and was not titrnted against calcium oxide under the experimental conditions. Some values from the titrations are listed in Table I. The method has some disadvantages: The solution must be I)oiled to remove the ammonia; the change of color of the indicator is dependent on the temperature of the solution and fading also occurs as the end point is approached; the ammonium acetate must be added successively with controlled boiling for some minutes between the portions added; and the values are a t least 5 t o 6% too low. The method has some advantages: The extraction of calcium oxide is fairly rapid and it is not always necesEary to remove the extracted residue. I t is not possible to have an independent control of the method in any way other than a repeating of the determination. The
1845
T a b l e I.
Emley-Lerch-Bogue-Dennis Method"
Calcium Oxide, Mg. Recovered % 39 4 95 42 9 95 41.5 93 29 37 9 .. 9 . _ ..R_ .5. a Titration of calcium oxide dissolved in glycerol-ethyl alcohol in the presence of 2 grams of strontium nitrate. titration solution, ammonium acetate in ethyl alcohol: indicator, phenolihthalein. Weighed 41 5 45 1 44 4
Table 11. Glycerol-Ethyl Alcohol Extraction w i t h S t r o n t i u m h-itrate Acceleratora Acid HC1 HC1 HCl HCI
H9901
Indicator &Ib hl hl
hl hI hI hl
Calcium Oxide, Mg. Present Found 31.8 32.5c 62.4 60.3: 40.1 39.3 49.3c 50.0 36.4c 38.1 48.6; 49 2 57.7 61 6 43.4c 46.4 33.0 24.ge 29.2e 36 4
HCI HCI HCl 11 HCl Df Benzoic D acid Glycerol-ethyl alcohol ratio of 1 to 4 (volume). b Methyl red. With comparison solution. d With comparison solution and addition of calcium chloride. Without comparison solution. f Mixed indicator.
Error,
%
+ 2
-
3 2 1 4
- 1
- 6 - 6 - 25 - 20
erior in the obtained values, however, seems to be constant; i t cannot be due to impurities in the chemicals used, which are of analytical reagent standard and which give correct values in determinations performed in other ways. The error must arise from the failure of the theoretical end point of the reaction of calcium glycerate and calcium acetate to coincide with the color change of red-violet to colorless under the given conditions. The ammonium acetate solution could be calibrated by carrying out control experiments with known amounts of calcium oxide. In order to obtain correctvaluesthetitrationsmust beperformed under identical experimental conditions, such as the concentration of calcium oxide in the extract and the proportion of ethyl alcohol-glycerol, so that the method is thus a purely empirical one. This method would be more useful if another indicator were used, if boiling of the solutions could be avoided during the titration by cooling down the extracts to room temperature, and if another titration solution could be substituted for the ammonium acetate. OTHER METHODS BASED ON GLYCEROL EXTRACTION
The other methods employing glycerol as extraction solvent differ from Emley's original method in the use of acids for the titrations instead of ammonium acetate. Rathke (11) and Schindler ( l e ) used tartaric acid and Bessey used (1, 2 ) benzoic acid, both acids in ethyl alcohol solution with phenolphthalein as indicator. I t is, of course, possible to extract the calcium oxide or hydroxide with or without accelerator, and the titrations can be performed at room temperature. They can be determined by other methods (pH and conductivity measurements). Since a decomposition of calcium silicate, aluminates, or their hydrates in the sample could be brought about by an excess of the titration liquid, the residue must be filtered off after the extraction. Extraction with Accelerator. Samples used for testing the method were 0.030 to 0.050 gram of calcium oxide. The extraction mixture consisted of 2 grams of strontium nitrate, 20 ml. of glycerol, 30 ml. of ethyl alcohol; 105 ml. of ethyl alcohol was used for washing and diluting the extract afterwards. The indicators used were the mixed indicator phenolphthalein-naphtholphthalein as described by Schlapfer-Bukowski (IS), and methyl red. The pH curves (Figure 1) show that the potential between the antimony and the calomel electrode is lowered successively with increasing amounts of acid. The curves are shaped as a double
1846
ANALYTICAL CHEMISTRY
wave when hydrochloric and sulfuric acids are used, and the end points of the reaction are a t the passage between the two parts of the double wave, about a t the same pH measured. The double wave is very marked, and it recurs in other experiments. The benzoic acid gives only a slow lowering of the pH with an estended single wave, and the true neutralization point is far from the inflection point of the curve. 12
-
m/ uc/Y
-
I -
Figure 1. Titration Curves of Glycerol Extracts Left.
Right.
Extraction with strontium nitrate accelerator Without accelerator A . Hydrochloric acid B. Sulfuric acid C. Benzoic acid T . Calculated end point of reaction
The conductivity measurements gave no distinct information concerning the neutralization point because of the large amount of accelerator present in the solutions (Figure 1). If a more sensitive bridge were used for the resistance determination it would probably be possible to find the neutralization point, but much more rigorous control of the experimental conditions vould then be necessary. Since the measurement of pH can also lead only to an approximate result, the methods suitable for the determination are limited to those in which the titration is carried out with an indicator. The results obtained with different indicators were not in complete agreement with the results mentioned in the literature. In Table I1 are recorded some of the results from these determinations. Only one of the indicators examined, methyl red, showed a distinct change of color close to the theoretical end point of the reaction when hydrochloric or sulfuric acids were used. This indicator can therefore be used in the determinations; it is not sensitive to benzoic acid in the solvents used. The mixed indicator as well as phenolphthalein showed considerable errors with the strong acids as well as with benzoic acid (10 to 20% too low). Extraction without Accelerator. This method has the disadvantage that calcium oxide or hydroxide dissolves slowly. The substance must be treated repeatedly and the completeness of the extraction controlled carefully, with the result that the analysis is laborious and time-consuming. However, some determinations were made by this method for comparison. The titrations were performed as described above and the change of color of the indicators was studied. The titrations with hydrochloric and sulfuric acids were followed by pH measurements. The same double wave curve found earlier is observed, and the end point of the reaction of calcium glycerate and sulfate or chloride is a t the part of the curve between the two waves (Figure 1). The titration q-ith benzoic acid shows a single wave which is rather extended, and because of this the inflection point is difficult to determine. The end point of the reaction of calcium gIycerate and benzoate, however, is far from the inflection point of the curve.
The conductivity measurement curves are of two types (Figure 1). Hydrochloric acid gives a curve, having two branches, and
shorving a conductivity increasing with increasing acid added. The break between the branches is distinct and corresponds exactly with the end point (Table 111). The sulfuric acid Ion-ers the conductivity because of the low solubilityof the calciumsulfate formed. When the neutralization point is passed, the conductivity increases rapidly. The end point of the reaction appears as a very sharp peak on the conductivity curve, corresponding exactly to the calculated value. Benzoic acid, too, has a conductometric curve with two branches, between which the reaction end point appears as a break. The agreement between the calculated and the found values showed that no objections could be raised concerning the quality of the chemicals. The indicator methyl red also gave the best values here, while the mixed indicator and phenolphthalein gave results which were 10 to 30% too low. If this estraction method is to be used, the most accurate determination is accomplished by a conductometric titration with sulfuric acid. Methyl red is the best indicator, and if the change of color is used for finding the neutralization point the most accurate procedure appears to be that described below. Extract the sample for 1 hour under reflux conditions with 50 nll. of glycerol-ethyl alcohol mi-iture (ratio of 1 t o 2 parts by volume) in a 100-ml. round-bottomed flask heated on a steam bath or in a heating jacket. Cool, filter, and wash with small portions of ethyl alcohol. As the extracFigure 2. Titration tion is not always complete Curves of ;icetoacetic after 1 hour, retain the residue Ester-Isobutyl Alcohol for repeating the extraction. Extracts Titrate the solution with hyA . Hydrochloric acid drochloric acid (1 ml. equivaB . Sulfuric acid lent to 10 mg. of calcium oside) C. Benzoic acid using a microburet and methyl T . Calculated end point of reaction red as indicator. To obtain the exact end ooint. compare the sample sohtion nit11 a solution containing the same portions of ethyl alcohol, glycerol, and methyl red together with 0.05 ml. of the same hydrochloric acid. Titrate the sample solution to the same color as the comparison solution. Then add to the comparison liquid a calcium chloride solution containing the weight of calcium chloride calculated to be present in the titrated solution and having approximately the same volume as the h drochloric acid added. Continue the titration of the sample sotktion until it is the same color as the comparison solution. Read the volume of hydrochloric acid consumed by the sample and correct for the 0.05 nd. in the comparison solution. The
Tahle 111. Glycerol-Ethyl Alcohol Extraction without Accelerator Calcium Oxide, >I& Titration with Conductometric Indicator Titration Arid HC1 H&04
HC1 HC1 HCl HC1 Benzoic rlcid b
Indicator Db Db
Present 34.7 45.3 30.8 32 10 . 59
Db Db
25.6 38.0
M a M 5
Vethyl red. Mixed indicator.
Found 32.9 43.3 20 8 11.2 2.5.9
Error,
%
-
5 0
-32 -32 -18
19.8
-22
34.0
-11
Found 34.4
...
...
3.2...5
%:E
Error,
“0
1
,..
...
... +3
5,:
V O L U M E 2 5 , NO. 1 2 , D E C E M B E R 1 9 5 3
1a41
error seldom exceeds 1% of the correct calcium oxide value. If an accuracy of about 3 ~ 3 %of the calcium content is sufficient, the corrrection for the calcium chloride dissolved in the sample solution can be omitted. For control, repeat the extraction of the retained residue until calcium can no longer be extracted. EXTRACTION WITH ACETOACETIC ESTER AND ISOBUTYL ALCOHOL
The instructions given by Franke ( 7 ) were followed. The sample was placed in a flask in an electrically heated jacket and extracted under reflux conditions for 1 hour with a mixture of acetoacetic ester and isobutyl alcohol. The solution was cooled, filtered, and washed a i t h isobutyl alcohol, and the filtrate was titrated with hydrochloric acid using bromophenol blue as indicator. It was observed during the investigation that the calcium compound of acetoacetic ester was readily deposited on the walls of the flask during boiling. The indicator has a rather indefinite and faint color change, so that there was some difficulty in discerning the exact end point of the reaction. I n some of the determinations ethyl alcohol was used for washing and diluting the filtrated solutions without any disadvantage. The pH curves for the titration with hydrochloric and sulfuric acids show a single wave, the inflection points of which are very near the end point of the reaction of the organic calcium compound with inorganic salt (Figure 2 ) . The benzoic acid gives a curve of very slight slope nithout any inflection point. The conductometric titrations Kith hydrochloric and sulfuric acids show very distinct breaks at the neutralization points quite as was described with regard to the glycerol-ethyl alcohol estraction. The benzoic acid curve has no break (Table IT). Although the indicator bromophenol blue recommended by Franke has an indefinite color change, the reaction end point was normally found t o be within 2% of the theoretical value. The other indicators gave no color change. The conductometric titration gives accurate results and takes only a little more time than the indicator titration, especially if the greater part of the solution is first titrated approximately with bromophenol blue and only the final titration up to the end point of the reaction followed by conductivity measurements.
Table IV.
Extraction with Acetoacetic Ester and Isobutyl AlcoholR Calcium Oxide, hlg. Titration with Conductometric Indicator Titration Found Error, % Found Error, %
IndiPresent cator Acid 50.9 -2 HCI B5 51.8 51.5 -0 6 51.8 -2 51.5 -2 HCI B5 52.8 33.6 -1 84.0 0 HCl Bb 34.0 41.7 0 41.7 0 HC1 Bb 41.7 35.0 -1 35 6 4-0.5 H&Or B5 35.4 Benzoic Bb 32.9 S o color changeb Indeterminable acid H&Or DC 37.3 Nocolorchangec 37.3 0 a Titration solution 6 rnl. acetoacetic ester diluted with 100 rnl. isobutyl alcohol. b Bromophenol blue. C Mixed indicator.
EXTRACTION WITH ETHYLFhE GLYCOL
Schlapfer, Bukowski, and Esenwein recommend the extraction of calcium oxide and hydroxide with pure ethj lene glycol a t 75" to 85" C., the extraction being complete after 30 minutes ( I S , Id). I n some of the experiments pure ethylene glycol was used, but extractions were also attempted with ethylene glycol diluted with methanol, ethyl alcohol, and ethylene glycol monoethyl ether in order to try the use of a heating jacket instead of the 75" C. bath. Khen diluted ethylene glycol was used, calcium oxide, but not calcium hydroxide, could be dissolved completely within reasonable time. All the diluted solutions, however, showed that the solubility of calcium glycolate has a negative temperature coefficient.
Table V.
Acid HCl HCI His01 HCI HCI HCI HCI HzSOI HzSOI
Indicator
Extraction with Ethylene Glycol
Present 35 0 36 7 42 9 42 1 57 8 34 0
36 7 35.1 35 5 43.5 34 4
Benzoic acid a Methyl red. b Ethylene glycol extract C Ethylene glycol extract d Ethylene glycol extract e RIixed indicatnr. I Ethylene glyrol extract Pure ethylene glycol.
--
Figure 3.
.
Titration Curves of Ethylene Glycol Extracts
A f B and C diluted with 96% ethyl alcohol; D diluted with a&soiute ethyl alcohol; E pure ethylene glycol; F diluted with water 1 t o 1 A . Hydrochloric acid B, D , E, F. Sulfuric acid C. Benzoic acid T. Calculated end point of reaction
Calcium Oxide, Mg. Titration with Conductometric Indicator Titration Error, Error, Found % Found % 33.5b - 4 ... ... - 3 35.6: 0 36.8 - 3 -1.5 42.2 41.6 32.9d 22 ... - 9 +0:3 58.0 52.61, 12 33.8 30.0 -0.6 3.5 4 Q - 3 ... ... - 10 31.46 ... ... 10 -1.5 31.8; 35.0 39.9 43.8 - 8 +0.7 31.0b 10 ... ...
-
-
diluted 1 t o 4 with ethanol. diluted 1 t o I with ethanol. diluted 1 t o 1 with water. diluted 1 t o 0 . 5 with ethanol.
After the extraction with pure ethylene glycol the solutions may be filtered and diluted with one of the solvents mentioned. Therefore, in cases where the sample contains calcium hydroxide, the solvent should be pure ethylene glycol and the extraction should be performed a t about 75" C. The ethylene glycol used in these experiments was an Eastman Kodak product labeled "For Chemical Purposes" (9). The pH curves for hydrochloric and sulfuric acids show the double wave (Figure 3), earlier mentioned in connection with the glycerol extraction. The end points of the reaction of calcium glycolate to chloride or sulfate are on the lower part of the second wave. As this curve branch has a steep slope, the position of the end point of the reaction can be determined rather accurately. The titration with benzoic acid gives a rather extended single wave curve with the theoretical end point of the reaction far from the inflection point of the curve. The glycerol extracts as R-ell as the ethylene glycol extracts have pH titration curves containing a double wave. A brief
ANALYTICAL CHEMISTRY
1848 investigation into the cause of this phenomenon was made. A solution containing pure glycerol is viscous and difficult to handle, while the ethylene glycol solutions are more suitable for experiments. Some titrations were therefore performed in ethylene glycol solutions, since it was assumed that the characteristics of the glycerol and ethylene glycol curves probably have the same explanation. The results and the curves from these titrations are given in Table V and Figure 3. Two titrations were performed in pure ethylene glycol; for one of them hydrochloric acid (aqueous solution) was used, for the other sulfuric acid (ethyl alcohol solution) was used. These two titrations gave the same curve; a single wave of the same shape (only one of them is drawn in Figure 3). They contain, however, a very slight flexure in the middle of the Rrave, an indication of the double wave. The kind of acid and the small amount of water and ethyl alcohol added during the titration have no significance for the shape of the curve. Another titration was carried out after dilution of the ethylene glycol extract with absolute ethyl alcohol. The curve obtained has a very marked double Aave. The titration curve after dilution with 96% ethyl alcohol has a double wave with a less marked part between the two waves. The curve after dilution of the extract with an equal part of water also shows a double wave. The part of this curve a t lower pH values has a less steep slope, and there is a large volume of acid required between the point for the color change of the methyl red indicator and the theoretical end point for the reaction. This shows the importance of defined experimental conditions. Similar curves containing double waves were obtained when titrations of glycerol-ethyl alcohol extracts were followed by measuring the pH with a glass electrode instead of the antimony electrode. From these experiments it can be concluded that the double wave is caused by the dilution of the ethylene glycol solution, which apparently involves a partial decomposition of the calcium glycolate. However, this explanation cannot beaccepteddefinitely until new experiments are conducted which do not lie within the scope of this investigation. The conductometric titrations gave curves similar to those described above in connection with the glycerol and acetoacetic ester extracts. Hydrochloric and sulfuric acids gave a distinctive end point for the reaction (Figure 3). The indicators showed very different points for the change of color. Methyl red gave very good values with hydrochloric or sulfuric acid when the solutions were in pure ethylene glycol or were diluted uith ethyl alcohol; a dilution with water caused considerable errors. The mixed indicator, which is recommended by the authors of the original description of the method ( I S , 1 4 , showed correct values when ethylene glycol alone was the solvent, but showed values which were 10 to 20% too low if the solution was diluted with an equal part of ethyl alcohol. The mixed indicator gave an indistinct change of color with benzoic acid after dilution with ethyl alcohol; when the extract was diluted with water the color change was highly dependent on the proportions in the mixture. If this extraction method is to be used, the best results will be achieved if pure ethylene glycol is used for the extraction and the titration is performed conductometrically with hydrochloric or sulfuric acid after thorough cooling and dilution of the extracts with ethyl alcohol. It is also possible to obtain reliable values correct within 1 to 2% of the dissolved calcium compound by USing methyl red as indicator in the extracts diluted with ethyl alcohol. A correction for the calcium chloride formed during the titration can be applied as described in connection with the titration of the glycerol-ethyl alcohol extracts. DISCUSSION OF RESULTS
4 method in practical use should give sufficiently accurate results, be easy to handle experimentally, and not be too tinie-con-
suming. When several methods exist, their advantages and drawbacks must be taken into consideration from these points of view. The greatest degree of accuracy in the calcium determination which could be achieved in the experiments described above was obtained by conductometrical titration of the ethyl alcohol-diluted extracts. It is absolutely objective, without errors of the kind which occur, for example, in the indicator titrations. The best titration fluid used is sulfuric acid in a solution of ethyl or any other alcohol. The low solubility of the calcium sulfate formed increases the control of the end point of the reaction of organic calcium compound with the acid added, so that this point can be established very distinctly and the titration is very easy to perform. The titration can be applied in the majority of cases. In the method where the substance is extracted with glycerol in the presence of an accelerator, the conductivity measurements are not suitable, however. The end point of the reaction must then be referred to the change of color of an indicator. Only a few indicators were examined in connection with this investigation, but a suitable one must be chosen carefully because its color change is dependent on the properties of the solvents. Some of those recommended in the literature do not meet the requirements, particularly when water is used for the dilution. The best indicator for the titration of the glycerol-ethyl alcohol extracts containing strontium nitrate seems to be methyl red, giving an end point very close to the theoretical one. The extracts from the other methods can be titrated conductometrically very rapidly and accurately, but indicators may beused in these cases for approximate determinations and for following the reactions during the titration. The pH measurements cannot generally be used for direct determinations, as they do not give accurate values but only an orientation with regard t o the end point. The methods described are generally easy to perform. The extraction with glycerol without the accelerator seems to be more difficult and more time-consuming than the others. The extraction in the presence of the accelerator is preferred, if it can be shown that the accelerator does not influence the calcium silicate hydrates or aluminates considerably. However, it would appear to be advisable to titrate the filtered cooled extracts with a strong acid and methyl red as indicator, instead of titrating the boiling extract with ammonium acetate solution. ACKNOWLEDGMENT
The authors wish to express their gratitude to the International Ytong Co. of Stockholm for a grant which made possible a wider investigation in connection with the technical research. LITERATURE CITED
(1) Besszy, G. E . , D e p t . Sei. >\led. Research, Building Research Tech. P a p . , KO. 9 (1930). (2) Bessey, G . E., J . SOC.C'hem. Ind. ( L o n d o n ) , 52, 219 (1933). (3) Bogue, R. E., "The Chemistry of Portland C e m e n t , " S e w York, Reinhold Publishing Corp., 1947. (4) Brandenburg, H. R . , Rock Products, 32, 76 (1929): 34, 68 (1931). (5) Dennis, J. W., Ibid., 41, 43 (1938). (6) Emley, W.E., T r a n s . Am. Cerninic Soc., 17, 720 (1915). (7) Franke, B., 2. anorg. ZL. allgem. Chcm., 247, 180 (1941). (8) Lerch, W., and Bogue, R . H., Ind. Eng. Chem., 18, 739 (1926); IND. ENG.CHEM., . h h L . ED.,2, 296 (1930). (9) RlacPherson, D. R., and Forbrick, L. R., Ibid., 9, 457 (1937). (10) NylBn, P., Svensk Kern. Tidskr., 48, 77 (1936). (11) R a t h k e , H., Tonind. Ztg., 32, 1318 (1928). (12) Schindler, K., Zement, 20, 389 (1931). (13) Schlapfer, P., a n d Bukowski, R., Lab. fdddral essais mate'riaux annex4 &ole polytech. fdddrale a Zurich, 63 (1933). (14) Schlapfer, P., and Esenn-ein, P., Schweiz. Arch. angew. Wiss. Tech., 2, S o s . 2 a n d 12 (1936).
RECEIVED for review March 19. 1953.
Accepted August 6 , 1953.
