48 
LABORATORY MANUAL 
EXERCISE NO. 35 
Object: To become acquainted with the preparation and use of the Farrington 
rapid method of acidity testing. 
Procedure: Milk which gives the highest acid test contains, as a rule, 
a larger number of bacteria and spores not destroyed by pas¬ 
teurization than does milk giving a low acid test. (F. & W. 
If 129.) It is therefore important that milk be properly se¬ 
lected at the receiving room in order to have the best material 
for manufacturing purposes, and any simple test which will 
aid in the proper selection is of immense value. 
A. The Farrington rapid method alkaline tablet solution for se¬ 
lecting milk containing less than .2% acid, is prepared by dis¬ 
solving two Farrington alkaline tablets in one ounce of distilled 
water. This solution is ready for use after about 12 hours, or 
after all tablets have entirely disentegrated. 
If, when equal amounts of this solution and milk are taken 
and mixed, the mixture turns pink, the per cent is below 
.2%. Conversely, if the mixture remains white, the acidity is 
above .2%. 
B. Test all samples provided, using the dippers and cups supplied. 
Note: With a little practice in using fractional parts of a dip¬ 
per of solution, the approximate acidity of a sample may be 
determined. 
Observations: 
Sample 
Approximate acidity 
1 
2 
3 
Conclusions: 
Questions: (1) How is an NaOH solution, equal in strength to the Far¬ 
rington rapid method solution, prepared? 
(2) How is a Farrington alkaline tablet solution prepared so that 
it will indicate an acidity exceeding .25%? 
(3) If 25 cc. of milk requires 5 cc. of an n / # NaOH solution for 
neutralization, what per cent of acidity does milk contain? 
(4) If 20 cc. of an n / 9 NaOH solution is necessary to neutralize a 
17.6 cc. sample of milk, what per cent acidity has it? 
References: F. & W. If 128-150, inclusive; Van S. pp. 131-151; Jud pp 
102-111; St. pp. 132-136. 
