DETERMINATION Of NICKEL AND COPPER CHRCMATES AND NICKEL, COPPER, AND MAGNESIUM ARSENATES IN TREATED WOOD March 1941 i , i RiS'J n r - 1 / ■ • l.' .,.■!.■, vie £Ui ^ -_.-;-. X.l , ■ ■'V of~?P UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin Digitized by the Internet Archive in 2013 http://archive.org/details/determOOunit DETERMINATION OF NICKEL AITD COPPER CHRd'AT ES AN D NICK EL , COPPER, AND MAGNESIUM ARSENATES IN TREATED WOOD By R. H. BAECHLER, Chemist and PHILIP SERVAIS, Student Assistant The methods to be described were required for a study of wood treated with different combinations of salts. They will be recognized in their separate parts as common procedures of chemical analysis ivith some minor changes which were found to be necessary. Preference has been given to methods requiring apparatus commonly available and also to volumetric methods as being more suitable than gravimetric methods for routine analyses. Destruction of Organic I late rial The destruction of organic matter by a mixture of sulfuric, nitric, and perchloric acids was found satisfactory in the determination of any of the ions present separately or in combination. The procedure used is as follows : To a 5"G ram sample of sawdust in a Kjeldahl flask add kO ml, of con- centrated nitric acid, 15 ml. of G.F.S. oxidant (2 parts SO percent sulfuric acid to 1 part fO percent perchloric acid by volume) and a few glass beads. Heat slowly at first, then with a medium flame until all the nitric and most, of the perchloric acid have boiled off, leaving the inorganic salts in a clear, sulfuric-acid solution. If the sample contains chromium, add an additional 5 wl . of 70 percent perchloric acid at the start of the digestion. Prolonged boiling at the end should be avoided as it causes the formation of chromic ion which precipitates as chromic sulfate and is hard to redis solve . In the determination of heavy metals and chromium, but not arsenic, dry ashing may be used. It requires less attention than the acid digestion, but does not yield results in so short a period of time and with some combi- nations an occasional sample yields an ash that is difficult to dissolve. The sample is ashed at 55O 2 C. After cooling, the ash is taken up with 10 ml. of dilute sulfuric acid (l:j)» anci if chromium is present, 2.5 ml. of 70 percent perchloric acid is added and the solution heated until it turns oran; Copper and Chromium The electrolytic deposition of copper leaving chromium in solution in reduced form is the most convenient separation. Should the necessary R1260 equipment be unavailable, copper may bo separated from chromium by the Haen-Low method and determined iodometrically. Chromium in the remaining solution is determined iodometrically. Average recoveries of known amounts were 99*7 percent for copper and 99*2 percent for chromium. The procedure is as follows: Transfer the digestion solution to a beaker and dilute to 100 ml., add two 1-inch squares of sheet aluminum, cover the beaker with a watch glass and keep the solution just below boiling for 15 minutes when the copper will be deposited on the aluminum. Filter and wash by decantation, keeping as much of the copper as possible in the beaker. Collect the filtrate and washings in a 500 ml. glass-stoppered, Erlenmeyer flask and save for the determination of chromium. Dissolve the copper in the beaker by warming it with 6 ml . of dilute nitric acid (1:1). Pour this acid solution of copper over the filter paper to dissolve any copper which may have been retained, collecting the solution in an Erlenmeyer flask. Wash beaker and filter with hot water. Copper Add 5 ml . of bromine water to the acidic copper solution and boil until the solution is again blue and no more bromine remains (starch-iodide paper test). Make the solution ammoniacal by the careful addition of ammonium hydroxide, then remove the excess ammonia by boiling. Adjust the volume of the solution to ^>0 ml., add 6 ml, glacial acetic acid, cool, and add 3 grams of potassium iodide. Titrate the liberated iodine with sodium thiosulfate, using starch indicator, lcc. M/lO Na2S20? = O.OI596 g m » CuSO^ = O.OO6357 gm. Cu. Chromium Make the filtrate from the copper separation alkaline with sufficient sodium hydroxide to dissolve the aluminum hydroxide which first forms. Add an excess of bromine water and boil after which the solution should be yellow and clear. A greenish color indicates chromic ion and the need of more bromine water. A precipitate indicates ferric hydroxide which must be removed by filtration. Acidify the solution with sufficient sulfuric acid to dissolve the aluminum hydroxide which first forms. Add 20 ml. of JO per- cent sodium bisulfate and boil to expel bromine. Adjust the volume to about 150 ml., cool, add 10 ml. of concentrated hydrochloric acid and 3 grams of potassium iodide. After allowing the stoppered flask to stand 5 minutes, dilute and titrate with sodium thiosulfate, using starch indicator. 1 ml. N/lO Na 2 S 2 0, = 0.005!+ gm. Na 2 Cr(fy = 0.00173^ gm. Cr. Copper and Arsenic Copper and arsenic are separated by precipitating magnesium ammonium arsenate from an ammoniacal solution in which copper is soluble. Both R1260 -2- copper and arsenic are then determined iodometrically. Average recoveries of known amounts were 100.1 percent for copper and 99*0 for arsenic. The procedure is as follows: Transfer the digestion solution to an Erlenmeyer flask and dilute to about 75 m ^ • For each 0.1 gram of arsenic add 10 ml. of magnesia mixture (66. k gm. MgS0^.6H 2 0, S6.4 gm. (NlfyJgSO^, 5 ml. H 2 S0j| diluted to 1 liter). Neutralize with ammonium hydroxide and add an excess equal to one-third the volume of the neutral solution. Allow the solution to stand for 12 hours, then filter and wash by decantation with 1.5 N ammonium hydroxide, keeping most of the precipitate in the flask. Save the filtrate and washings for the determination of copper. Arsenic Dissolve the precipitate on the filter with 20 ml. dilu!;e sulfuric acid (1:3) ana collect in the flask in which the precipitation was made originally. IVash the filter with hot water and dilute to 2 r ;j0 ml. Add 2 grams of potassium iodide and boil to a volume of l }0 ml. Add starch solu- tion and decolorize with a minimum of sodium thiosulfate solution. Neutralize to litmus with sodium hydroxide, then make just acid with a few drops of sulfuric acid. Neutralize with sodium bicarbonate, cool, and add an excess of 5 grams of the latter. Dilute to about 25O mi . and titrate with N/lO iodine solution. 1 cc. Il/l0 Io = 0.0093 gm. Na 2 IIAsOk = O.OO375 gm. As. Copper Proceed as above under "Copper," omitting the treatment with bromine Nickel and Chromium The determination of nickel and chromium does not require their separation. Chromium may be determined iodometrically in the presence of nickel. Nickel in wood, either alone or in the presence of chromium or arsenic, may be determined accurately by precipitation from the acid diges- tion solution with dimethyl glyoxime . However, for a large number of samples the titration of nickel with potassium cyanide is more rapid. Chromium in chromic state interferes with this titration and must be oxidized to chromate. Average recoveries of known amounts were from 99*5 for nickel and 100.2 for chromium. The procedure is as follows: Chromium Dilute the acid digestion mixture to 1 liter. Take a 100 r.l . aliquot Precipitate any iron present with ammonium hydroxide, adding sufficient excess to redissolve nickel hydroxide. Filter, if necessary, and determine chromate iodometrically as outlined above. R1260 -3- Nickel Take a 100 ml. aliquot, make slightly ammoniacal, and dilute to about 250 ml. Tartaric acid may be added to dissolve any precipitated ferric hydroxide. Make the solution slightly acid with hydrochloric acid and add exactly 5 ml. of 0.1 percent silver nitrate solution. Dissolve the precipi- tate of silver chloride with a minimum of ammonium hydroxide. Add 10 ml. of 1.6 percent potassium iodide solution which forms a precipitate of silver iodide. Titrate with a standardized solution of potassium cyanide (k-.b gm. KCN and 1.7 gm. NaOH per liter) until the precipitate of silver iodide has just dissolved, leaving a clear solution. Note the titration and deter- mine the blank to correct for the amount of potassium cyxnide used to dissolve the silver iodide indicator. This is done by aiding an additional 10 ml. of potassium iodide solution and ^> ml . of silver nitrate solution and titrating until the precipitate has again dissolved. Repeat and sub- tract the average blank from the original titration. Standardize the potas- sium cyanide solution against a solution of nickel sulfate which has been assayed by the precipitation of nickel with dimethyl glyoxime. Nickel and Arsenic The determination of nickel and arsenic, like nickel and chromium, does not involve a separation of the elements. The arsenic in the acid digestion mixture is reduced with potassium iodide and titrated with iodine after which the solution is titrated with potassium cyanide to determine nickel. Average recoveries of known amounts were 99*6 percent for nickel and 99*0 percent for arsenic. Magnesium and Arsenic In the acid digestion solution containing both magnesium and arsenic, the latter is first determined by reduction with potassium iodide and titration with iodine. Magnesium is then determined in the same solution by precipitating magnesium ammonium arsenate, filtering, dissolving the precipitate with acid, reducing the arsenic with potassium iodide and titrating with iodine. Magnesium is then calculated from the arsenic. This is a modification of a method described by Meadei who titrated the iodine liberated by the acid solution of magnesium ammonium arsenate. More accurate results were obtained than by the us^ of Meade's original procedure This was ascribed to the difficulty of controlling the quantitative libera- tion of iodine by arsenate. Average recoveries of known amounts were 100. k percent for magnesium. The procedure is as follows: -Jour. Amer. Chem. Soc . 21:7^6 (1899). R1260 -k- Magnesium Acidify the solution whicli remains from the titration of arsenic with sulfuric acid. Boil to a volume of about 75 ^ • an d add 25 ml • of 10 percent di sodium arsenate. Add a suitable excess of ammonia to precipitate magnesium ammonium arsenate and proceed as under the deter- mination of arsenic in the presence of copper. 1 ml. N/10 l£ = 0.00602 gm. MgS0] + = 0.001216 gm. Mg. R1260 Umm^L OF f lor,da 3 1262 08927 3378