Reproducibility of Weighings Made on Microchemical Balances CLEnlENT J. RODDEN, Chairman, JULIUS A. KUCK, Secretary, A. BENEDETTI-PICHLER, ALSOPH CORWIN, 4 Y D EDW. W. D. HUFFMAN, Committee on Microchemical Balances, Division of Analytical and 3Picro Chemistry, AMERICAN CHEMICAL SOCIETY
A‘r
THE Detroit Meeting of the AJIERICAKCHEMICAL TABLE11. COSDEXSED SUMMARY OF RESTLTS (OCTOBER,1912) SOCIETYin September, 1940, G. E. F. Lundell,
chairman of the newly organized Division of Analytical and Micro Chemistry, asked the divisional secretary to form a Conimittee on Microchemical Balances, for the purpose of formulating specifications for performance and suggestions for materials and construction of these instruments for the benefit of American balance manufacturers. At the meeting of this committee a t Atlantic City in September, 1941, it was decided first to find out what performance, as regards reproducibility of Tyeighing, vas actually being obtained b y microanalysts in the course of their regular work. T o obtain this information, the members of the division n-ere circularized Kith the request that they perform a test under definitely stated conditions and report their results to the committee secretary. These have been suiiiniarized and the necessary statistical calculations performed by the divisional secretary, Rev. Francis IT-.Power. S.J., Ford1i:tni University, and 11-ere presented in subqtniitially this form a t the Buffalo hleeting in Septembei 1942. TABLEI. Sr W I A R Y
S u m b P r of microbalances studied
I
K (not givm)
OF REPRODUCIBILITY EXPERIVEXTS o\ %IICRoB.iLAXClX
K 2790 K 2423 B 26,730 B 30,833 K 2877 K 2545 B 21,406 S-R 6164 A 12.745
5.1
1937 1940
1937 1937 1933 1939 1941 1938
1934 1930 1938 1940
....
iY40
1940 1939
.. .. .. .
.... ...
1935
. . ..
,
2.5 1.2 1.8 4.7 2.2 3 .. 03 4 5.9 2.2 8.0 3.5 1.3
=
- 3 4 micrograms * 2 , 3 micrograms j=3 2 niicrograrns
-7
micrograms
d n l- 1.5r
where d is the deviation in micrograms of each of the n( = 10) n-eighings from the mean. The number in the denominator is decreased by 1.5 to correct for the underestimation of a standard deviation derived from such a short series. Further calculation indicates that there is no significant difference between the v d u e s for 1-gram load and those for 10-grain load. I n the summary, Table 11, the median values for the standard deviations are also expressed as “probable errors” i n order to make the data more easily comparable with a somewhat similar study made b y Corner and Hunter (4) which came to our notice about the time that our requests for data were being sent out. They give figures for the probable error of weighings of two 1-gram weights made on 8 different microbalances, one of which (an Oertling) was studied extensivery. The standard deviations in their paper vary from 2.5 to 12.6 micrograms, the poorer values being obtained on rather old balances used by students. On the whole their study leads to a probable error for the individual wighings of about + 3 micrograms, which is in very good agreement with the results of the present survey. They also conclude t h a t most of this error is due to the setting of the rider. Our figures, as well as those of Corner and Hunter, obviously upset a n y chemist’s pious belief that because his
5.2 6.4 0.7 2.3 6.7 1.7 25 .. 52
5.6 3 6 6.5 21 .. 3 0
Results on Other Balances 1941 4 8 A semimicro B X 11,661 B macro 1931 21 (2-gram load) 27 (50-gram load) 22,984 1915 5 2 (20-gram load) S macro 20,450 ..... B semimicro 1933 6 (5-gram Sept., 1938) load, 24,675 B semimicro 1933 16 (5-gram load, Oct., 1942) 24,675 a Designations of balances: X, Kuhlmann; B. Becker; A , Ainsworth; S-R, Sartorius (RambeFg):
.....
bc
I
The test consisted in u-eigliing two 1-gram weights one against the other, each weighing being follou-ed by a zero point determination. The rider n-as required to be res:t for each weighing and for each zero point. Details of reading the scale, proper conditions of temperature equilibrium. minimizing vibrat,ion effects, etc., ’were left to the discretion of each analyst, who vias asked simply to perform these weighings ryith the usual careful technique he would use in a regular series of micro n-eighings such as those involved in a succession of carbon and hydrogen determinations. The cooperators \vex asked to make a series of ten such weighings on the 1-gram w i g h t s and another series of ten using two 10-gram lyeights; the number ten for the series was selected as a compromise between the requirements of statistical stability for the standard deviations and the amount of time that a busy analyst could be reasonably. espect,ed to spend . on such a task. The standard deviations which are giren in Table I are those of the individual weighings, and were calculated from the formula
(Requested b\ the lliciobalance Committee of t h e Di\ision of Analytical and Micro Chemistrv, 1942) Uate RecondiStandard Deviation Purchase tioning I-gram load 10-gram load Micrograms K 2i4l 1935 ... 7.4 7.2 1935 1937 2.8 ... Ii 2531 1936 . . . . 3 . 2 K 2776 13‘Oh 1941 ... 12,8b B (special) X 2340 1930 193G 6.6 6.3 1938 ... 3.1 4.9 I< 2943 2 . 8 (recheck) 3.9 1936 .... 3.6 5.2 B 2731 X 2413 1931 . .. 6 1 8.3 X 2447 1933 1.0 1.6 n 27,225 1937 iiig 22 3 b 17.0b 1933 1939 14.nh 38.05 K 2561 B none 1933 .... 5.9 3.8 1937 .... 1.7 2.1 K 2769 1942 .... 1.2 2.1 A 14,772 K 2881 1937 ,... 2.2 21 .. 81 1936 .... 0.9 A (special) n 31,636 1942 .... 9.9b 28.3b 8 . 4 ; (recheck) 22.25
B 29,734 K 2863
29
Standard deviation of individual aeighings (median value) Probable error of individual weiKhings Probable error of individual weizhines. Corner and Hunter Largest error to be expected in any one weighing (twice standard deviation)
After Not used elimination in establishing of vibration. median.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
microchemical balance will respond to a difference in weight of 1 microgram i t will also reproduce weight differences as closely as this on repeated trials. If we assume that the outside tolerance or maximum error to be expected is twice the standard deviation, we find that b y and large our analysts should work on about 7-mg. samples if they wish to be reasonably sure of their sample weights to 1 part per 1000, the value to be used b y a n y given analyst depending on the reproducibility figure which he gets. The actual sample weight for the over-all process is also controlled b y its chemical factor and of course b y the error that one is willing to tolerate for the whole analysis. This matter has been fully discussed b y Benedetti-Pichler (1, S), who gives figures for the reproducibility of weight b y some analytical balances. The discussion in his book should also be read in this connection (9). Seeing t h a t only about one third of the reported figures are such as would permit 5-mg. samples to be weighed to within 1 part in 1000, it is recommended that the analyst check his balance b y some such reproducibility test as described here, and be guided by what he may find.
Conclusion While n-e do not as yet wish to go on record as reconimending a certain maximum reproducibility as a specification, we feel that the foregoing figures warrant a n expression of surprise a t the poor performance of so many microchemical
Detection of Gold in Plating MELVIN LERNER New York Customs Laboratory, Treasury Department, New York, N. Y.
T
ESTS to detect gold in platings must frequently be made i n U. S. Customs Laboratories, as the Tariff Act (3) prescribes different rates of duty for plated and unplated articles. Various methods have been proposed for the electrographic detection of gold without material injury to the surface. Each is limited in either failing to reveal gold in very thin “flash” plates or in not providing a sharp distinction between gold and other metals. Arnold (1) uses a solution of benzidine in acetic acid for the electrolysis, but differentiation between gold and copper in flash plates on brass is extremely difficult, as both produce similar greenish-blue colorations. Calamari, Hubata, and Roth (2) use a peroxide solution containing nitrate ion, but their method, as admitted by the authors, is inapplicable to thin gold plates on copper or brass because the characteristic gold coloration is masked by a strong greenish brown from the copper. Yagoda f4)electrolyzes with a 1 per cent sodium chloride solution and follows with a spot test using stannous chloride, but results are negative with very thin gold plates.
For the past 6 months, this laboratory has used the following method, which yields a distinctive gold test even for thin plates without interference from other metals or alloys yet encountered. Apparatus and Test Solution Three No. 6 dry cells, connected in series, are used (approximately 4.5 volts). A small clamp is connected to the anode, a piece of platinum wire (B. & S. gage 18) to the cathode. The test solution is repared by adding approximately 10 grams of stannous chlorixe to 100 ml. of dilute sulfuric acid (1 to 9). Any precipitate formed on mixing can be ignored. A piece of metallic tin should be added to the solution to prevent its oxidation.
balances (including some domestic ones) and a recommendation to the analyst to ascertain for himself just how well his own balance will actually perform. The work of the committee along lines of performance specifications and suggestions for materials and construction is being continued, and the chairman would appreciate suggestions from those interested. We wish also to express our gratitude t o all those who have cooperated in making these tests: Cooperators Rieman (Rutgers) Kuck (C. C. N. Y . ) Nichols (Cornell) Tuemmler (Shell Development Co.) Power (Fordham) Alicino (Fordham) Royer (Calco) Shrader (Dow) Van Etten (Northern Regional Lab.) Huffman (Denver)
Cooperators Rodden (N. Bur. Standarda) Chapman (Cyanamid) Haagen-Smit (Cal. Tech.) Bushey (Aluminum Co.) Hallett (Eastman Kodak) Duncan (W. Va. University) Galitzine (G. E., Pittsfield) Clarke (Wesleyan) Signeur (Canisius)
Literature Cited (1) Fenedetti-Pichler, A. A., IXD.ENG.CHEM.,ANAL. ED.,11, 226 (1939). (2) Benedetti-Pichler, A. 8.. “Mirro-technique of Inorganic Analysis”, pp. 173-84, New York, John Wiley & Sons, 1942. (3) Benedetti-Pichler, -4.A,, Mikrochemie, 27, 339 (1939). (4) Corner, Mary, and Hunter, Harold, Analyst, 66, 149 (1941). P R E a m r m before the Division of Analytical and RIicro Chemistry a t t h e 104th Meeting of the AMERICANCHEMICAL Socrmu, Buffalo, N. Y .
Procedure The anode clamp is attached to the article to be tested after the removal of any lacquer present thereon. A piece of spot-test paper or absorbent filter paper moistened with a drop of the test solution is then placed against the spot on the metal surface to be tested. The platinum cathode is used to press the moist paper against the metal surface for a period of about a second. If any doubt arises as to whether or not a closed circuit has been obtained, a milliammeter may be inserted in the circuit. An intense purplish-brown stain on the side of the paper in contact with the specimen indicates the presence of gold. If silver is present in the object under examination, the test should not be performed in direct sunlight, nor be exposed too long to diffused light, as the purple color of the partially reduced silver chloride will develop in a short time.
Interferences Approximately forty common metals and alloys have been tested by the above method and negative results obtained in each case. Nickel, cadmium, zinc, tungsten-nickel-tin alloy, brass, copper-vanadium alloy, titanium, silver, phosphor-tin alloy, copper-molybdenum alloy, sterling silver, tungsten-nickel alloy, copper, molybdenum, tungsten, iron, carbon steels, stainless steels, tungsten-tin-copper alloy, chrome-titanium alloy, nickel-silicon alloy, platinum, rhodium, tin-nickel alloy, lead, pewter, tin solder, Monel metal, bronze, German silver, antimonial lead, type metal, tantalum, Babbitt metal, aluminum, bismuth-lead alloy, magnesium, tin, and indium gave no perceptible colorations. Chromium gave a light yellow and cobalt a very light pink color. NOTE.TT-hile extensive tests have not been made on gold alloys, this method gives excellent results with the usual lo-, 14-, and 18-carat jewelry.
Literature Cited (1) Arnold, E., Chem. L i s t y , 27, 73-8 (1933). (2) Calamari, J. A., Hubata, R., and Roth, P. B., IND.ENG,CHEM., ASLL. ED.,14, 535 (1942). (3) Tariff Act of 1930, Paras. 339, 367, and 397. (4) Yagoda, H., private communication, 1941.
