diff --git a/js/common/PHScaleConstants.js b/js/common/PHScaleConstants.js
index 6f390a1c..553f9f02 100644
--- a/js/common/PHScaleConstants.js
+++ b/js/common/PHScaleConstants.js
@@ -50,6 +50,7 @@ const PHScaleConstants = {
 
   // logarithmic graph
   LOGARITHMIC_EXPONENT_RANGE: new Range( -16, 2 ),
+  LOGARITHMIC_MANTISSA_DECIMAL_PLACES: 1,
   LINEAR_EXPONENT_RANGE: new Range( -14, 1 ),
   LINEAR_MANTISSA_RANGE: new Range( 0, 8 ),
 
diff --git a/js/common/view/graph/GraphIndicatorDragListener.js b/js/common/view/graph/GraphIndicatorDragListener.js
index 41bf100f..5546a629 100644
--- a/js/common/view/graph/GraphIndicatorDragListener.js
+++ b/js/common/view/graph/GraphIndicatorDragListener.js
@@ -7,6 +7,7 @@
  */
 
 import Utils from '../../../../../dot/js/Utils.js';
+import ScientificNotationNode from '../../../../../scenery-phet/js/ScientificNotationNode.js';
 import DragListener from '../../../../../scenery/js/listeners/DragListener.js';
 import Tandem from '../../../../../tandem/js/Tandem.js';
 import phScale from '../../../phScale.js';
@@ -48,15 +49,30 @@ class GraphIndicatorDragListener extends DragListener {
           // Adjust the y-coordinate for the offset between the pointer and the indicator's origin
           const y = targetNode.globalToParentPoint( event.pointer.point ).y - clickYOffset;
 
-          // Convert the y-coordinate to a model value
+          // Convert the y-coordinate to a model value.
           const value = yToValue( y );
+          assert && assert( value > 0 );
+
+          // Round the model value to the first 2 non-zero decimal places. This prevents continuous dragging from
+          // creating values that have too much precision, which can result in pH = 7.00 with unequal amounts of 
+          // H3O+ and OH-. See https://github.com/phetsims/ph-scale/issues/225.
+          const scientificNotation = ScientificNotationNode.toScientificNotation( value, {
+            mantissaDecimalPlaces: PHScaleConstants.LOGARITHMIC_MANTISSA_DECIMAL_PLACES
+          } );
+          const exponent = scientificNotation.exponent - PHScaleConstants.LOGARITHMIC_MANTISSA_DECIMAL_PLACES;
+          const interval = Math.pow( 10, exponent );
+          const adjustedValue = Utils.roundToInterval( value, interval );
 
           // Map the model value to pH, depending on which units we're using.
-          let pH = ( graphUnitsProperty.get() === GraphUnits.MOLES_PER_LITER ) ? concentrationToPH( value ) : molesToPH( value, totalVolumeProperty.get() );
+          let pH = ( graphUnitsProperty.get() === GraphUnits.MOLES_PER_LITER ) ?
+                   concentrationToPH( adjustedValue ) :
+                   molesToPH( adjustedValue, totalVolumeProperty.get() );
 
           // Constrain the pH to the valid range
           pH = Utils.clamp( pH, PHScaleConstants.PH_RANGE.min, PHScaleConstants.PH_RANGE.max );
 
+          phet.log && phet.log( `value=${value} adjustedValue=${adjustedValue} pH=${pH}` );
+
           // Set the solution's pH
           pHProperty.set( pH );
         }
diff --git a/js/common/view/graph/GraphIndicatorNode.js b/js/common/view/graph/GraphIndicatorNode.js
index 8719d5cc..393fa2ea 100644
--- a/js/common/view/graph/GraphIndicatorNode.js
+++ b/js/common/view/graph/GraphIndicatorNode.js
@@ -55,7 +55,7 @@ class GraphIndicatorNode extends Node {
       valueYMargin: 3,
       xSpacing: 8,
       ySpacing: 4,
-      mantissaDecimalPlaces: 1,
+      mantissaDecimalPlaces: PHScaleConstants.LOGARITHMIC_MANTISSA_DECIMAL_PLACES,
       exponent: null, // use this to request a specific exponent, otherwise the exponent is computed
       isInteractive: false,
       arrowFill: 'rgb( 0, 200, 0 )',