US2332497A - Determination of sodium hydroxide in presence of the aluminate - Google Patents

Description

Patented Oct. 26, 1943 UNITED STATES PATENT OFFICE DETERMINATION OF SODIUM HYDROXIDE IN PRESENCE OF THE ALUIVHNATE Perle N. Burkard, Wyandotte, Mich., assignor, by mesne assignments, to Wyandotte Chemicals Corporation No Drawing. Application January 6, 1941, Serial No. 373,323

8 Claims.

their conditions and circumstances of usage, have become contaminated with an alkali aluminate. My invention is equally well applicable to caustic alkali solutions in which the caustic alkali is sodium or potassium hydroxide and the corresponding contaminating compound is sodium or potassium aluminate. Sodium hydroxide, however, is the caustic alkali most commonly used commercially in detergent, cleaning and washing solutions. with metallic aluminum, a chemical reaction takes place whereby some or all of the alkali is converted into sodium (or potassium) aluminate, thereby decreasing the alkaline intensity of the solution and its corresponding detergent and germicidal ability.

My invention is based upon the discovery that the addition of sodium fluoride to such solutions which have become contaminated with sodium or potassium aluminate, results in a substantial increase in the alkalinity thereof; in fact, in tripling the alkalinity or hydroxyl ion concentration of.

the aluminate. This discovery is primarily of a two-fold utility.

First it provides a new convenient and reliable analytical method for determining both the caustic alkali and aluminate content of alkaline solutions which have become contaminated with alkali metal aluminate; and secondly, as already indicated it provides a convenient method of regenerating the alkalinity of such solutions of deficient alkaline capacity.

In the art of alkaline cleaning for the food and beverage industries, food containers, beer bottles, milk bottles, and numerous other articles are washed in alkaline solutions containing sodium or potassium hydroxide in combination with alkali metal salts, such as trisodium phosphate,

As such solutions come in contact ingredient in bottle washing compounds. Various other alkalies, such as the alkali metal phosphates, carbonates, bicarbonates, borates and silicates, are used in sodium hydroxide to improve its rinsing properties and to condition the water for more effective cleaning.

Such alkali detergent solutions, when coming in contact with metallic aluminum, such as that found in metal foil labels, bottle caps, utensils and the like, reacts to form alkali metal aluminate according to the following equation:

The other alkali metal salts present also react with the aluminum in a similar manner, forming additional aluminate. The presence of the aluminatein such solutions is undesirable, first, because it is a weaker and much less effective alkali, thus depreciating the alkaline capacity or hydroxyl ion concentration of the solution as a whole; and secondly, because it is impossible, with analytical testing methods heretofore known, reliably to determine the actual amount of alkali metal hydroxide present in the solution. It is, therefore, the general object and nature of my invention to provide a method for regenerating the alkalinity of alkaline,.detergent soluof my analytical method may be accurately determined in dirty and soil-contaminated solutions, To the accomplishment of the foregoing and related ends, said invention, then consists of the steps hereinafter fully described and particularly pointed out in the claims.

The following description sets forth in detail one approved method of carrying out the invention, such disclosed method, however, constituting but one of the various ways in which the principle of the invention may be used. Although, as above indicated, the invention is equivalently applicable to alkali metal compound solutions in which the hydro des d c nating aluminates are those of sodium orpotasslum, the followingdetailed description will suitably, and for the sake of convenience, be confined to the sodium compounds.

As previously pointed out, the contamination oi alkali, detergent solutions with sodium aluminate has presented a serious problem in the art. The seriousness of this problem can be appreciated by considering that the aluminum labels from 210,000 average iii-ounce beer bottles will completely convert all the sodium hydroxide in 600 gallons of a 3 percent sodium hydroxide solution into sodium aluminate, a milder and much less eflective alkali. In addition, by the methods commonly used for titrating alkaline solutions, this solution will still show a 3 percent sodium hydroxide contentwhen no sodium hydroxide is present.

For operating efllciency and to comply with certain public health regulations, it is desirable to maintains deflnite sodium hydroxide content in the solution. Some State laws. for instance, require a 3 percent sodium hydroxide content. To do this, a reliable method of testing the solution must be employed. The methods used in the pasthave been:

(1) By determining the specific gravity of the solutions with a hydrometer and reading the sodium hydroxide content from a conversion table.

(2) By the double titration method employing a standard acid solution, phenolphthalein indicator, and methyl orange indicator and calculating the sodium hydroxide content.

(3) By precipitating the other alkaline salts with neutral barium chlorideand titrating the sodium hydroxide which remains in solution with a standard acid using phenolphthalein indicator solution.

Method (1) is accurate for sodium hydroxide solutions, and, since sodium hydroxide-is seldom used alone,.this method is not accurate enough for accepted use.

Method (2) is accurate only ior solutions con taining sodium hydroxide in combination with sodium carbonate and/or trisodium phosphate.

Method (3) has been most widely used and for general use is the most accurate, but it still is not sufllciently accurate for solutims contaminated with sodium aluminate.

1'. have discovered that the alkalinity of a soduim aluminate solution above a pH of 8.2 can be tripled by adding an excess of sodium fluoride, preferably in the form oi. a powder. The chemicalreactionthattakupiacecannotatthis time be fully explained. Sodium aluminate, NaAlOz, reacts inthe presence of sodium fluoride as it its iorrnula were NasAlOa. Thus, by adding a chemical equivalent amount of powdered sodium fluoride to a solution containing sodium aluminate, the spent alkalinity due to the presence oi the sodium aluminate is regenerated. Such a chemical equivalent amount of the sodium fluoride is, oicourae, estimateduponthebasisoitheprobtetrasodium pyrophosphatc, sodium hexametaphosphate, borax, sodium bi-carbonate, sodium silicate and sub-silicates. The solution is then titrated with an acid solution of known strength using a few drops 01' 0.5 percent alcoholic phenolphthalein solution as the indicator. When the end point is reached at a pH of approximately 8.2, only the alkalinities or the sodium hydroxide and the sodium aluminate have been neutralized. The other alkaline materials have reacted with the barium chloride precipitating barium salts and forming neutral sodium chloride. If at this point an excess, usually about 0.5 to 1.0 gram for every 10 ml. of alkaline solution, of sodium fluoride powder be added, the solution will again turn pink due to the alkalinity formed by the reaction of the aluminate and the sodium fluoride. I have discovered that this increase in alkalinity upon the addition of the sodium fluoride powder is equal to twice the alkalinity oi the sodium aluminate present in the original solution.

Applying the above-stated facts to chemical analysis, I have discovered that, by titrating an aliquot portion of an alkaline solution with an acid of known strength in the presence of barium chloride and phenolphthalein indicator solution, the number of milliliters of acid used in this titration minus 5 the number of milliliters of acid required to reach the second end point after the addition of sodium fluoride powder is equal tothe number of milliliters of acid required to neutralize the sodium hydroxide present in the aliquot portion of the alkaline solution. If 1.000 normal acid were used, each milliliter of acid is equivalent to 0.040 gram 01' sodium hydroxide.

For example, in titrating 10 milliliters of an alkaline solution having a specific gravity of 1.03

to which barium chloride has been added in ex-' cess of that amount required to precipitate carbonates, phosphates, etc., with 1.000 normal suli'uric acid using 0.5% alcoholic phenolphthalein as the indicator solution; 16.5 milliliters of the acid were required to reach the end point. After the addition of sodium fluoride, 10.4 milliliters of additional acid was required to reach the second end point. Then:

(16.5- 10.4/2) X .040X 100 "1o 1.o3 In practical fleld testing. I employ 2.56 normal sulfuric acid and titrate lo-milliliter portions 01 the cleaning solution. The use or 2.58 normal acid in place of 2.50 normal acid compensates apable amount or sodium aluminate present and is added in excess thereoiinorder to insure asate the preceding paragraph then become:

1st end point. 6.44 milliliters of 2.58 8 acid 2nd end point at. the addition or sodium fluoride---- 4.07 milliliters of 2.56 8 acid %NaOH=6.44--4.07/2=4.4

The sodium aluminate can be obtained by multiplyins the milliliters of acid used in themond titration by a constant factor. factor is derived as inflows:

Since all oi the acid used in the second titration is used in titrating the alkali liberated by the re- I action oi sodium aluminate and sodium fluoride, it means that 1 molecule of sulphuric acid is regfggX .l255=0.104ii grams N aAlO;

The above factor is converted into terms of per cent of sodium aluminate by multiplying it by 100 and dividing by 10.3, viz.

The percent of sodium aluminate in our illustrative example is thus calculated as follows:

% NaAlOz=L019 X 4.07

In some instances, it might beyfound necessary to adjust the above derived factor of 1.019 to compensate for the presence of other chemicals in solution and in practical field tests. This ad- .iustment can be made in the factor by titrating a number of solutions of known strength and calculating the factor.

'The method herein described works equally well for solutions from their freezing to their boiling points and for dirty solutions, as well as clean solutions. To overcome the necessity of filtering dirty solutions, I have purposely used barium chloride and sulfuric acid to form an internal, white background oi white barium sulfate to make indicator color changes easily visible.

It will be apparent to those skilled in the art that equivalent indicators other than the phenolphthalein indicator hereirabove mentioned may be employed to indicate the pr p end point during the neutralizing or titrating steps of my analytical method.

By the 'same token, equivalent neutralizing acids, made up in standard solutions, may likewise be employed in lieu of the standard sulfuric acid solutions hereinabove described.

The above-described method for determinin the caustic alkali and sodium aluminate content of solutions is highly accurate and reliable for practically all alkaline detergents encountered in the fleld, even those containing silicates in the amount of up to 1% sodium silicate. Above that amount, silicates will have some eifect upon the accuracy of the sodium hydroxide content determinations. but any such higher percentage of sodium silicate is rarely encountered in solutions contaminated with aluminum, for the simple reason that detergents containing silicates are not ordinarily employed if the solutim is likely to be placed in contact with aluminum.

Other modes of applying the principle of my invention may be employed instead of the one explalned, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims 01' the equivalent of such stated step or steps be employed.

1, therefore, particularly pointout and distinctly claim as my invention:

1. The method of determining the amount of alkali metal hydroxides in solutions thereof contaminated with alkali metal aluminate, consisthis in the steps of neutralizing the alkalinity of said hydroxides and said aluminate, adding sodium fluoride thereto in an amount suiilcient to react completely with the aluminate, thereby regenerating the alkalinity of said aluminate to twice its original amount, and then neutralizing the resultant solution.

2. The method of determining the amount of sodium hydroxide in solutions thereof contaminated with sodium aluminate, consisting in the 7 steps of neutralizing the alkalinity of said hy-' droxide and said aluminate, adding sodium fluoride thereto in an amount sufllcient to react completely with the sodium aluminate, thereby regenerating the alkalinity of said aluminate to twice its original amount, neutralizing the resultant solution, and measuring the sodium hydroxide chemical equivalent amount of neutralizing reagent required in each of the aforesaid neutralizing steps.

3. The method of determining the amount of sodium hydroxide and sodium aluminate present in the same solution, consisting in the steps of neutralizing the alkalinity of said hydroxide and said aluminate, adding sodium fluoride thereto in an amount suflicient to react completely with the sodium aluminate. thereby regenerating the alkalinity 01 said aluminate to twice its original amount. neutralizing the resultant solution, and measuring the sodium hydroxide chemical equivalent amount of neutralizing reagent required in each of the aforesaid neutralizing steps.

4. The method of determining the amount of sodium hydroxide in a solution containing alkali metal salts of weak acids in addition thereto, and contaminated with sodium aluminate, consisting in the steps of precipitating said alkali metal salts. whereby the sodium hydroxide and sodium aiuminate remain in solution, neutralizink the alkalinity of said hydroxide and said aluminate, adding sodium fluoride thereto in an amount sufiicient to react completely with the sodium aluminate, thereby regenerating the alkalinity of said aluminate to twice its original amount,- and then neutralizing the resultant solution.

5. The method of determining the amount 0 sodium hydroxide in a solution containing alkali metal salts of weak acids in addition thereto and contaminated with sodium aluminate, consisting in the steps of precipitating said alkali metal salts, whereby the sodium hydroxide and sodium aluminate remain in solution, neutralizing .the

alkalinity of said hydroxide and said aluminate, adding sodium fluoride thereto in an amount sumcient to react completely with the sodium aluminate, thereby regenerating the alkalinity of said aluminate to twice its original amount,

then neutraliziing the resultant solution, and measuring the sodium hydroxide chemical equivalent amount of neutralizing reagent required in each 'of the aforesaid neutralizing steps.

6. The method of determining the amount of sodium hydroxide and. sodium aluminate, both present in the same solution, and in which alkali with the sodium aluminate, thereby regeneratin the alkalinity of said aluminate to twice its original amount, neutralizing the resultant solution,

and measuring the sodium hydroxide chemical equivalent amount of neutralizing agent required in each of the aforesaid neutralizing steps.

7. The method of determining the amount of sodium hydroxide in a solution containing alkali metal salts of weak acids in addition thereto and contaminated with sodium aluminate, consisting in the steps of adding barium chloride to said solution in excess of the amount required to precipitate said alkali metal salts. thereby leaving the sodium hydroxide and sodium aluminate in solution, adding an indicator to said solution and titrating to the neutral end point with a standard solution of sulfuric acid, thereby forming a white background of barium sulphate in the solution, adding sodium fluoride to the solution in an amount suiiicient to react completely with the sodium aluminate, thereby regenerating the alkalinity of said aluminate to twice its original amount, and again. titrating to the neutral 20 normal solution of sulfuric acid, thereby forming a white background of barium sulphate in the solution, adding sodium fluoride to the solution in an amount suiiicient to react completely with the sodium aluminate. thereby regenerating the alkalinity of said aluminate to twice its original amount, and again titrating to the neutral end point with a 2.58 normal solution of sulfuric acid and measuring the respective amounts of said sulfuric acid solution required in both of the aforesaid titration steps.

PERLE N. BURKARD.