Part 24

2CuCl₂ + 4KI = Cu₂I₂ + 4KCl + I₂.

The analytical process is carried out as follows: In a 100 cubic centimeter flask are boiled fifty cubic centimeters of the copper solution, ten cubic centimeters of about one-tenth per cent reducing sugar solution are added, the boiling continued for five minutes, the flask filled to the mark with boiling water and its contents filtered. Fifty cubic centimeters of the hot filtrate are cooled, slightly acidified, potassium iodid solution added in slight excess; and the iodin set free determined by titration with sodium thiosulfate. The quantity of iodin obtained corresponds to the unreduced copper remaining after treatment with the reducing sugar. The number of cubic centimeters of thiosulfate used subtracted from twenty-five will give the number of cubic centimeters of the copper solution which would be reduced by five cubic centimeters of the sugar solution used.

Example.—In the proportions given above it was found that eleven cubic centimeters of thiosulfate were required to saturate the iodin set free. Then 25 - 11 = 14 cubic centimeters of copper solution reduced by five cubic centimeters of the sugar solution. Since one cubic centimeter of the copper solution is reduced by 0.0036 gram of dextrose the total dextrose in the five cubic centimeters = 0.0036 × 5 = 0.0180 gram.

The above method does not seem to have any practical advantage over those based on noting the disappearance of the copper and is given only to illustrate the principle of the process. While the titration of the iodin by sodium thiosulfate is easily accomplished in the absence of organic matter, it becomes difficult, as shown by Ewell, when organic matters are present, as they always are in the oxidation of a sugar solution. Ewell has therefore proposed to determine the residual copper by a standard solution of potassium cyanid, but the method has not yet been developed.

GRAVIMETRIC COPPER METHODS.

=135. General Principles.=—In the preceding pages the principles of the volumetric methods of sugar analysis by means of alkaline copper solution have been set forth. They depend either on the total decomposition of the copper solution employed by the reducing sugar, or else on the collection and titration of the cuprous oxid formed in the reaction. In the gravimetric methods the general principle of the process rests upon the collection of the cuprous oxid formed and its reduction to metallic copper, the weight of which serves as a starting point in the calculations of the weight of reducing sugar, which has been oxidized in the solution.

The factors which affect the weight of copper obtained are essentially those which influence the results in the volumetric method. The composition of the copper solution, the temperature at which the reduction is accomplished, the time of heating, the strength of the sugar solution and the details of the manipulation, all affect more or less the quantity of copper obtained. As in the volumetric method also, the kind of reducing sugar must be taken in consideration, dextrose, levulose, invert sugar, maltose and other sugars having each a definite factor for reduction in given conditions. It follows, therefore, that only those results are of value which are obtained under definite conditions, rigidly controlled.

=136. Gravimetric Methods of the Department of Agriculture Laboratory.=—The process used in this laboratory is based essentially on the methods of Maercker, Behrend, Morgen, Meissl, Hiller and Allihn. Where dextrose alone is present, the table of factors proposed by Allihn is used and also the copper solution corresponding thereto.

For pure invert sugar, the tables and solutions of Meissl are used. For invert sugar in the presence of sucrose, the table and process proposed by Hiller are used.

The reduction of the copper solution and the electrolytic deposition of the copper are accomplished as follows:

The copper and alkali solutions are kept in separate bottles. After mixing the equivalent volume of the two solutions in a beaker, heat is applied and the mixture boiled. To the boiling liquid the proper volume of the cold sugar solution is added. This must always be less than the amount required for complete reduction. The solution is again brought into ebullition and kept boiling exactly two minutes. A two-minute sand glass is conveniently used to determine the time of boiling. At the end of this time an equal volume of freshly boiled cold water is added, and the supernatant liquor at once passed through a gooch under pressure. The residual cuprous oxid is covered with boiling water and washed by decantation until the wash water is no longer alkaline. It is more convenient to wash in such a way that, at the end, the greater part of the cuprous oxid is in the gooch. The felt and cuprous oxid are then returned to the beaker in which the reduction is made. The gooch is moistened with nitric acid to dissolve any adhering oxid and then is washed into the beaker. Enough nitric acid is added to bring all the oxid into solution, an excess being avoided, and a small amount of water added. The mixture is again passed under pressure through a gooch having a thin felt, to remove the asbestos and the filtrate collected in a flask of about 150 cubic centimeters capacity. The washing is continued until the gooch is free of copper, when the volume of the filtrate should be about 100 cubic centimeters. The liquid is transferred to a platinum dish holding about 175 cubic centimeters and the flask washed with about twenty-five cubic centimeters of water. From three to five cubic centimeters of strong sulfuric acid are added and the copper deposited by an electric current.

=137. Precipitating the Copper.=—When no more nitric acid is used than indicated in the previous paragraph, it will not be necessary to remove it by evaporation. The platinum dishes containing the solutions of the cuprous oxid are arranged as shown in the figure for the precipitation of the copper by the electric current. Each of the supporting stands has its base covered with sheet-copper, on which the platinum dishes rest. The uprights are made of heavy glass rods and carry the supports for the platinum cylinders which dip into the copper solutions. The current used is from the city service and is brought in through the lamp shown at the right of the figure. This current has a voltage of about 120. After passing the lamp it is conducted through the regulator shown at the right, a glass tube closed below by a stopper carrying a piece of platinum foil, and above by one holding a glass tube, in the lower end of which is sealed a piece of sheet platinum connected, through the glass tube, with the lamp. The regulating tube contains dilute sulfuric acid. The strength of current desired is secured by adjusting the movable pole. A battery of this kind easily secures the precipitation of sixteen samples at once, but only twelve are shown in the figure. The practice here is to start the operation at the time of leaving the laboratory in the afternoon. The next morning the deposition of the copper will be found complete. The wiring of the apparatus is shown in the figure. The wire from the regulator is connected with the base of the first stand, and thence passes through the horizontal support to the base of the second, and so on. The return to the lamp is accomplished by means of the upper wire. This plan of arranging the apparatus has been used for two years, and with perfect satisfaction.

Where a street current is not available, the following directions may be followed: Use four gravity cells, such as are employed in telegraphic work, for generating the current. This will be strong enough for one sample and by working longer for two. Connect the platinum dish with the zinc pole of the battery. The current is allowed to pass until all the copper is deposited. Where a larger number of samples is to be treated at once, the size of the battery must be correspondingly increased.

=138. Method Used at the Halle Station.=—The method used at the Halle station is the same as that originally described by Maercker for dextrose. The copper solution employed is the same as in the allihn method, viz., 34.64 grams of copper sulfate in 500 cubic centimeters, and 173 grams of rochelle salt and 125 grams of potassium hydroxid in the same quantity of water. In a porcelain dish are placed thirty cubic centimeters of copper solution and an equal quantity of the alkali, sixty cubic centimeters of water added and the mixture boiled. To the solution, in lively ebullition, are added twenty-five cubic centimeters of the dextrose solution to be examined which must not contain more than one per cent of sugar. The mixture is again boiled and the separated cuprous oxid immediately poured into the filter and washed with hot water, until the disappearance of an alkaline reaction. For filtering, a glass tube is employed, provided with a platinum disk, and resembling in every respect similar tubes used for the extraction of substances with ether and alcohol. The arrangement of the filtering apparatus is shown in Fig. 44. In the Halle method it is recommended that the tubes be prepared by introducing a platinum cone in place of the platinum disk and filling it with asbestos felt, pressing the felt tightly against the sides of the glass tube and making the asbestos fully one centimeter in thickness. This is a much less convenient method of working than the one described above. After filtration and washing, the cuprous oxid is washed with ether and alcohol and dried for an hour at 110°, and finally reduced to metallic copper in a stream of pure dry hydrogen, heat being applied by means of a small flame. The apparatus for the reduction of the cuprous oxid is shown in Fig. 45. The metallic copper, after cooling and weighing, is dissolved in nitric acid, the tube washed with water, ether and alcohol, and again dried, when it is ready for use a second time. The percentage of dextrose is calculated from the milligrams of copper found by Allihn’s table.

=139. Tables for Use in the Gravimetric Determination of Reducing Sugars.=—The value of a table for computing the percentage of a reducing sugar present in a solution, is based on the accuracy with which the directions for the determination are followed. The solution must be of the proper strength and made in the way directed. The degree of dilution prescribed must be scrupulously preserved and the methods of boiling during reduction and washing the reduced copper, followed. The quantity of copper obtained by the use of different alkaline copper solutions and of sugar solutions of a strength different from that allowed by the fixed limits, is not a safe factor for computation. It must be understood, therefore, that in the use of the tables the directions which are given are to be followed in every particular.

=140. Allihn’s Gravimetric Method for the Determination of Dextrose.=—Reagents:

I. 34.639 grams of CuSO₄.5H₂O, dissolved in water and diluted to half a liter:

II. 173 grams of rochelle salts } dissolved in water and diluted 125 grams of KOH, } to half a liter.

Manipulation: Place thirty cubic centimeters of the copper solution (I), thirty cubic centimeters of the alkaline tartrate solution (II), and sixty cubic centimeters of water in a beaker and heat to boiling. Add twenty-five cubic centimeters of the solution of the material to be examined, which must be so prepared as not to contain more than one per cent of dextrose, and boil for two minutes. Filter immediately after adding an equal volume of recently boiled cold water and obtain the weight of copper by one of the gravimetric methods given. The corresponding weight of dextrose is found by the following table:

ALLIHN’S TABLE FOR THE DETERMINATION OF DEXTROSE.

(A) = Milligrams of copper. (B) = Milligrams of dextrose.

(A) (B) (A) (B) (A) (B) (A) (B) (A) (B) 10 6.1 46 23.9 82 41.8 118 60.1 154 78.6 11 6.6 47 24.4 83 42.3 119 60.6 155 79.1 12 7.1 48 24.9 84 42.8 120 61.1 156 79.6 13 7.6 49 25.4 85 43.4 121 61.6 157 80.1 14 8.1 50 25.9 86 43.9 122 62.1 158 80.7 15 8.6 51 26.4 87 44.4 123 62.6 159 81.2 16 9.0 52 26.9 88 44.9 124 63.1 160 81.7 17 9.5 53 27.4 89 45.4 125 63.7 161 82.2 18 10.0 54 27.9 90 45.9 126 64.2 162 82.7 19 10.5 55 28.4 91 46.4 127 64.7 163 83.3 20 11.0 56 28.8 92 46.9 128 65.2 164 83.8 21 11.5 57 29.3 93 47.4 129 65.7 165 84.3 22 12.0 58 29.8 94 47.9 130 66.2 166 84.8 23 12.5 59 30.3 95 48.4 131 66.7 167 85.3 24 13.0 60 30.8 96 48.9 132 67.2 168 85.9 25 13.5 61 31.3 97 49.4 133 67.7 169 86.4 26 14.0 62 31.8 98 49.9 134 68.2 170 86.9 27 14.5 63 32.3 99 50.4 135 68.8 171 87.4 28 15.0 64 32.8 100 50.9 136 69.3 172 87.9 29 15.5 65 33.3 101 51.4 137 69.8 173 88.5 30 16.0 66 33.8 102 51.9 138 70.3 174 89.0 31 16.5 67 34.3 103 52.4 139 70.8 175 89.5 32 17.0 68 34.8 104 52.9 140 71.3 176 90.0 33 17.5 69 35.3 105 53.5 141 71.8 177 90.5 34 18.0 70 35.8 106 54.0 142 72.3 178 91.1 35 18.5 71 36.3 107 54.5 143 72.9 179 91.6 36 18.9 72 36.8 108 55.0 144 73.4 180 92.1 37 19.4 73 37.3 109 55.5 145 73.9 181 92.6 38 19.9 74 37.8 110 56.0 146 74.4 182 93.1 39 20.4 75 38.3 111 56.5 147 74.9 183 93.7 40 20.9 76 38.8 112 57.0 148 75.5 184 94.2 41 21.4 77 39.3 113 57.5 149 76.0 185 94.7 42 21.9 78 39.8 114 58.0 150 76.5 186 95.2 43 22.4 79 40.3 115 58.6 151 77.0 187 95.7 44 22.9 80 40.8 116 59.1 152 77.5 188 96.3 45 23.4 81 41.3 117 59.6 153 78.1 189 96.8

(A) (B) (A) (B) (A) (B) (A) (B) (A) (B) 190 97.3 233 120.1 276 143.3 319 167.0 362 191.1 191 97.8 234 120.7 277 143.9 320 167.5 363 191.7 192 98.4 235 121.2 278 144.4 321 168.1 364 192.3 193 98.9 236 121.7 279 145.0 322 168.6 365 192.9 194 99.4 237 122.3 280 145.5 323 169.2 366 193.4 195 100.0 238 122.8 281 146.1 324 169.7 367 194.0 196 100.5 239 123.4 282 146.6 325 170.3 368 194.6 197 101.0 240 123.9 283 147.2 326 170.9 369 195.1 198 101.5 241 124.4 284 147.7 327 171.4 370 195.7 199 102.0 242 125.0 285 148.3 328 172.0 371 196.3 200 102.6 243 125.5 286 148.8 329 172.5 372 196.8 201 103.1 244 126.0 287 149.5 330 173.1 373 197.4 202 103.7 245 126.6 288 149.4 331 173.7 374 198.0 203 104.2 246 127.1 289 150.9 332 174.2 375 198.6 204 104.7 247 127.6 290 151.0 333 174.8 376 199.1 205 105.3 248 128.1 291 151.6 334 175.3 377 199.7 206 105.8 249 128.7 292 152.1 335 175.9 378 200.3 207 106.3 250 129.2 293 152.7 336 176.5 379 200.8 208 106.8 251 129.7 294 153.2 337 177.0 380 201.4 209 107.4 252 130.3 295 153.8 338 177.6 381 202.0 210 107.9 253 130.8 296 154.3 339 178.1 382 202.5 211 108.4 254 131.4 297 154.9 340 178.7 383 203.1 212 109.0 255 131.9 298 155.4 341 179.3 384 203.7 213 109.5 256 132.4 299 156.0 342 179.8 385 204.3 214 110.0 257 133.0 300 156.5 343 180.4 386 204.8 215 110.6 258 133.5 301 157.1 344 180.9 387 205.4 216 111.1 259 134.1 302 157.6 345 181.5 388 206.0 217 111.6 260 134.6 303 158.2 346 182.1 389 206.5 218 112.1 261 135.1 304 158.7 347 182.6 390 207.1 219 112.7 262 135.7 305 159.3 348 183.2 391 207.7 220 113.2 263 136.2 306 159.8 349 183.7 392 208.3 221 113.7 264 136.8 307 160.4 350 184.3 393 208.8 222 114.3 265 137.3 308 160.9 351 184.9 394 209.4 223 114.8 266 137.8 309 161.5 352 185.4 395 210.0 224 115.3 267 138.4 310 162.0 353 186.0 396 210.6 225 115.9 268 138.9 311 162.6 354 186.6 397 211.2 226 116.4 269 139.5 312 163.1 355 187.2 398 211.7 227 116.9 270 140.0 313 163.7 356 187.7 399 212.3 228 117.4 271 140.6 314 164.2 357 188.3 400 212.9 229 118.0 272 141.1 315 164.8 358 188.9 401 213.5 230 118.5 273 141.7 316 165.3 359 189.4 402 214.1 231 119.0 274 142.2 317 165.9 360 190.0 403 214.6 232 119.6 275 142.8 318 166.4 361 190.6 404 215.2

(A) (B) (A) (B) (A) (B) (A) (B) (A) (B) 405 215.8 417 222.8 429 229.8 441 236.9 453 244.0 406 216.4 418 223.3 430 230.4 442 237.5 454 244.6 407 217.0 419 223.9 431 231.0 443 238.1 455 245.2 408 217.5 420 224.5 432 231.6 444 238.7 456 245.7 409 218.1 421 225.1 433 232.2 445 239.3 457 246.3 410 218.7 422 225.7 434 232.8 446 239.8 458 246.9 411 219.3 423 226.3 435 233.4 447 240.4 459 247.5 412 219.9 424 226.9 436 233.9 448 241.0 460 248.1 413 220.4 425 227.5 437 234.5 449 241.6 461 248.7 414 221.0 426 228.0 438 235.1 450 242.2 462 249.3 415 221.6 427 228.6 439 235.7 451 242.8 463 249.9 416 222.2 428 229.2 440 236.3 452 243.4

=141. Meissl’s Table for Invert Sugar.=—Invert sugar is usually the product of the hydrolysis of sucrose. The following table is to be used when the hydrolysis is complete, i. e., when no sucrose is left in the solution. The solution of copper sulfate and of the alkaline tartrate are made up as follows: 34.64 grams of copper sulfate in half a liter, and 173 grams of rochelle salt and 51.6 grams sodium hydroxid in the same volume. The quantity of sugar solution used must not contain more than 245 nor less than ninety milligrams of invert sugar.

In the determination twenty-five cubic centimeters of the copper solution and an equal volume of the alkaline tartrate are mixed and boiled, the proper amount of sugar solution added to secure a quantity of invertose within the limits named, the volume completed to 100 cubic centimeters with boiling water, and the mixture kept in lively ebullition for two minutes. An equal volume of recently boiled cold water is added and the cuprous oxid at once separated by filtration on asbestos under pressure, and washed free of alkali with boiling water. The metallic copper is secured by one of the methods already described.

TABLE FOR INVERT SUGAR BY MEISSL AND WIEN.

(A) = Milligrams of copper. (B) = Milligrams of invert sugar.