APRIL 15, 1939
ANALYTICAL EDITION
If the relay performs erratically, which it may if long unshielded leads go to the control contacts, the values of R3, R4,and Rsmay be changed to one tenth of those shown, and CZincreased in the same ratio. The current through the control contacts then becomes 0.8 milliampere. It is important that the grounded side of the line be connected as shown. If one side of the regulator is grounded, it must obviously be connected to the same point. A mercury regulator is preferably connected so that the lead which remains in contact with the mercury when the mercury column falls is on the grounded side. One side of a bimetal regulator, or other controller requiring the relay to make the control contacts close, may be grounded only if a break-when-energized relay is used in circuit A . A 50-watt light globe may be substituted for RI, but it is not recommended because the first cost is only a few cents less, and the resistor is more dependable. If a quicker acting relay is wanted, condenser CI may be changed to the smallest value which will keep the relay from buzzing (about 1 microfarad), condenser C) shorted out in circuit A , and the straps readjusted on resistor Rz. With these changes the circuit becomes nearly identical with one described by Hersh, Fry, and Fenske ( I ) , except for the use of the beam power tube and of resistor Rs to limit the current of the control grid during the part
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of the cycle when it is positive with respect to the cathode. This modification is suitable for use on direct current power lines but for alternating current use it imposes about twice the voltage across the regulator contacts for any particular plate current requirement. This circuit may be used with a bimetal regulator by interchanging the grid leads a t points x and g. The magnetic relay is rated to handle 500 watts. Should larger amounts of power be required, there is available a current of about 40 milliam eres a t voltages up to 50 volts (about 20 milliamperes a t 100 ~ 0 8 s for ) operation of a heavier relay, if the contact voltage may be increased to 16 volts. Condenser CI should be as large as is practical (say, 32 microfarads) to obtain maximum power to operate the relay. The total cost for the parts is less than $8.00, more than half of which is for the magnetic relay. Parts are available from any radio supply house.
Literature Cited (1) Hersh, R. E., Fry, E. M., and Fenske, M. R., IND. ENG.CREM., 30,363 (1938).
Correspondence-Determining Gold in Cyanide Plating Solutions SIR: Kushner’s article on a method for determining gold in cyanide plating solutions [IND.ENG.CHEM.,Anal. Ed., 10, 641 (1938) ] might lead some to think that the Weisberg method there referred to was originated by me. That is not the case. The sulfuric acid decomposition method has been employed in this laboratory for a number of years. Its use here was started by William B. Stoddard, Jr. We seldom encounter difficulty due to failure of the gold to coagulate. However, our procedure is a little different from that described by Kushner. We seldom use more than 25 ml. of sulfuric acid to effect the decomposition. The decomposition is carried out in a Kjeldahl flask. If the mixture is not overheated at the beginning, the gold can be readily coagulated by adding water after decomposition is completed and boiling the mixture for a few minutes. LOVISWEISBERG 71 WEST 45TH 8T. NEW YORH, N. Y.
November 28, 1938
SIR: At the time of writing, I was aware that the method might not be original with Dr. Weisberg, but since he communicated it to me, I credited him with suggesting it. If the method was initiated by Stoddard, he deserves full recognition. Regarding the second paragraph of Dr. Weisberg’s letter, the difficulty was not that the gold failed to coagulate, but that some of the finer particles of the metal failed to precipitate and remained in colloidal suspension in the sulfuric acid solution. It may have been a negligible amount, but it was almost always there, I tried varying the rate of heating, but had no success in completely eliminating the effect. Another objection to adding water after decomposition has been completed is the danger involved in adding it to hot acid. While the Kjeldahl flask is now in use in Dr. Weisberg’s laboratory, I was under the impression that an Erlenmeyer flask was employed. The long neck of the Kjeldahl is no doubt an advantage in condensing the acid vapors and conserving the sulfuric needed to effect decomposition. However, the sloping walls of the cone flask bring about the same result to a large degree, besides permitting easy washing and transfer of gold to the Gooch crucible.
I feel that I did not stress sufEciently the fact that fumes given off during the reaction are violently poisonous. The list of constituents of these fumes reads like a Who’s Who of toxic and irritating gases. Any or all of the following may be evolved: HCN, (CN),, CO, CO,, NH3, SO*. The first five are given off a t the beginning of the process as the sulfuric acid decomposes the cyanides, formates, and carbonates that are usually present in the plating solution, and the last gas is generated as the sulfuric acid starts to boil, just before the gold is precipitated. It is therefore very important that a good draft hood be used for the decomposition. Under no circumstances should the face be brought close to the mouth of the flask during addition of the sulfuric acid or a t any time thereafter. If these precautions are taken there is no danger. Metals of the platinum family if present will interfere. Platinum, rhodium, ruthenium, and iridium never occur in commercial gold plating solutions and need not be considered. Rarely some palladium may be found-generally in what is known as an “optical pink” gold solution. When present it precipitates partially with the gold. The gold no longer coagulates into a sponge, but comes down as a brownish black powder contaminated with palladium. I n such a case, it is best to use the evaporation method (Scott, W. W., “Technical Methods of Metallurgical Analysis,” pp. 718-19, D. Van Nostrand Co., New York, 1923) and the regular fire assay. Of the more common metals, iron, if present in large quantities, also interferes. While the commercial gold plating baths of today seldom if ever contain sodium or potassium ferrocyanide, single-cell (commonly termed “salt water”) gold solutions still make use of these salts. If present, they are decomposed by the sulfuric acid with the formation of ferric sulfate. Ferric sulfate, being insoluble in sulfuric acid, comes down as a grayish white powdery coating over the gold sponge. After decanting most of the sulfuric acid and carefully washing the precipitate in the flask with water, decanting repeatedly, several alternate washings with dilute sodium hydroxide and hydrochloric acid, finally ending with boiling hot water, will completely remove the ferric sulfate. The gold can then be filtered onto a Gooch crucible as previously ENQ.CHEM.,Anal. Ed., 10, 641 (1938).] described [IND. 304 ECHO PLacs N E W YORK,
N. Y.
December 23, 1938
J. B. KUSHNER
