March 15, 1935
ANALYTICAL EDITION
The space between the collar and the piston in a 2-ml. syringe was filled with 20 per cent hydrochloric acid. The syringe was filled and emptied 20 times in 2.5 minutes from 10 ml. of distilled water in a basin. An indicator would show the addition of 1 volume of the acid in 100,000 volumes of water, The acidification observed was 1 in 50,000. A similar experiment with a 10-ml. syringe filled 20 times in 5 minutes from 20 ml. showed no detectable acidification. Syringes which are not used constantly should be washed out with water to prevent the piston’s sticking fast. T h e author has found it possible, however, in such cases to loosen it by treatment with hot water. It is possible with syringe pipets to work several times more rapidly and at the same time with much greater pre--
1 The Laboratory of Zoophysiology is prepared to supply precision syringes mounted with a n ordinary stainless-steel cannula at a price of 25 Danish kroner; glass cannula, 2 Danish kroner; and oannula made entirely of stainless steel, 12 Danish kroner. The prices of the ordinary syringe pipets here described are: for 1 and 2 ml., 10 Danish kroner; 5 ml., 12 Danish kroner; 10 ml., 15 Danish kroner, st the laboratory. The 10-ml. pipet is provided with an extra movable stop on one of the guide roda for preparing definite mixtures.
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cision than with pipets of the same volume, but the higher price will prevent their being generally used.’ They have been found especially suitable for the following purposes: Serial measurements of the same fluid, for which purpose it i8 often advantageous that the syringe can be easily arranged to deliver any desired volume. Measurement of viscous fluids. Delivery of fluids which must not come into contact with air. All measurements in which more precision is desired than can be obtained by means of ordinary pipets. (The author has made many determinations of specific gravity of solutions by measuring with a syringe and weighing as described for the calibration, and has used syringes in combination with a small buret for precision titrations.) In many cases chemical reactions can with advantage be arranged to take place in syringes, especially when it is essential to exclude atmospheric air. LITERATURE CITED (1) Krogh and Keys, J . Chem. SOC.,1931, 2436. R E C ~ I Y ESeptember D 18, 1934.
Precise Determination of Oxygen in Water by Syringe Pipets AUGUSTKROGH,Laboratory of Zotiphysiology, University of Copenhagen, Copenhagen, Denmark
0
XYGEN titration in water is carried out according to Winkler by adding a known small amount of alkali containing potassium iodide (reagent I) and then a
corresponding volume of manganous chloride (reagent 11). Manganous hydroxide is thereby formed as a voluminous precipitate which absorbs the free oxygen. After acidification with hydr&hloric acid, iodine equivalent to the oxygen is liberated and is titrated with thiosulfate. It is customary to utilize water samples of 100 ml. or more, measured in glassstoppered bottles. For years in the author’s laboratory a micro-Winkler method has been used, in 7- to 15-ml. bottles titrating 5-ml. samples from a 2-ml. buret with 0.005 N thiosulfate. A micromethod described b y v a n Dam (3) utilizes a 1-ml. syringe and a Rehberg microburet. In oxygen determinations as hitherto performed there are two main s o u r c e s of e r r o r which should be eliminated. One is the diffusion of oxygen that takes place whenever the sample is not in equilibrium with the atmosphere, and the other is the uncertainty of the correction for oxygen d i s s o l v e d i n the reagents. These sources of error are completely eliminated in the procedure d e s c r i b e d below.
n
The water sample is collected and t h e reaction carried out in a 10-ml. s y r i n g e pipet, slightly modified as shown in Figure 1, by arranging a fixed stop on one of the guide rods. This stop
is placed so as to correspond as nearly as possible to 10.00 ml. and the screw stop is set a t a definite distance above, corresponding to about 0.1 ml. more. For a determination, the dead space of the syringe is first washed out and filled with reagent I, care being taken that no air bubbles remain in the syringe or cannula. As much water as possible is removed from the syringe before taking in the reagent, but since this cannot be done completely the reagent in the bottle becomes slowly diluted. The tube through which the reagent is taken in therefore reaches only halfway down the bottle and the rest of the content is discarded. As pointed out by Yoder and Dresher (4), rubber may give off sulfur and a ground joint in the top of the bottle will be preferable to the rubber stopper shown. The sample of water is drawn in slowly until the top of the piston comes against the fixed stop, and finally the piston is turned free of the stop and reagent I1 drawn in quickly. The tip of the cannula is wiped off between each intake and is finally closed by means of a small piece of rubber tubing with a glass stopper. It is convenient to put this on filled with water. The syringe is well shaken and laced with the tip upward, so that the precipitate collects on t\e piston. The syringe should be left in this position for 0.5 hour to allow the precipitate to settle. If left longer a firm clot of precipitate may form in the cannula, which may have to be pushed down by means of a fine wire before the syringe can be emptied. It is now possible to drive out almost all the fluid without losing any of the precipitate, to take hydrochloric acid into the syringe, and titrate in a small volume of fluid, but the manganous hydroxide is not sufficiently insoluble to make this procedure advisable (except perhaps when the amount of oxygen is very small). It is therefore necessary to titrate on the total volume of fluid. A suitable quantity of hydrochloric acid (0.4 ml., 20 per cent) is measured off into a flat-bottomed test tube of 30 X 100 mm. (as used for Hagedorn blood-sugar determination). Some fluid from the syringe is driven out into this and taken back again, so that the main reaction takes place in the syringe. The content of the syringe can be titrated from a 2-ml. buret with 0.01 N thiosulfate and, in comparative determinations an accuracy of 0.005 ml., corresponding t o about 0.04 ml. of oxygen per liter, is easily obtained. The author has endeavored t o obtain a definite improvement in the accuracy of this determination and has come across several sources of error which can be overcome by a suitable technic.
INDUSTRIAL AND ENGINEERING CHEMISTRY
132
NECESSARY PRECAUTIONS
1. Since it is difficult to add less than 0.003 ml. from a 2-ml. buret, the author has used a weaker solution of thiosulfate made up to 0.003572 N so that l ml. corresponds to 0.02000 ml. of oxygen (O', 760 mm.). When 0.1 gram of cyanide of mercury per liter is added to this solution, it is perfectly stable and only has to be guarded against evaporation. Titrating with this solution the result when multiplied by 2 gives the milliliters of oxygen per liter. 2. When the iodine liberated in a sample corresponds to more than 0.5 ml. of this thiosulfate, a loss of iodine, which may reach 1 per cent, will take place by evaporation. This is avoided, however, when the titration is carried out by a combination of syringe pipets and a 2-ml. buret, which will, moreover, increase materially the rapidity and accuracy of the determination. In all precision determinations it is necessary to have a fair approximation of the result beforehand. The thiosulfate solution should be ready in the buret and also in a three-necked Woulff bottle mounted with 3 syringes delivering 2.000, 1.000, and 0.500 ml., respectively. A suitable quantity of thiosulfate is measured off by means of 1 or more of these syringes, and starch (3 drops) is added before the syringe containing the free iodine is emptied; the mixture can now be titrated from the buret with not more than 0.5 ml. until the blue color disappears. The syringe is washed out at this stage by filling and emptying it once from the titrated mixture and the titration is finished. The exact point of disappearance of the color is obtained by means of a comparison vessel. Performed in this way, a series of titrations of the same fluid will agree within +0.005 ml. or 0.01 ml. of oxygen per liter, 0.01 ml. of oxygen per liter corresponding to 0.0001 ml. of oxygen in the syringe or the oxygen in an air bubble 1 mm. in diameter. Usually the results are even better. It is evident therefore that even small bubbles of air make the accuracy illusory.
3. As shown by Yoder and Dresher (h), the quality of the starch is an important consideration. Certain starches have a greater affinity for iodine than others, as measured by the quantity necessary to produce a just visible blue color. As is well known, this affinity is a function of temperature. The ideal starch should show the highest possible affinity, which should be practically independent of temperature within the range usually met in the laboratory. The starch as used by the author is prepared according to Hagedorn by dissolving 1 gram of the starch by heating in 100 ml. of saturated sodium chloride, giving a solution which is not attacked by bacteria and is perfectly stable. Three drops (0.12 to 0.15 ml.) of this solution are added to 10 to 15 ml. of fluid. The starch is tested as follows: To 10 ml. of water with 0.1 to 0.2 ml. of reagent I, 0.3 ml. of 20 per cent hydrochloric acid, and 3 drops of the starch, add from a 2-ml. buret 0.0002 N iodate until the color can be just distinguished from pure water. This test is performed at different temperatures, each time on a fresh sample because the blue color when already present is scarce1 changed by temperature inside the range from 16' to 30" which is of
8.
practical importance.
The following starches were tested: -IODATE 16'
ADDED220
30'
iM2.
M1.
1. Merck's c. P. soluble in sodium chloride
0 07
0 085
0 10
3. Soluble starch in sodium chloride
0.10 0 08
0 125 0.135
0 155 0.18
2. Starch of,low 6plubillty stabilized wlth mercuric iodide
iM1.
Merck's soluble starch shows the highest sensitivity and the lowest temperature coefficient. The temperature coefficient of No. 2 is definitely lower than of No. 3. The result is calculated as a correction which must be added to the oxygen directly determined by the titration. The correction is proportional to the total titration volume, which
VOl. 7, No. 2
can be taken as 10.5 ml. plus the volume of thiosulfate used. For Merck's soluble starch this correction works out as 0.0010 ml. of oxygen per liter for each milliliter of total volume a t 16O, 0.0012 a t 22O, and 0.0014 a t 30" C. This correction can also be calculated from a series of titrations of known dilutions of iodate measured off by means of a syringe pipet. Values for starch No. 3 a t 22" are given in Table I. TABLEI. TITRATIOX VALUESFOR STARCH No. 3 IODATE
N 0.00200 0.00120 0.00100 0,00080 0 00040 0.00020 0.00010 0.00008 0.00004 0.00002 I
TITRATIOX CORRECTEDCALCULATED VARIATION M1. 1w. 1M1. 5.588 5.671 5.675 + 4 3.333 3.404 3.405 + 1 2.772 2.840 2.837 - 3 2.206 2.272 2.270 - 2 1.074 1.134 1.135 + 1 0.565 0.568 0.508 + 3 0.284 0.229 0.284 0 0.227 0.175 0.230 - 3 0.123 0.114 0.069 - 9 0.026 0.057 0.080 -23
The results between 0.002 and 0.00008N correspond to a correction figure of 0.0104 ml. of oxygen per liter per milliliter of titration volume. The figure is much higher than the value deduced above and was found to be valid only for oxygen concentrations from 0.4 ml. of oxygen per liter upwards. For starch KO.1 a corresponding test showed substantial agreement with the simple color test, giving a t 22' a correction figure of 0.0014. 4. During the titration direct sunlight should be avoided, because it may accelerate the liberation of iodine from the iodide present to such an extent as to interfere with the titration. Most constant results have been obtained in artificial light. 5. In the oxygen determination proper allowance must be made for the oxygen present in the reagents. The quantity of reagent depends upon the dead space of the syringe and may vary somewhat from one syringe to another. The volume is determined by titration. The dead space is filled with some suitable normal solution washed out with distilled water, and titrated. I n his syringes the author has found volumes between 0.18 and 0.25 ml. The proportion between reagent I and I1 can be varied within wide limits. The author prefers to set the top screw so as t o make the quantity of I1 just half that of I, which is amply sufficient for any ordinary quantity of dissolved oxygen. The setting is easily performed by connecting the syringe filled with water with a 1-ml, pipet divided in 0.01 and observing the movement of the meniscus when the piston is moved from stop to top screw. The oxygen present in the reagents was measured in one series of determinations by filling up the syringe with oxygenfree water and titrating this. Absolutely oxygen-free water is difficult to prepare and it is simpler to mix the concentrated reagents in the right proportions directly in a 2-ml. syringe pipet. After 0.5 hour the content is poured into 10 ml. of suitably diluted hydrochloric acid and titrated as usual. The two methods have given identical results. The reagents as usually employed are:
I. 33 grams of sodium hydroxide and 10 grams of potassium iodide made up with water to 100 ml. 11. 40 grams of manganous chloride made up with water t o 100 ml. When I and I1 are mixed in the proportion 2 to 1and saturated with atmospheric air at 20' to 22' C., the solution will contain 3.4 mI. of oxygen per liter as against 6.4 in pure water. 6. The acid added to liberate the iodine must be present in fairly large excess to secure complete solution of the precipitate within the syringe. A large excess acceIerates the secondary reaction of liberating iodine from the iodide, but when strong light is avoided, this reaction is too slow to interfere. The author has tried only hydrochloric acid and has chosen
ANALYTICAL EDITION
March 15, 1935
20 per cent as the most convenient concentration; 0.25 ml. is theoretically sufficient when the volume of the reagents is 0.3 ml., but 0.4 cc. is taken when the reagent volume is less than 0.3, and 0.5 ml. when it is above. 7. Results obtained on the same water by the author have varied by ==0.007ml. of oxygen per liter. The accuracy is sufficient to warrant calibration and correction of the 2-ml. buret employed, as errors on these burets up to 0.01 ml. are not uncommon. A comparison of the absolute values for distilled water a t different temperatures with Fox's (1) tables revealed a systematic discrepancy of 1 per cent which has to be added to the titration results. A similar discrepancy has been observed before (2), but an explanation has not, so far, been found. CONCLUSIONS The method described was worked out mainly for biological purposes, but should also be useful industrially for the determination of oxygen in deaerated water. A serious source of error in the Winkler titration of water with a very low oxyaen content is the raDid absorwtion of air from the atmosphere. This error is counteracted, but cannot be abolished,
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by taking very large samples. I n the syringe method here described any possibility of contamination with air is excluded. When the amount of oxygen is below 0.5 ml. per liter there is no danger of loss of iodine, the buret only is required for the titrations, and the corrections for final volume and for oxygen in the reagents can be combined.
SUMMARY By the use of 10-ml. syringe pipets for the Winkler determination of oxygen in water, contamination with air from the atmosphere can be completely avoided. The accuracy can be increased a t least to *0.007 ml. of oxygen per liter, by avoiding loss of iodine and by corrections for the final concentration of iodine and for oxygen in the reagents. LITERATURE CITED (1) F ~L a~ n d o,l & B ~ r n s t e i n ,5. ~ ~ fI,l p.. 715. (2) Jacobsen, Medd. Kommiss. Havunders$gelser Series Hydrografi I, No. 8 (1905). (3) Van Weekbzad*301 No. 2o (1933). (4) Yoder and Dresher, Combustion, April, 1934. R ~ C I ~ I VSeptember BD 18, 1934.
Fluorescence of Gaseous Acetone as a Test for Traces of Oxygen GLENN H. DAMON, Michigan College of Mining and Technology, Houghton, Mich. N A RECENT investigation by Damon and Daniels (1) it was noted that the photolysis of pure acetone vapor was
I
accompanied by a relatively intense green fluorescence. However, when even a trace of oxygen was present in the reaction cell, the fluorescence was a faint blue instead of green. It was also found that a sharp change in color from blue to green took place when the irradiation was continued for a period of time, roughly dependent upon the amount of oxygen present in the cell. This change in fluorescence suggests the possibility of its use as a sensitive semi-quantitative test for traces of oxygen. A method for the detection of traces of oxygen in samples of various gases should have a wide range of application, provided the gases with which the oxygen is mixed have no effect on the fluorescence of the acetone. Qualitative tests (1)show that nitrogen, hydrogen, carbon monoxide, carbon dioxide, chlorine, ethylene, methane, ethane, ethyl ether, and water have no effect on the fluorescence of pure acetone vapor. The time for the color change from blue to green was not materially changed by the addition of the above-mentioned gases.
s.,
length less than 3000 but the 3130 1.line of the mercury arc is transmitted with sufficient intensity for this test. In order to observe the fluorescence change, the reaction cell must be either in a dark room or in some type of enclosure which can be made relatively dark. hfAX1hfUR.Z SENSITIVITY O F
The maximum sensitivity of the fluorescence test for oxygen is determined by introducing known quantities of air into a cell containing pure acetone vapor. Air is sealed into a thin glass capillary tube, I, of known volume. The side container, H , is then sealed onto the main cell at G. Liquid acetone is placed in E and the entire system evacuated until most of the liquid acetone is evaporated and all air removed.
APPARATUS The apparatus is relatively simple and inexpensive. Figure 1 shows the essential materials for this work. Auxiliary apparatus for the purification, storage, and transfer of gases may be required. A capillary quartz mercury vapor lamp ( d ) , A , is the most satisfactory source of ultraviolet light, but a commercial lamp was found to give sufficient intensity for the test, and has the advantage of a more uniform intensity over a long period of time. A Corning Red Purple Corex No. 986 light filter, B, satisfactorily cuts out the visible light in the mercury arc. C is a 6-cin. quartz focusing lens (f = 10 em.). The reaction cell, D, is made from a round-bottomed Pyrex flask. Pyrex glass cuts out all radiation of a wave
TEST
FIGURE 1
