Spectrophotometric Determination of Nitrates in Plating Baths ALBERT DOLANCE
AND
PAUL W. HEALY, The
Cleveland Graphite Bronze Company, Cleveland, O h i o
A method for the determination OF nitrates in silver plating baths, using a quartz spectrophotometer, is described, which for routine analysis i s accurate to within 2.5 grams per liter.
tilled water in each, and noting the readings. These readings were then applied as correction to the readings taken on the standard solutions and unknowns. USE OF DATAIN ANALYSISOF PLATING SOLUTIONS. The contents of the bath for which the analysis was desired were potassium silver cyanide, potassium cyanide, potassium carbonate, potassium hydroxide, and potassium nitrate (4). Since the cyanides, carbonates, and hydroxides show light absorption in the region of interest, it became necessary to remove these ions. The agent best suited for removing all these substances from the solution by precipitation was found to be a mixture of perchloric acid, barium perchlorate, and silver perchlorate, since the perchlorate causes no interference. It was necessary to dilute the bath to a standard volume after the precipitating agent had been added, since different baths required different amounts of the precipitating solution. A dilution of 1 to 10 proved most practicable.
THE
wet method (2) for the determination of potassium nitratecyanide plating baths is long, tedious, and unsuited to the control work required in bath maintenance. The possibility of making use of the absorption of light due to the nitrate ion a t 305 mp was considered and successfully developed to form a rapid method. APPARATUS AND MATERIALS
Beckman quartz spectrophotometer with tungsten bulb light source and 10-mm. Corex glass cells. The precipitating solution is made up to contain 13.5 grams of perchloric acid, 93.6 grams of silver perchlorate, and 258 grams of barium perchlorate per liter.
PROCEDURE
EXPERIMENTAL WORK SPECTROPHOTOMETRIC
DATAON
Five milliliters of the plating bath are placed in a 100-ml. beaker, and the precipitating solution is added in 1- or 2-ml. portions until no further precipitate is formed. A convenient way to make this addition is to use a medicine dropper, allowing the precipitate to settle for 2 or 3 minutes after each addition. When more than 3 samples are run at once, this will cause no delay if additions are made to each solution in turn. After an addition of precipitating solution causes no further precipitate to form, the beaker is covered with a watch glass and allowed to stand for a t least one hour. The solution is then filtered through KO.42 Whatman paper. The precipitate is washed down at least 3 times with distilled water, the washings being collected in the
POTASSIUM NITRATE SOLU-
TION. I n order to determine the best choice of wave length, a
series of absorption measurements was made throughout the wave-length range 300 to 350 mp a t concentrations of 10, 7.5, 5.0, and 2.5 grams per liter. The absorption data shown in Figure 1 indicate that the maximum in the absorption curve is found a t 302.5 mp.
Table I. Potassium Nitrate Corrected Density Readingso 10 grams 7.5 grams 5 grams per liter per liter per liter 300.0 0.638 0.482 0.322 0.323 302.5 0.638 0.483 0.628 0.478 0.318 305.0 307.5 0.612 0.466 0.307 310.0 0.578 0.437 0.298 0.406 312.5 0.535 0.268 0.358 315.0 0.475 0.239 0.204 0.410 0.310 317,s 0,169 0.257 0.340 320.0 0.156 0.104 0.206 325.0 0,080 0.051 0.106 330.0 0,020 0.034 0.047 335.0 0.004 0.011 0.018 340.0 0,000 0.000 0,002 350.0 a Beckman quartz spectrophotometer. MP
2.5 grams per liter 0.168 0.166 0.159 0.157 0.148 0.137 0,120 0.104 0.086 0.052 0,025 0,009 0.004 0.000
L'?
.600
1
However, examination of the data from which the curves were drawn (Table I) will show that better agreement with Beer's law is obtained when the constant (calculated by dividing the concentration value by the density) is obtained from the values a t 305 mp. Higher concentrations of potassium nitrate than those shown in Figure 1 give the same type of curve but with the maximum shifted slightly toward the higher wave lengths-e.g., the maximum is a t 310 mp for 30 grams per liter of potassium nitrate. The values given in Table I and the curves obtained are in agreement with the curve for potassium nitrate in International Critical Tables in the range 302.5 to 312.5 mp (3). Corex cells were used a t these wave lengths and therefore it was necessary to correct the readings obtained. This correction was computed for each cell simply by running through a set of cells with dis-
5
Figure 1.
P18
Absorption Curves for Potassium Nitrate
ANALYTICAL EDITION
November, 1945
same beaker used for the filtrate. The filtrate is now diluted to 50 ml. An ordinary graduate will do very well for the dilution. The sample should be run soon after dilution, since standing for more than an hour will cause some additional silver salts to precipitate. The solution is run in 10-mm. Corex glass cells. No. of grams per liter = Ko CALCULATION.Using the formula density (the reciprocal of the constant usually obtained when the BeerLambert law applies), we have from the third column of Table I, 7.5 oppobite 305 mp: o,478 = 15.7. Then (KOX 10) X density = grams per liter of K N 0 3 or 157 X D = grams per liter of I
