SO78
INDUSTRIAL AND ENGINEERING CHEMISTRY
VoI. 15, No. 10
Preliminary Examination of Water Samples’’z Calcium and Sulfate by Turbidity and Hardness by the Soap Method By W. D. Collins and Margaret D. Foster U. S. GEOLOGICAL SURVEY,
DEPARTXENT OF
THB INTERIOR, WASHINGTON, D. C .
N a laboratory where using the rapid methods The preliminary examination of a water sample will save time samples of water redescribed by Hale,4 or by in making a complete mineral analysis. and may show that a comceived for analysis may using the turbidimeter in plete analysis is not necessary. The examination will include contain from 25 to 25,000 the determination of sulfate titrations of chloride and alkalinity by the usual methods, turbidparts per million of disand calcium. The turbidimetric determinations of sulfate and calcium by comparison with solved mineral matter, it is imetric determinations destandards in test tubes, and sometimes a determination of hardness well worth while to make scribed below are made by the soap method. a preliminary examination without the turbidimeter The turbidimetric tests are made with volumes of IO cc. for the before proceeding to evapand require only 10 cc. or sample and standards. For the sulfate precipitation I cc. of acid orate the part of the samless of the sampIe. barium chloride solution is used (48 cc. of hydrochloric acid, specific ple that is to b e used for SULFATE gravity 1.19,and IO0 grams of BaC12.2H20 in 1 liter). For calcium the completemineral analyEstimation of quantities 1 cc. of acetic acid (equal parts of 99.5 per cent acid and water) is sis. The preliminary examof sulfate by the turbidity added, the solution is shaken, I cc. of potassium oxalate solution ination ordinarily involves (0.2 gram KaCZOp per cc.) is added. the solution is shaken again produced on the addition the determination of the of barium chloride to an and after IO minutes the turbidities are compared. acid radicals present in conacidified solution is probaTests of one hundred seventy-four waters varying in composition siderable quantities and the over a wide range show that. the preliminary results for sulfate, calbly one of the oldest of the calcium and magnesium or simple rapid quantitative cium, and hardness are reasonably close to the results obtained in their equivalent. From methods. A number of the complete analysis. In general, the error is likely to be about these results it is possible IO per cent of the quantity determined, and may be plus or minus. authors6 have described vato calculate the sodium rious ways of increasing the For preliminary examinations or for field work8 this accuracy is needed to balance the analaccuracy of the turbidimetsuficient. ysis and thus have roughly ric determination of sula com~leteanalysis. fate, and it is probable that In Gost natuial waters the only acid radicals to be con- in many laboratories where large numbers of routinesulfate desidered are chloride, sulfate, and bicarbonate or carbonate. terminations are made the results obtained with the turbidimOnly a few waters contain enough nitrate to enter into the eter are as accurate as gravimetric results. Such accuracy calculation of the quantity of sodium, or to cause the solu- is not generally found in the results of turbidimetric detertion of platinum when the waters are evaporated with hydro- minations of sulfate in miscellaneous samples of water, nor chloric acid in a platinum dish. It is always well, however, is it necessary in preliminary examinations. Equally useful t o determine the nitrate before the acidified sample is evap- results are obtained by the simple procedure outlined below. orated. The regular methods for the titration of chloride and bicarbonate are sufficiently rapid to serve for the preliminary examination. Sulfate is the only acid radical for which a special method must be used in the preliminary examina tion, The determination of total hardness of a water‘by the soap method gives a measure of the combined calcium and magnesium and serves as a basis for calculation of the sodium, A turbidimetric determination of calcium takes less time than a soap test and often is sufficient for determining the quantity of a sample to take for evaporation. In order to have definite data in regard to the reliability of simple turbidimetric sulfate and calcium determinations and the soap test as applied in regular laboratory work, these three tests were made on each of a series of one hundred seventy-four samples which were analyzed in the Water Resources Laboratory of the U. S. Geological Survey over BY TURBIDITY : FIG. I-ERRORSIN DETERMINATIONS OF SULFATE a period of about a year. The results of these tests as discussed below, and as shown in Figs. 1, 2, and 3, demonstrate the usefulness of the methods for preliminary examinations. Samples of 10 cc. are taken in test tubes (100 X 10 mm.) The data show clearly that the methods as used cannot be and standards are made up containing from 0.05 to 0.35 mg. trusted to give results that can be used in place of regular of sulfate (SOJ in 10 cc. To each tube is added 1 cc. of an analytical determinations. If greater accuracy is desired acid solution of barium chloride (48 cc. of hydrochloric acid without making a complete analysis, it can be obtained by of specific gravity 1.19 and 100 grams of BaC12.2H20 in 1
I
Received May 4, 1923. by permission of the Director, U. S. Geological Survey. 3 Collins, “Field Examination of Water,” special circular, U. S. Geol. Survey, March, 1922. 1
* Publislied
J. Am. Chem. SOL, 29, 1078 (1907). Hinds, Ibid., 18, 661 (1896);22, 269 (1900);Jackson, Ibid., 28, 799 (1901); Muer, THIS JOURNAL, 3, 553 (1911). 4
6
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October,, 1923
liter). The tubes are shaken vigorously and the turbidities compared by looking through the depth of the liquid a t a black background. If the sample contains over 35 parts per million of sulfate, a smaller portion is diluted to 10 cc. with distilled water. If the sulfate is over 700 parts per million, a dilution is made and the proper quantity of the diluted sample is taken for the determination.
FIG.%-ERRORS
IN
DETERMINATIONS OF CALCIUM
BY
TURBIDITY
Fig. 1 shows the differences between the sulfate found by the rough turbidimetric method and the quantity found by the regular gravimetric method in the course of the complete analysis. The complete analyses all balanced well enough to indicate that the differences were due almost entirely to the error of the turbidimetric comparisons. The points which represent 10-cc. samples show the actual quantity of sulfate determined by analysis plotted against the difference between this quantity and that determined by turbidity. For the points representing samples of 5 cc. the analytical result has been divided by 2 and plotted against half the difference between the gravimetric and turbidimetric results. Similar reduction has been made for the smaller samples. The average of the percentage differences for all the samples is 16 per cent of the analytical results. For the seventyseven samples of 10 cc. the average difference of 22 per cent corresponds to only 2.4 parts per million of sulfate (SO4), which is of no practical significance. An average difference of 12 per cent for the fourteen samples of 06 cc. represents an average difference of 50 parts per million of sulfate, which is considerably more than the allowable error in the gravimetric determination of from 350 to 700 parts per million of sulfate. The turbidimetric determination of sulfate in test tubes is thus shown to be sufficiently accurate for a preliminary examination of a water sample, but not reliable enough to take the place of a gravimetric determination. It calls for no special apparatus, is rapid, and requires only a small sample, which is sometimes an important consideration.
CALCIUM Although the turbidimetric determination of calcium after precipitation with ammonium oxalate in neutral solution was reported by Hinds6 to give accurate results, the method has not been found generally applicable to natural waters. A number of experiments were made with oxalic acid and po-
1079
tassium oxalate to find the proportions which would give the most accurate turbidimetric determination of calcium, but the best proportions of these reagents did not give results of the accuracy of the turbidimetric determination of sulfate. When acetic acid was substituted for oxalic acid much better results were obtained. The results of a large number of tests with different quantities of the reagents led to the selection of the details of the method given below. Samples of 10 cc. of the water to be tested are taken in test tubes and standards are prepared in other tubes containing in 10 cc. from 0.04 to 0.24 milligram of calcium. To each tube 1 cc. of acetic acid (equal volumes of 99.5 per cent acid and water) is added and the contents are well shaken. After the addition of 1 cc. of potassium oxalate solution (containing 0.2 gram K2Cz04) the tubes are again shaken. The turbidities are compared after 10 minutes. The relative turbidities are quite constant for 3 or 4 hours, so it is not necessary to make the comparison at once. The upper limit for the determination of calcium as described is about 24 parts per million. Samples of less than 10 cc. are taken for the more concentrated waters, as is done for sulfate, and the volumes made to 10 cc. before the addition of reagents. The differences between the analytical results and the turbidimetric results for calcium in one hundred seventy-four samples of water are shown in Fig. 2. The differences and the analytical results are divided by the appropriate factors for the samples of which less than 10 cc. were used for the turbidimetric determination. It is evident that the accuracy is about the same as for the determination of sulfate. The average percentage error is 19 per cent of the analytical result. The error of 28 per cent for the sixty-eight samples of 10 cc. corresponds to an average error of 2.2 parts per million of calcium, which is not generally of any significance. The results shown in Fig. 2 make it plain that the turbidimetric method described is sufficiently accurate for a preliminary determination, but cannot be depended upon to give results that can be used with the same confidence as the results obtained by the ordinary analytical methods.
HARDNESS The soap method for hardness, practically as originally devised by Clark,B is recognized in all texts as useful for approximate determinations. Some of the criticisms of the method have been based on results obtained with details of manipulation different from those generally recommended, and therefore do not apply to the method as now used.
391
0
I 10
PO
30
40
~*..N,,,‘r,~,~~~~~~
FIG.3-ERRORS
IN
I
I
50
60
I 10
I 80
I
I
90
Mo
ANALYSIS)
DETERMINATIONS OB HARDNESS BY
SOAP
METHOD
I n the preliminary examination of the one hundred seventyfour samples for which sulfate and calcium were determined by turbidity, the hardness was determined by following every “Repertory of Patent Inventions for 1841,” London, 1844; Goe., I , 100 (1847).
Chcm.
1080
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
VOl. 15,
K O . 10
ness in Fig. 3. The errors for determinations above 100 parts per million have been divided by a factor corresponding to the quantity of water used in the soap test, as was done with the sulfate and calcium results. It is evident that the soap method for hardness is more reliable than the rough turbidimetric methods for sulfate and for calcium. The average percentage or difference between the calculated and determined hardness for the one hundred seventy-four samples is 8 per cent of the calculated hardness.
detail of the soap method as given in Standard Methods of Water Analysis of the American Public Health Association. If the hardness of a sample was over 100 parts per million, less than 50 cc. were taken for the test and the volume made up to 50 cc. with distilled water. The hardness of each water was calculated from the calcium and magnesium determined in the regular analysis. The difference between the determined and calculated hardness for each sample is plotted against the calculated hard-a
T h e Titration of Hydrofluoric and Hydrofluosilicic Acids in Mixtures Containing Small Amounts of Hydrofluosi li ci c Acid’ By Paul H. M.-P.Brinton, Landon A. Sarver, and Arthur E. Stoppel UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MI”.
HE determination of hydrofluoric and hydrofluosilicic acids in mixtures of the two has been investigated by a number of workers,2 and the problem is of considerable importance because it is really encountered in every analysis of the hydrofluoric acid of commerce. A study of the method given by Scotta forms the subject of this paper. In this method the sample of the mixed acids, to which has been added some potassium nitrate, is first titrated ice cold with standard sodium hydroxide, using phenolphthalein as indicator; and then the titrated solution is heated to about 80’ C., and again titrated to a pink end point. The reactions for these two titrations are as follows: For the cold end point: HzFz + 2NaOH -+ 2NaF -I- 2H20 HzSiFef 2KNOs + 2NaOH + KzSiFG f 2NaNOa f 2H20 For the hot end point: KzSiFa f 4NaOH +4NaF f 2KF f Si(0H)d From the equations the results of the titrations for the two acids are readily calculated. Results of analyses of the same sample made in three different laboratories by this method showed wide divergence. Not only were the h a 1 results a t variance, but there was considerable difference in the ea88 with which the end points, particularly in the cold titrations, could be detected. Investigation of the causes of these differences has led to the uncovering of sources of error which the writers believe are responsible for the dissatisfaction with the method which has been expressed in some quarters. WEIGHINGOF SAMPLE It is customary in many laboratories to transfer the sample from the bottle to a platinum weighing vessel by means of a lead thief or lead pipet. The writers noticed in some of their early experiments that the tip of the lead tube became corroded, and found that if the tip of the pipet were allowed to touch the platinum as the acid was flowing from the tip, a brisk evolution of gas occurred and the lead was rapidly at-
T
1 Presented before the Division of Inorganic and Physical Chemistry a t the 65th Meeting of the American Chemical Society, New Haven, Conn , April 2 to 7, 1923. 2 Guyot, Compt rend., 71, 224 (1870); Katz, Chem. Ztg., 98, 356, 387 (1904); Greef, Ber., 46, 251 (1913); Dinwiddie, Am. J . Sci., 192,421 (1916); Huddleston and Bassett, J . Chem. SOG.(London), 119,403T (1921). 8 “Standard Methods of Chemical Analysis,” 1992, p. 1019, D. Van Nostrand Co.
.
tacked as a result of the galvanic couple. If the lead is not allowed to touch the platinum there is no action. With small platinum weighing vessels, even if opened under alkali, there is apt to be loss of hydrogen fluoride vapors when the vessel is first opened. This source of error has been corrected by weighing the sample in a rubber bottle4 which has been nearly filled with cracked ice and then weighed. To overcome the change of weight due to condensation of moisture on the cold bottle during weighing, the bottle is weighed in a stoppered weighing bottle, well lined with asbestos paper, and counterpoised by a weighing bottle of similar external form on the opposite balance pan. After weighing, the bottle is shaken once or twice to absorb any vapors of hydrogen fluoride, and the sample, then in the form of an ice-cold, dilute solution, can be poured and rinsed into the alkali without loss. ENDPOINTAND INDICATOR Methyl orange cannot be used because of the weak nature of the second hydrogen of H2F2. This leads to the formation of acid salts as intermediate reaction products, which have the strength of weak organic acids. Phenolphthalein, however, is satisfactory. ScottSsuggests as the end point for the cold titration a pink that will persist for 15 seconds. It is self-evident that an alkali solution free from carbonate must be used, for even a t this temperature the end point with phenolphthalein when using a solution contaminated with carbonate is not only misplaced, but is also faint and fleeting. If the solution is not icecold for the fist end point, the color will rapidly fade due to hydrolysis of potassium fluosilicate. K a t d has suggested that this hydrolysis proceeds much more rapidly in the presence of calcium salts, owing to disturbance of equilibrium due to formation of insoluble calcium fluoride, and it was thought that perhaps calcium oxide would be inadmissible as a reagent for removing carbonate from the standard alkali; but the amount of calcium oxide which can exist in a sodium hydroxide solution is found to have a negligible effect . EFFECT OF SILICA I N THE STANDARD ALKALI bpparently, the most serious source of error in the use of this method is the silica content of ordinary alkali solutions. In tracing out the cause of the divergence in the analytical results obtained in three laboratories on the same sample, 4 Hard rubber bottles of 0 5-ounce capacity have been found very satisfactory for this purpose. 6 Chem. Ztg., 98, 356 (1904).
