Turbidimetric Determination of Small Amounts
of Chlorides E. N. LUCE, E. C. DENICE, AND F. E. AKERLUND The Dow Chemical Company, Midland, Rlich.
A turbidimetric method, based on the Tyndall effect, is given for the determination of small amounts of chloride. The method is essentially one in which the nephelometric method has been adapted to the Hellige turbidimeter in order to eliminate the dis-
T
HE first accurate measurements on silver chloride sus-
pensions were made nephelometrically by Richards and Wells ( 3 ) . Lamb, Carleton, and Rfeldrum ( 2 ) later modified the earlier procedure for the preparation of the suspensions, and the method which they adapted to routine analysis has been put into general use ( 5 ) . Kolthoff and Yutzy ( I ) made a complete investigation of this method and a systematic study of the various factors affecting the results of nephelometric chloride determinations. From the results obtained by these and other workers, a tried and tested nephelometric method has been evolved. The nephelometer was designed originally to surmount the difficulties incurred in gravimetric and volumetric methods for small quantities of precipitate. When applied to small amounts of chloride, the nephelometric method is admittedly accurate; but the preparation of fresh standards for each determination has been found inconvenient and time-consi ming. Certain permanent, artificial standards scch as kieselquhr, ground or etched glass, etc., have been used, but only with limited sLccess. Since the advent of the nephelometer, micro and semimicro techniques have been developed ; and principally because of the difficulties involved in the preparation of standards, low chlorides have most often been determined by either microgravimetric or microvolmietric methods. Both methods are time-consuming and require considerable equipment and qpecial technique; as a result they are not readily adaptable to rapid routine work. It is the purpose of this paper to sholv how the nephelometric method may be used with the Hrllige turbidimeter to give a rapid and reliable method for the determination of small amounts of chloride. The method is accurate, and simple enough to be used by one unskilled in the use of elaborate and expensive equipment. I t is rapid, requiring less than 15 minutes’ working time per determination. A standard curve (Figure 1)’ covering the full range of the instrument, may be prepared in a few hours, and eliminates the need of reference standards. The instrument is easy to operate, and unique inasmuch as it combines the principles of the turbidimeter and the nephelometer. I t s range is greater than that of the turhidimetcr and equal to that of the nephelometer, and its use does not necessitate the preparation of fresh standards for each determination. This greatly simplifies the proc:dure by eliminating possible errors due to faclty preparation of individual standards, and also reduces the norking time to less than one-half that required for the nephelometer. The turbidimeter used in this investigation compares a beam of light with the Tyndall effect produced from a lateral illumination of the specimen by the same light source. The
advantages of reference standards. The method is rapid, requiring less than 15 minutes’ working time per determination, and is readily adaptable to routine work. The accuracy is comparable with that obtained with the nephelometer.
beam of light appears to the observer as a circular spot in the center of the Tyndall effect of the illuminated liquid, which is seen as lighter or darker than the Tyndall light, depending on the size of the opening of a precision slit. By matching the brightness of the two fields, the apparatus may be calibrated over a complete range of chloride concentrations. The construction and operation of this instrument for measuring the turbidity of barium sulfate suspensions have been given in complete detail by Sheen, Kahler, and Ross ( 4 ) .
Reagents Stock ch1ori:e solution, 4.12 grams of c. P. sodium chloride per liter, in distilled water. Diluted chloride solution, 10 ml. of the stock solution diluted to 1 liter with distilled water. This solution contains 25 mg. of
365
INDUSTRIAL AND ENGINEERING CHEMISTRY
366
chloride per liter; 1 ml. diluted to 50 ml., as in the procedure, gives a solution corresponding to 0.5 p. . m. of chloride. Xitric acid-silver nitrate reagent. solution approximately 0.2 N in nitric acid and 0.01 N in silver nitrate is prepared by diluting 12.5 ml. of concentrated nitric acid to 1 liter and adding 1.70 grams of ailver nitrate. Absolute ethanol. (Standard curves have also been prepared usin 95 per cent and Formula 30 alcohols.) A%lank determination must be made in order to eliminate the possibility of chloride contamination from the reagents.
1
TABLEI. STANDARDIZATION OF INSTRVMENT Chloride P. p . m. 5
4 3 2 1
0.75 0.50 0.25
Blank on reagents
(20-mm. optical cell) Standard 7 Dial Readings T o filter Milk filter Gray filter Solution MI. 10 72-73 8 62-63 6 51-52 4 36-37 80182 2 20-2 1 45-46 1.6 15-16 35-3 6 .. 1.0 12 26 70 0.5 7 16 48
.. .. ..
...
..,
8
21
Procedure Transfer the sample to a 50-ml. volumetric flask, adjust the pH with either 1 N nitric acid or 1 N sodium hydroxide until the solution is just neutral to phenolphthalein, and add distilled water until the volume approximates 20 * 1 ml. Add 20 ml. of absolute ethanol and then, dropwise from a pipet, 5 ml. of the nitric acid-silver nitrate reagent while swirling the contents of the flask. Make up to the mark with absolute ethanol and shake. If any initial turbidity is found before the addition of the nitric acidsilver nitrate reagent, the sample must be rejected, because turbidity at this point indicates contamination. Place the volumetric flask in a water bath at 40” C. for 30 minutes, and then cool the sample rapidly to room temperature. Pour into a 20-mm. optical cell (a manufacturer’s accessory for the turbidimeter), stir in an additional 10 ml. of absolute ethanol, and determine the turbidity as soon as the bubbles cease to form in the cell. The sample should be read within 30 minutes after it has been cooled to room temperature. The reading is compared with the standard curve, and the amount of chloride calculated.
Calculation Per cent of C1
=
p. p. m. of C1 (from the curve) X 0.005 ._ weight of sample, in grams
Vol. 15, No. 6
found that dial settings may easily be made to within *1 unit, by either the same or different operators. This limits the maximum deviation to less than 3 per cent for all concentrations of chloride, which is comparable with the accuracy obtained with the nephelometer. Kolthoff and Yutzy ( I ) determined the effect of certain foreign ions on the magnitude of opalescence of silver chloride suspensions. They found that different ions have different effects, and these changes should be taken into account n hen preparing standard curves. If the instrument is to be used for routine work, standard curves should be prepared by taking a given volume of the diluted standard chloride solution, adding an amount of purified electrolyte equivalent to that in the sample to be investigated, diluting to 20 ml., and then follon-ing the procedure given above. In this way accurate curves may be prepared for each compound in which the chloride content is being determined. I n many instances, hoivever, chloride concentrations of 0.005 per cent or less are of interest to an accuracy of only one significant figure, and a special reference curve is unnecessary. The volumes of the reagents used must be the same as those given in the procedure, because the alcohol serves to decrease the intensity of the opalescence as well as to stabilize the suspension. The final 10 ml. of alcohol which are added have no effect on the suspension other than dilution, but are added to adjust the solution to the proper depth in the 20-mm. optical cell. Other factors which affect the opalescence are heating, time of standing, mixing, and relative concentrations. Lamb and co-workers (2) found that the intensity approached a maximum upon heating the suspension for 30 minutes a t 40’ C., and a t that point it remained stable for an hour. Higher concentrations are stable for about 0.5 hour. The suspension should be mixed just before i t is placed in the water bath and just before i t is read, as continuous mixing will cause the silver chloride to coagulate. At present this instrument is being used in conjunction with the lamp combustion method for the routine analysis of organic compounds for small amounts of chloride and sulfur. It may also be used to determine the chloride content of wash liquors, and for the routine checking of many inorganic compounds which contain traces of chloride. The limited solubility of certain compounds in 1 to 1 wateralcohol mixtures makes the use of acetic acid solutions necessary. Good results may be obtained when acetic acid is substituted for alcohol, and the standard curves may be prepared by the same procedure as given above.
Discussion The standard curves w r e prepared from values obtained when known volumes of the standard chloride solution were treated in the same way as the sample. The dial readings were taken and these values plotted. Table I gives the results obtained, and Figure 1 s h o w a set of curves obtained in a typical calibration. Curve A represents the complete range of the instrument, 0 to 250 micrograms of chloride, and was obtained without the use of a filter. Suspensions of silver chloride made by the above procedure, which contain more than 250 micrograms of chloride, are too opaque to be read accurately with the instrument. Sample sizes taken must be such that the upper limit of the chloride concentration is not exceeded. Curve B was obtained b y using the milk-glass filter, and curve C with the gray glass filter (both filters supplied as manufacturer’s accessories). From the curves it follows that suspensions of less than 2 p. p. m. can be read more accurately on curve B than on curve A. Curve C may be used for concentrations of less than 1 p. p. m. The accuracy to which dial readings may be reproduced determines, in part, the accuracy of the method. It has been
TABLE 11. LIMITSOF ACCVRACY OF INSTRVMENT 7
Chloride
P. p .
m.
5 4
3 2 1 0.76 0.50 0.25
None
P. p . m. to.11 10.10 t0.08 *0.0G
....
... .... ....
Filters Milk
>
Gray
P. p . m.
P. p . rn
....
.... ....
.... ....
tO.04 10.04
1 0 04
.... ....
.... .. .
+0:01 +0.01 *0.01
Literature Cited (1) Kolthoff, I. M.,and Yutzy, H., J. A m . Chem. Soc., 55, 1918 (1933). (2) Lamb, A. B., Carleton, P. W., and Meldrum, W. B., Ibid., 42, 251 (1920). (3) Richards, T. W., and Wells, R . C., A m . Chem. J . , 31, 235 (1904). (4) Sheen, R. T., Kahler, H. L., and Ross, E. M.,ISD. ENG.CHEM., Bx.4~.ED.,7, 262 (1935). (5) Yoe, J. H., “Photometric Chemical Analysis”, Vol. 11. p. 137, “Nephelometry”, Kew York, John Wiley & Sons, 1929.
